Coal petrology and petrographic analysis 3.1
INTRODUC DUCTION
Close examination of coal in hand specimen generally shows it to be composed of different layers. Under the microscope, these layers in turn are seen to be composed of mixtures of discrete entities, each class of which is distinguished by having different optical characteristics. Coal petrology is the study of the origin, composition and technological behaviour of these different materials, while the systematic quantification of their proportions and characteristics under the microscope is sometimes known as ‘coal petrography’. he different layers or entities occurring in a single coal may possess quite different physical and chemical properties, and hence their relative abundance and manner of admixture is vital in determining the overall characteristics of a coal seam or mined coal product. Coal Coal petr petrog ogra raphy phy has has been been wide widely ly appl applie iedd to the the sele select ctio ionn and and blen blendi ding ng of coal coalss for for prod produc ucti tion on of metallurgical coke, ke, and is one of the ma!or cons consid ider erat atio ions ns in rese resear arch ch dire direct cted ed towa toward rdss coal coal liquefaction operations. he techniques of coal petrology are also also used used in geo geolog logica icall invest investiga igatio tions ns aimed aimed at assessing the potential of rocks and sedimentary basins as sources of petroleum. 3.2 MEG!CO"IC##$ RECOGNI%&#E CON!TITUENT!
' large part of the terminology used in coal petrology is derived derived from the work of &topes $()()% in recogni*ing recogni*ing four basic ‘ingredients’ of banded bituminous coal that can can be dist distin ingu guis ishe hedd in hand hand spec specim imen ens. s. hes hesee constituents, regarded in current usage as ‘lithotypes’, were identified by &topes, a palaeobotanist, as follows+ $a% Vitrain $. vitrurn, glass% -itrain is the black, glassy, vitreous material that is probably the most striking component of bituminous coals. t occurs as thin bands, commonly less than / or 0 mm in thickness and is usually very closely !ointed. -itrain tends to be more brittle than other megascopic coal coal con consti stitue tuents nts,, often often breaki breaking ng with with a con concho choida idall fraction $"ig. 1.(a%. $b% Clarain $. clarus, bright% his lithotype is represented by bright to semi#bright bands of finely laminated coal. Clarain generally exhibits an overall silky lustre, and commonly contains fine fine vitra vitrain in ban bands ds altern alternati ating ng with with a duller duller attri attrital tal groundmass $"ig. 1.(b%. $c% Durain $. durus, hard%
2urain occurs as grey to black bands with a dull to slightly slightly greasy greasy lustre. lustre. he material material is relative relatively ly hard he he petr petrol ology ogy of coal coal may be stud studie iedd at eith either er a compared to other lithotypes, and tends to break into megascopic or a microscopic scale. "rom a megascopic large large,, blocky blocky fragm fragment ents. s. 2urain 2urain may someti sometimes mes be point of view, coal may be classified into two broad confused with impure coal or carbonaceous shale, which groups, the banded or ‘humic’ coals and the non#banded are also often dull and hard, but it can be distinguished $massive% or ‘sapropelic’ coals. he humic coals are by its lower density density $"ig. 1.(c%. visibly stratified, consisting of layers or bands of organic material of varying appearance, with individual layers usually no more than a few centimetres in thickness. $d% Fusain $. fusus, a spindle% &uch coals are derived from a heterogeneous mixture of a wide range of plant debris. he sapropelic coals, on the n "rench, the word fusain means charcoal, which at one other hand, are homogeneo homogeneous, us, tough materials, materials, often often time was made from the wood of the spindle tree. he displaying a marked conchoidal fracture. hey are made suffix 3ain’ was adopted for the other lithotypes. 4here up of specif specific ic kinds kinds of fine fine graine grainedd organi organicc matter matter,, it is unminerali*ed, fusain is a soft, friable material that closely resembles the charcoal from which it takes its notably masses of spores or algal material. name. &oft, or unminerali*ed, fusain easily disintegrates int into a bla black fibr ibrou ouss po powd wder er,, bu butt hard ard fusai usain, n, 3.2.1 3.2.1 #ithot #ithotype ypess in in 'and 'anded ed 'it()i 'it()ino( no(ss coal coalss impregnated with mineral matter, may be found in some coals coals as well. well. "usain "usain usuall usuallyy occurs occurs as thin thin lenses lenses,, seldom more than a few millimetres thick, and is only a very minor constituent of most bituminous coal seams on a volumetric basis $"ig. 1.d%. he terms duroclarain $Cady ()56% and clarodurain
Coal Petrology and Petrographic Analysis
$a%
,c-
dark in colour with a dull to greasy lustre and typically displa displayy a marke markedd con concho choida idall fractu fracture re $"ig. $"ig. 1.6a%. 1.6a%. &apropelic coals may occur as layers or plies within seams of banded or humic coal, often at the roof. hey also occur as seams made up mainly of homogeneous, non#banded material in their own right. he two ma!or types of sapropelic coal are ‘cannel coal7, composed largely of spores or fine organoclastic detritus, and ‘boghead coa8 composed largely of algal material. hese are, however, effectively end#members of a range of materials representing mixtures of these two two type typess of comp compon onen ents ts,, and and tran transi siti tion onal al or inte interm rmed edia iate te form formss such such as cann cannel el## bo bogh ghea eadd and and boghead#cannel may be recogni*ed as well. 9ogheads may grade laterally or vertically into oil shales. 4hen viewed under the microscope, cannel coal can be distinguished from boghead coal both by the abundance of spores and by the presence of a regular microstratification. he materials are, however, almost impos impossib sible le to distin distingui guish sh from from each each other other in han handd specimen.
$b%
,d-
"ig. 1.( ithotypes in banded bituminous coal, $a% -itrain -itrain in polished surface. Aote that the three bands of vitrain vitrain at the top of of the block are more more highly !ointed $cleated% than the rest of the coal $xB1.0%. $b% Clarain in polished surface. he lower three quarters of the block are composed mainly of clarain the upper quarter consists of two durain bands $grey% and a vitrain band $black% $black% $x?.0%. $c% 2urain in hand specimen $x?.1%. $d% "usain in bedding surface of hand specimen $x?.6%.
3.2.3 3.2.3
have been added to this list by some workers to extend the number number of terms terms availa available ble for megasc megascopi opicc coal coal description. hey represent material that is intermediate in character between clarain and durain.
:ricr-it
Ul;,lll-ll-ll 4t -< 4ll
$vit vitrain rain and clar larain% ain% and du dull ll components compon component entss $durai $durainn and fusain fusain%% is most most appa appare rent nt in coal coalss of bitu bitumi mino nous us rank rank.. n anthracites, however, the lithotypes all tend to develop a relatively bright lustre, and the contrast between them decreases. 3.2.2. #ithotypes in sapropelic 'it()ino(s coals
Unli Unlike ke the the band banded ed or hu humi micc coal coals, s, whic whichh were were deposited as peats made up of large to small fragments of plan plantt debr debris is,, the the sapr saprop opel elic ic coal coalss repr repres esen entt accumulations of fine organic mud containing concen# trations of algae or spore remains. &apropelic coals are charac character terist istica ically lly fine fine graine grained, d, faintl faintlyy bed bedded ded to homogeneous, massive materials. hey are generally
*iel *ield d desc descri ript ptio ion n o+ coal coal sea sea)s )s
'lthough the terms vitrain, clarain, durain and fusain are are wide widely ly used used for for the the desc descri ript ptio ionn of indi indivi vidu dual al specimens or discrete hori*ons within a coal seam, a number of difficulties arise with their employment in routine logging of seam sections in bore cores or coal exposures $2avis ()=0a%. ;ne ma!or problem is that these four terms refer to quite different kinds of units within within the coal. coal. -itra -itrain in ban bands ds repres represent ent coalif coalified ied fragments of wood or bark and are generally no larger than an individual tree trunk in si*e. Clarain and durain, on the other hand, are usually more extensive units, each possibly representing a aepositional environment within the peat swamp. n a rather exaggerated analogy, the vitrain bands might be compared to an individual pebble, while the clarains and durains are like the conglomerate in which the pebbles occur. 'nother disadvantage associated with use of &topes’ terminology lies in the fact that the individual layers or lenses of the four lithotypes may be very thin, usually only only some some milli millimet metres res in thickn thickness ess.. >ven >ven with with the accept accepted ed minimu minimum m layer layer thickn thickness esses es of differ different ent countries $1#(? mm%, strict application of the &topes terms terms can result result in extremely extremely detailed descriptions descriptions.. @any @any fiel fieldd desc descri ript ptio ions ns of coal coal seam seams, s, ho howe weve ver, r, especially those of very thick seams, are based, for expediency expediency,, on sub#divis sub#division ion into a relatively relatively small number of megascopically distinct units. he
75
Coal Petrology and Petrographic Analysis
$a%
,c-
dark in colour with a dull to greasy lustre and typically displa displayy a marke markedd con concho choida idall fractu fracture re $"ig. $"ig. 1.6a%. 1.6a%. &apropelic coals may occur as layers or plies within seams of banded or humic coal, often at the roof. hey also occur as seams made up mainly of homogeneous, non#banded material in their own right. he two ma!or types of sapropelic coal are ‘cannel coal7, composed largely of spores or fine organoclastic detritus, and ‘boghead coa8 composed largely of algal material. hese are, however, effectively end#members of a range of materials representing mixtures of these two two type typess of comp compon onen ents ts,, and and tran transi siti tion onal al or inte interm rmed edia iate te form formss such such as cann cannel el## bo bogh ghea eadd and and boghead#cannel may be recogni*ed as well. 9ogheads may grade laterally or vertically into oil shales. 4hen viewed under the microscope, cannel coal can be distinguished from boghead coal both by the abundance of spores and by the presence of a regular microstratification. he materials are, however, almost impos impossib sible le to distin distingui guish sh from from each each other other in han handd specimen.
$b%
,d-
"ig. 1.( ithotypes in banded bituminous coal, $a% -itrain -itrain in polished surface. Aote that the three bands of vitrain vitrain at the top of of the block are more more highly !ointed $cleated% than the rest of the coal $xB1.0%. $b% Clarain in polished surface. he lower three quarters of the block are composed mainly of clarain the upper quarter consists of two durain bands $grey% and a vitrain band $black% $black% $x?.0%. $c% 2urain in hand specimen $x?.1%. $d% "usain in bedding surface of hand specimen $x?.6%.
3.2.3 3.2.3
have been added to this list by some workers to extend the number number of terms terms availa available ble for megasc megascopi opicc coal coal description. hey represent material that is intermediate in character between clarain and durain.
:ricr-it
Ul;,lll-ll-ll 4t -< 4ll
$vit vitrain rain and clar larain% ain% and du dull ll components compon component entss $durai $durainn and fusain fusain%% is most most appa appare rent nt in coal coalss of bitu bitumi mino nous us rank rank.. n anthracites, however, the lithotypes all tend to develop a relatively bright lustre, and the contrast between them decreases. 3.2.2. #ithotypes in sapropelic 'it()ino(s coals
Unli Unlike ke the the band banded ed or hu humi micc coal coals, s, whic whichh were were deposited as peats made up of large to small fragments of plan plantt debr debris is,, the the sapr saprop opel elic ic coal coalss repr repres esen entt accumulations of fine organic mud containing concen# trations of algae or spore remains. &apropelic coals are charac character terist istica ically lly fine fine graine grained, d, faintl faintlyy bed bedded ded to homogeneous, massive materials. hey are generally
*iel *ield d desc descri ript ptio ion n o+ coal coal sea sea)s )s
'lthough the terms vitrain, clarain, durain and fusain are are wide widely ly used used for for the the desc descri ript ptio ionn of indi indivi vidu dual al specimens or discrete hori*ons within a coal seam, a number of difficulties arise with their employment in routine logging of seam sections in bore cores or coal exposures $2avis ()=0a%. ;ne ma!or problem is that these four terms refer to quite different kinds of units within within the coal. coal. -itra -itrain in ban bands ds repres represent ent coalif coalified ied fragments of wood or bark and are generally no larger than an individual tree trunk in si*e. Clarain and durain, on the other hand, are usually more extensive units, each possibly representing a aepositional environment within the peat swamp. n a rather exaggerated analogy, the vitrain bands might be compared to an individual pebble, while the clarains and durains are like the conglomerate in which the pebbles occur. 'nother disadvantage associated with use of &topes’ terminology lies in the fact that the individual layers or lenses of the four lithotypes may be very thin, usually only only some some milli millimet metres res in thickn thickness ess.. >ven >ven with with the accept accepted ed minimu minimum m layer layer thickn thickness esses es of differ different ent countries $1#(? mm%, strict application of the &topes terms terms can result result in extremely extremely detailed descriptions descriptions.. @any @any fiel fieldd desc descri ript ptio ions ns of coal coal seam seams, s, ho howe weve ver, r, especially those of very thick seams, are based, for expediency expediency,, on sub#divis sub#division ion into a relatively relatively small number of megascopically distinct units. he
75
76
Chapter 3
&top &topes es syst system em was was no nott desi design gned ed for, for, and and is no nott this is summari*ed in able 1.6. "usain bands or lenses particularly effective effective in, this kind of usage. thicker than about 7 mm are recorded separately, as are Decogni*ing these and other difficulties inherent in non#coal non#coal bands or partings, partings, for which conventional conventional the &topes &topes termin terminol ology ogy of coal coal litho lithotyp types, es, &chopf &chopf sedimentary rock terms may be used. 'nother system, $()/?% established a descriptive system for use by the used extensively extensively to describe describe 'ustrali 'ustralian an bituminou bituminouss U.&. Eeological &urvey, and this has been subsequently coals in outcrops, mine exposures and drill cores, is employed by many others for field use. &chopf s terms disc discus usse sedd in &ect &ectio ionn /.7. /.7./. /. 2isc 2iscus ussi sion onss of the the are out in able 1.(. n summary, three constituents are preparation and use of megascopic coal seam logs in described, namely ‘vitrain’, ‘fusain’ and ‘attrital coal’. field field studi studies es are are also also given given by &chopf &chopf $()/?% $()/?% and he he firs firstt two two are are rega regard rded ed as larg larger er clas clasti ticc un unit itss 2utcher $()=0%. occurring within a matrix of finely divided attrital coal. he thickn thickness ess and con concen centra tratio tionn of the vitra vitrain in and 3.2. .2. I)p(re co coals fusain fusain are described described in terms terms which are quantitat quantitatively ively precise, and the attrital coal is described as having one 4here the coal contains a significant amount of mineral of five levels of lustre, ranging from bright to dull. matt matter er,, its its ov over eral alll dens densit ityy and and ash ash yiel yieldd incre increas asee 'nother alternative has been to describe the coal significantly. 'lthough the distinction between clean with with refe refere renc ncee only only to its its rela relati tive ve brig bright htne ness ss.. coal coal and and impu impure re coal coal is gene genera rall llyy base basedd on the the >mploying terms used in the Eerman coal industry, economic constraints of mining, marketing and use, 2iesse 2iessell $()/7% $()/7% has descr describe ibedd coal coal seams seams as being being most materials regarded in the latter category have an compos composed ed of megasc megascopi opical cally ly disti distinct nct layers layers of the ash yield greater greater than 67F, and sink when placed placed in a following types of materials+ liquid with a relative density of (./?. Gowever, where $a% bbrright coal 3 the coal has more than about 5?#7?F ash, it is usually $b% ban banded ded bbrig right ht coal coal 3 more more corr correc ectl tlyy desc descri ribe bedd in no non# n#co coal al term terms, s, for for $c% $c% band banded ed coal coal example as a carbonaceous shale or a coaly shale. $d% ban banded ded dull dull coal coal he mineral impurity in the coal may be in the form $e% $e% dull dull coal coal.. of discrete bands, streaks or layers interbedded with the here are many similar descriptive systems in use organic constituents, or it may occur as nodules or as around the world, and all can be used on as broad or as fracture infillings. t may be made up of clay or shaley material, or of pyrite, siderite or calcite. Coal with a narrow narrow a scale scale as necess necessary ary or con conven venien ientt for the the particular purpose for which the description is being significant amount of fine clay disseminated throughout prepared. ' seam may be described on a centimetre the organic matter, rather than in discrete layers, is often described as stony coal or, in the U.&.'., as one scale $e.g. for research purposes%, or on a metre scale, coal’ $"ig. 1.6b%. &uch material is characteri*ed by a as desired. '9> 1.( U.&. Eeological &urvey terms for megascopic description dull appearance and, commonly, a grey, rather than a of banded bituminous coal $&chopf ()/?%. black, coloured streak. 'n indication of the terms that may be used to describe impure coals is given in able -itrain "usain 1.1. 'ttrital coal J
f bright bright moderately bright midlustrous
moderately dull ( dull hi hickn ckness ess cl classes $m $mm% thin bands ?.7#6 medium bands 6#7 thic k bands 7#7? very thick bands I7?
Concent entrati ation cl classes $F $F% sparse B(7 moderate (7#1? abundant 1?#/? dominant I/?
' combination of the bright#dull system and the &chopf system has been devised by the Coal Desearch &ectio &ectionn of he 8ennsy 8ennsylva lvania nia &tate &tate Univer Universit sityy for effective seam descriptions at a megascopic level, and
3.2./ 3.2./
Megasc Megascopic opic +eat(r +eat(res es o+ lo0ra lo0ran n coals coals
"or most practical purposes, the distinction between ‘hard’ coals, of bituminous rank or higher, and ‘soft’ or low rank coals is based on the specific energy and other chemical properties of the materials concerned $&ection 6.(? 6.(?.6 .6%. %. n >uro >urope pean an term termin inol olog ogyy, the the low low rank rank H mate materi rial alss are are gene genera rall llyy desc descri ribe bedd as brown coaf, whereas in the U.&.'. and elsewhere they are classed either as ‘lignite’ or ‘sub#bituminous coal’, depending mainly on their chemical characteristics. 'lthough the term ‘brown coal’ is, strictly speaking, applicable to a wider range of material than ‘lignite’, it is also used as a synonym for ‘lignite’ in many contexts. ignite is a dull, soft, earthy material, ranging from
'9> 1.6 2escriptive terms for coals $he 8ennsylvania &tate University%. Coarsely banded coals
'ttrital coals
bright $banded%< $banded%< coal $I )? -%f -%f bright interbanded interbanded with dull coal $/7#)? -% interbanded dull and bright coal+%+ $17#/7 -%
bright $attrital% $attrital% coal< $I )? )? -%
midlustrous coal J
fusain
dull interbanded with bright coal $(?#17 -% dull coal $B (? -% sapropelic $non#banded% coal K
< n practice, only one category of bright coal has been employed. Aumbers in parentheses refer to approximate percentages of vitrain bands and streaks. 3 Contains approximately equal proportion of bright coal and dull attrital coal. J @idlustrous attrital coal may contain many fine vitrain streaks, but obvious vitrain bands should be less than (?F.
brown to black in colour. t may occur in a massive sapropelic form or, more commonly, as a humic material banding, but some are almost massive in hand specimen. 9oth lignites and subbituminous coals also tend to crack with recogni*able wood, leaf and other plant and fall fall apart apart on drying drying#ou #outt with with exp exposu osure, re, a proces processs known generally as ‘slacking’.
"ig. "ig. 1. 1.66 $a% &aprop &apropeli elicc coal. coal. Aote Aote the the massiv massivee struc structur ture, e, faint bedding and conchoidal fracture $
fragments of various si*es set in a finer grained organic matri matrix. x. ' sub#bi sub#bitum tumino inous us coal, coal, on the other other han hand, d, is generally black in colour and relatively hard, ranging from dull to vitreous in lustre. Gumic varieties may display
ow rank coals, especially lignites, are very difficult to descri describe be by megasc megascopi opicc examin examinati ation. on. he colour colour and lustre may vary with different degrees of dessication, and
no generally generally accepted classificat classification ion of lithotyp lithotypes es has yet been devised. ' classification of brown coal lithotypes is currently being prepared by the nternational Committee for Coal 8etrology $.C.C.8.%, with the different categories distinguished on the basis of colour and texture rather than chemical composition. he terms that have been proposed in this classification are+ $a% xyliti xyliticc $woody $woody% % $b% $b% attr attrit itic ic $c% fusi usitic tic $d% $d% mine minera rali li*e *ed. d. 3.2. pplications o+ )egascopic sea) descriptions
2etailed megascopic logging of coal seam sections is a time consuming and often difficult task. Conditions of lighti lighting, ng, surfac surfacee moistu moisture re and access accessibi ibilit lityy may pose pose problems for the geologist working at a mine face, and even in the somewhat less harried situation of bore core logging, the friability or degree of oxidation of the coal may impede the logging process.
'9> 1.1 2escriptive terms for impure coals. impure coal, undifferentiated bone coal< impure coal, with shale bandsMstreaks $alternative name+ shaley c oal% impure coal, with pyrite layer$s%Mband$s%Mnodule$s%Metc. impure coal, with carbonate band$s%Mnodule$s%Metc. < ‘9one coal’ is an 'merican miners’ term for describing coal in which a significant amount of fine clay is disseminated through the coal rather than occurring in obvious shale partings. Conse# quently, bone coal has a dull appearance and a grey streak.
"resh, clean surfaces are needed for good descriptions and cores should be carefully broken open, while in situ seam exposures should be cleared of any weathered debris or fire retardant stone dust $as used in underground mines% prior to logging. ' geologist inspecting a seam exposed at a mine face or in an exploratory bore core has a unique opportunity to record the exact structural characteristics of that seam before it is destroyed either by mining or by the analysis process. >ven though, in some instances, there may be a lack of consistency between results from different workers, experienced personnel are often able to obtain a considerable amount of useful data that may be evaluated in con!unction with other ply#by#ply analyses for very little additional cost. &ome coal seams have characteristic lithotype profiles that remain more or less constant over wide areas, or contain marker beds of distinctive character that can be recogni*ed in many parts of the field. ' detailed megascopic log, perhaps expressed in graphic form, may be very useful in correlating the individual seams in a coal#bearing succession, and in the interpretation of displacements in faulted strata. 2urains and fusains have proved especially useful in this regard $Cameron ()=( 'ustin N 2avis ()=)% as have individual bands of non# coal material $e.g. @ackowsky ()/0a%. @egascopic profiles or logs of the seam are also potential sources of information on variations in coal quality that may affect the mining, preparation or utili*ation of the material. ' brightness log based on the descriptive system of 2iessel $()/7% has been used, for example, as a rough guide to the coking potential of individual seams in 'ustralia $Gawthorne N weedale ()/=%. 'lthough it is not necessarily proper nor wise to draw inferences on the maceral composition of seams from such data, it has proved possible to make at least some correlation with micropetrographic characteristics in a number of cases $e.g. 2iessel ()/7 Cameron ()=0 @archioni ()0?%. 3.3 MICRO!CO"IC ""ERNCE O* CO# MCER#!
4hen viewed under the microscope, coal is seen to consist of particles and bands of different kinds of carbonaceous
material. hese discrete entities represent the coalified remains of the various plant tissues or plant#derived substances that existedO at the time of peat formation. hey are distinguished from each other on the basis of their morphology, hardness and optical properties, and also exhibit differences with respect to their chemical characteristics or technological behaviour in coal utili*ation. he different entities that make up a coal in this way are known as ‘macerals’, a term coined by &topes $()17% as an analogy to the minerals of inorganic rocks. &everal of the maceral names suggested by &topes were adopted at the ()17 Geerlen Congress, and as a consequence the universally adopted classification of these components is referred to as the &topes#Geerlen system. @uch of the early work on the micropetrology of coal, including &topes’ original classification, was carried out using thin sections viewed in transmitted light. Gowever, &tach $()6=% pioneered the use of polished sections studied in reflected light, under oil immersion. his development paved the way for more efficient and consistent practice of coal petrography. 'lthough the techniques tend to complement each other to some extent, almost all routine petrological work at present is based on polished section methods. he criteria by which the various macerals are identified depend mainly on their appearance and optical characteristics under reflected light illumination. 'll maceral names in the &topes#Geerlen system have the suffix ending finite’. wo of these macerals are entities which, when observed under the microscope, are seen to make up the bulk of the lithotypes vitrain and fusain, and they have been named vitrinite and fusinite, respectively. he other lithotypes, namely clarain and durain, are generally composed of a heterogeneous mixture of macerals of different kinds. Coal macerals in the &topes#Geerlen system are classified into three groups on the basis of their physical appearance, chemical characteristics and biological affinities $able 1.5%. he appearance of the members of each group, however, changes with advance in coal rank, and the distinctions between the groups that are easily seen in low rank bituminous coal may, for example, be lost in semi#anthracite. n some cases, particularly with brown coals $lignites and sub#bituminous materials%, different names may be used for macerals of similar origin to emphasi*e these characteristics.
he appearance of the different macPrais in transmitted light is the antithesis of that in reflected light, since a material that is relatively transparent is inherently a poor reflector. Coal for thin section study must be ground to a thickness of about (?Mun $&ection 1.=.(%, one#third that needed for other rocks, and this makes the preparation process a highly skilled and time#consuming operation. ' comprehensive summary of the features that characteri*e the various members of the macPrai groups is given in the !nternational "and#oo$ of Coal Petrography published by the nternational Commission on Coal 8etrology $.C.C.8. ()/1, ()=(, ()=/%. his all#embracing reference gives a complete definition for each macPrai, including the derivation of its name and a list of any synonyms, as well as its morphography, IOLIVO
anu
nrnnprfi 'C rhprmrol phoroM3tpricti3c or#i rl Q 14CU
-ll-llll-Ul -liUia-Cl
'9> 1.5 &topes#Geerlen classification of maceral groups, macerals and submacerals of hard coals. @aceral group telocollinite
@aceral
) telinite -
vitrinite
&ubmaceral gelocollinite desmocollinite corpocollinite
collinite
liptinite $‘exinite’%
%& sporinite cutinite suberinite resinite alginite
liptodetrinite
botanic affinities. t also includes theories on the mode of origin of the various macPrais, and an indication of their respective significance to commercial processes. 3.3.1
fluorinite bituminite exudatinite
The 4itrinite gro(p
-itrinite is the preponderant macPrai in most coals. t originates mainly from the preservation of the stems, roots and leaves of plants, including the wood, periderm and leaf mesophyll tissues and some cell fillings, all with varying degrees of mechanical degradation but relatively minor oxidative alteration. -itrinite is also formed from colloidal humic gels. 8lant cell structure may often be observable under the microscope $"ig. 1.1a%, and sometimes even by the naked eye $"ig. 1.6c% in the larger vitrinite occurrences. n thin section, vitrinite is moderately transparent and appears coloured in various shades of red, orange and brown. n reflected light under oil immersion, however, it appears medium grey in contrast to the darker liptinite and lighter inertinite macPrais $"ig. 1.1b%. -itrinite occurring in vitrain bands of about 1#(6 mm in thickness represents the mummified and coalified products of larger roots, bark and stems of plants. his type, or sub#maceral of vitrinite is known as ‘telocollinite’. 4here a distinct cell structure is visible the term ‘telinite’ has been used, although some authors prefer to restrict this particular term to the cell wall material only. n some instances it is possible to identify the actual plant genus from which such vitrinite was derived. 'part from the relatively thick bands derived from woody components, other vitrinite originates from
inertinite
s fusinite semifusinite ' macrinite ( micrinite sclerotinite 3 inertodetrinite
smaller plant tissues, such as grass and reeds, and tissues that have been degraded into finer si*ed fragments. his material often occurs in attrital admixtures with other macerals and minerals, and is known as ‘desmocollinite.’ -itrinite of this type is a typical constituent of clarains. he individual particles have lost much of their integrity, and are cemented together as a result of gPlification. 2esmocollinite often appears darker in reflected light than the telocollinite in the same coal $"ig. 1.1c% and this may be due in part to derivation from a more cellulose#poor type of plant material. Gowever, the fact that desmocollinite layers sometimes fluoresce in blue light illumination $&ection 1.1.5% suggests that some lipid substances may have been absorbed into the material. he breakdown of ligno#cellulosic tissues by bacterial, fungal or chemical action produces a colloidal gel, of which the ‘dopplerite’ found in peats is an example. 4here it is possible to recogni*e that cell lumens of vitrinite or fusinite, or cracks and other cavities, have been filled with a substance which must have precipitated as a gel, the type of vitrinite that forms the infilling material is known as ‘gelocollinite’. Ret another type of vitrinite, called ‘corpocollinite’, is represented by circular, elliptical or rod#shaped
bodies, occurring either in isolation or as cell fillings. his is the high#rank equivalent of ‘corpohuminite’, a maceral of low rank coals described more fully in &ection 1.1.7. 3.3.2
The inertinite gro(p
he macerals of the inertinite group are characteri*ed by a high reflectance, and have higher carbon and lower hydrogen contents than other macerals in coals of equivalent rank. hey are essentially opaque in thin section, but appear white or light grey in polished section. he inertinite macerals are mainly derived from the same basic types of organic matter as vitrinite, but owe their properties to oxidation of .t _
____. 1 — 1 —
r* «-> QilflT ctOtTP
tnose materials ai an wn% formation.
tViP mill’s
t#v S
he name inertinite was selected as a group name to imply the relative inertness of these macerals in technological processing $e.g. coke manufacture% in comparison to the members of the other two maceral groups. Gowever, one of these macerals $semifusinite% sometimes displays sufficient reactivity in processes, including carboni*ation, that the term ‘semi#inert’ has been used by some workers to describe such materials. @icroscopic observations made of a heated coal sample by Aandi and @ontgomery $()/=% also suggest the possibility that another maceral $micrinite% is far less ‘inert’ than had originally been thought, but this conclusion has been disputed by other investigators in the field. $a% "usinite and semifusinite 'lthough, in certain circumstances, plant materials are transformed into vitrinite, in other circumstances identical materials are seen to have undergone a radically different process, giving rise to a brittle, opaque maceral called ‘fusinite’. ' plant origin is recogni*able for this material from the well#preserved cell structure, and. fine detail of the cell walls may be commonly seen $"ig. 1.1d%. he cell lumens of fusinite may be open cavities or they may be infilled with minerals $carbonates, pyrite, clays% or with gelocollinite. 4here a lack of infilling has resulted in the eventual collapse of the brittle cell walls due to compression, a broken ‘bogen’ structure may be developed $"ig. 1.1b%. "usinite is opaque in thin section and in reflected light is the most highly reflecting maceral in most ranks of coal. t generally appears white, sometimes even yellowish in polished section studies. 4here there is the same detail of preservation of cell
structure, but the level of reflectance is grey and intermediate between that of the vitrinite and fusinite in the same coal, the maceral, which also usually appears brown in thin section, is known as ‘semifusinite’ $"ig. 1.1b and d%. "or convenience, some petrologists have set an arbitary reflectance threshold of 6.?F to distinguish between fusinite and semifusinite in petrographic analysis. 3 ;rganic cell fillings which were emplaced early in the coal’s history may also be sub!ect to the fusini*a# tion process. hus, oval resin bodies $&ection 1.1.1% may be converted into masses which, because of their shape, high reflectance and the presence of cavities $"ig. 1.1e%, can be easily mistaken for fungal sclerotia $&ection 1.1.6e%. hese bodies may be called fusinite, semifusinite or macrinite $aylor N Cook ()/6%, sclerotinite’ $.C.C.8. ()=(% or ‘scierotioids’
( 7( R ‘serretinn
-----------
‘resino#sclerotinite $&tach ()// yons et al ()06%. &everal authors, including :osanke and Garrison $()7=% and yons et al $()06%, believe that many of the resin rodlets from which resino#sclerotinite was derived were probably /i marlnllnCilf 01*11!1.
-
$b% @acrinite 'lthough it has a similar level of reflectance to fusinite, macrinite occurs most often as small, rounded but irregularly shaped bodies without cell structure, usually ranging from (? to 5? MTm in diameter. @acrinite often appears homogeneous, but it is also apparent that some macrinite has originated through the fusini*ation of gelified tissues. Certain durains contain relatively large amounts of macrinite in association with sporinite $"ig. 1.5a%.
$c% @icrinite @icrinite is another highly reflecting coal maceral. t occurs as very small, rounded grains, rarely more than a few microns in si*e $"ig. 1.5b%. he grains are in fact discrete particles, but they tend to form accumulations, either as lenses or layers or in cell lumens. n transmitted light, micrinite is opaque and, in large aggregates it may be difficult to distinguish from fusinite or macrinite. @icrinite is a ubiquitous component of bituminous coal, particularly in durains and sapropelic coals, but it is rarely present in large quantities. t commonly occurs in association with sporinite and other liptinite group macerals. @icrinite occurs far less commonly in lignites and sub#bituminous coals, a fact which supports the hypothesis that it is generated as a result
"ig. 1.1 Deflected#light photomicrographs of coal $under oil immersion%, $a% -itrinite showing the cell structure of lycopod periderm $x6=?%. $"rom 2avis et al ()=/.% $b% -itrinite appears medium grey in contrast to the light grey fusinite and semifusinite and the dark grey liptinite macerals sporinite and cutinite $centre%. he semifusinite seen at the top of the photomicrograph has a lower reflectance and less distinct cell wall outlines than the fusinite, which displays ‘bogen’ structure.8yrite, occurring mainly as small euhedral crystals in the centre, appears white $x50?%. $c% he telocollinite in coarse bands a t the top left and bottom right has a higher reflectance than the fine desmocollinite occurring in the central attrital layer with fragments of other minerals $x 50?%. $d% "usinite $white% and semifusinite $light grey%, both showing well defined plant cell structure $x50?%. $"rom 2avis et al ()=/.% $e% Desino#sclerotinite or sclerotioid in 8ermian high volatile bituminous coal from southern 'frica. Aote the deep notch which can be a feature of these bodies $x50?%. $f% &clerotinite $white% derived from fungal sclerotia with isolated resinite bodies $dark, oval% in a ate >ocene sub#bituminous coal from 4ashington $x11?%.
Coal Petrology and Petrographic Analysis
of the chemical changes that accompany coalification $&tach ()/0 eichmiiller ()=5a%. Gowever, it can also be observed in cell fillings of very low rank coals, in which case it may represent an end product of the decay of woody tissues.
*
@aterial derived from the outer layer, or exine of spores and pollens was originally referred to by &topes $()17% as ‘exinite’. Gowever, the meaning of the term was expanded to include coalified cuticular $leaf cuticle% material $Vongmans et al ()17%, and eventually macerals derived from algae and i"#$ bodies as well. he term ‘liptinite’ is more appropriate " 5 resin to encompass all of these macerals, together with the more $d% nertodetrinite recently identified components suberinite, liptodetrinite, fluorinite, bituminite and exudatinite. nertodetrinite is composed of broken fragments of he macerals of the liptinite $or exinite% group appear inertinite macerals $"ig. 1.5f, 1.7a and d%. 'ccording to darker in reflected light than the associated vitrinite, and are .C.C.8. $()=(% a fragment of fusinite or semifusinite generally pale in colour under transmitted illumination. hey which has less than one complete cell should be classed represent a diverse assemblage of small organic particles, as inertodetrinite. Gowever, when a petrographic study characteri*ed, particularly in low rank coals, by a high is being conducted for the purpose of interpreting the hydrogen content and a high proportion of volatile matter. depositional environment of the coal, it may be he optical properties and chemical characteristics, however, reasonable to identify as inertodetrinite any small piece change significantly as rank advances from high volatile to of fusinite which has been detached from a larger mass medium volatile bituminous coal, and many of the and deposited away from the immediate vicinity of distinguishing features are lost in the higher rank materials. other similar fragments in any attrital coal band. 2iscrete fragments of fusinite $or semifusinite% which represent less than an entire cell may also be observed in the resin binder of polished section grain mounts. &uch particles are, however, better classed as fusinite $or semifusinite% because they are more likely to have he most common of the liptinite macerals in humic coals been detached from a fusain lens than from an attrital are the coalified remains of spore and pollen exines called coal lithotype during sample preparation. ‘sporinite’. he typical appearance of these materials is that of a flattened oval shaped particle with a central cavity or line indicating that the inner layer and protoplasm of the structure has decayed $"ig. 1.5a%. he si*e of these particles $e% &clerotinite ranges from about five to several hundred microns, while the shape, including thickness and ornamentation, is extremely 4ith the exception of most micrinite, all of the macerals varied. described above have probably been derived, in one n thin section, the colour of sporinite is yellow or orange way or another, from the ligno#cellulosic tissues of in high volatile coals. ike the other low# reflecting macerals plants. &clerotinite is another maceral that is opaque in described below, sporinite can display a high level of thin section and highly reflecting, but this material fluorescence under blue or ultraviolet irradiation $&ection originated instead from fungal remains. he high 1.1.5%. &porinite is abundant in some durains and clarains reflectance in this case is due to the presence of dark and is the characteristic constituent of cannel coals. pigment $melanin% rather than the main component of n coals with a volatile matter yield of less than 6).7F such bodies, the polymer chitin $&tach et al ()=7%. $d.a.f.%, a point known as the ‘coalification !ump’, the @aterial classed as sclerotinite includes all coalified reflectance curve of sporinite $&ection 1./.5% begins to sclerotia, fungal spores, hypae and plectenchyme. rue converge with that of vitrinite. he two curves are coincident sclerotinite is a ubiquitous component of ertiary coals, at a volatile matter of about 6(F and a reflectance of about occurring as rounded spores and sclerotia some tens of (.7?F, and it is not possible to distinguish between the two microns in diameter and having one or more cell macerals under oil immersion in these circumstances. n high cavities $"ig. 1.1f%. 9odies which appear similar to rank coals, optical differentiation of these macerals may be sclerotinite may be observed in Carboniferous coals improved to some extent by using methylene iodide $"ig. 1.1e%, but these are usually oxidi*ed or fusini*ed immersion techniques. resin rodlets $&ection 1.1.6a%. he cavities of these bodies can generally be discerned as vesicles rather than $b% Cutinite a regular cell structure $&tach et al ()=7 yons et al he waxy cuticular coatings on certain aerial epidermal ()06%. tissues, notably leaves, are preserved in coal as cutinite. he 3.3.3 The liptinite ,or e6inite- gro(p functions in life of this cutin, an insoluble polymer, are to prevent the delicate tissues from rapid desiccation, and to
+
Chapter 3
give them physical support and protection from biochemical agencies. 'n entire organ with a double layer of cutinite may be preserved as in "ig. 1.5c, where the inner mesophyll of a leaf has been converted into vitrinite. "requently, however, the resistant cutinite is all that remains of the parent plant structure. Cutinite usually occurs as very thin elongate bodies. ' series of cusps or teeth may be seen on one side of the cuticle to indicate where it originally extended between the radial walls of epidermal cells. Cutinite can also be relatively thick $"ig. 1.5d%, indicative in some instances of a dry, sunny climate. hick cuticles may result from extensive cutini*ation of the epidermal tissues or the build#up of many layers. Unusually rich accumulations of cutinite occur in rare but well#known occurrences of paper or leaf coals $'uerbach N rautschold (0/? Euennel N Aeavel ()7) Cook N aylor, ()/1%. $c% &uberinite he corky cells of the plants that contributed to coals, particularly those of the ertiary, contained the waxy polymer suberin, which is similar in many ways to the cutin of cutinites. 9oth are mixtures of substances, mainly fatty acids, and are consequently impervious to water. he functions of suberin are similar to those of cuticle, but suberin is deposited within the cell walls rather than outside them. n reflected light, suberinite stands out as the dark walls of the relatively large cork cells. hese are filled with more highly reflecting materials $"ig. 1.5e%.
he colour in thin section and the reflectance resinite can vary widely, even within a single coal. t can approach that of the vitrinite with which it may be intimately associated, so that microscopic differentiation of the two becomes difficult. 8olished sections sometimes show internal reflections, while in transmitted light, resinite may be shades of yellow, orange, red or brown. ;xidi*ed resinite bodies can have rims which are relatively higher in reflectance than the interiors. Desino#sclerotinite is fusini*ed resin with a high reflectance and is often vacuolated $&ection 1.1.6a%. &ome discrete resin#rich bands are also encountered in coals. hese may be layers of secondary resinite or attrital accumulations of weathered resin bodies. ertiary and Cretaceous coals contain relatively large amounts of resinite because of the contribution from conifers. he resinite may be concentrated from low rank coals for commerical use either by a special coal preparation process or, because it is more soluble in ben*ene than other liptinite macerals, by a solventextraction technique. $e% 'lginite
'lginite represents the coalified remains of algae. &uch material is rare but not unknown in humic coals, and abundant in the variety of sapropelic coal known as boghead coal or torbanite. t is also abundant in some oil shales. he individual algal colonies are oval in shape, often with scalloped outlines, and these help to differentiate alginite from sporinite in reflected light $"ig. 1.7a%. 'lginite also has a somewhat lower reflectance than other $d% Desinite liptinite macerals. n thin section alginite has a pale straw Coalified resins occur in coals as more or less oval or rod# colour in low rank coals, but is somewhat orange at higher shaped bodies at their original sites of deposition in cell ranks. >xamination in blue or ultraviolet light reveals more of lumens $these are primary resinite%. ;ther material the details of the colonial structure of alginite than can be $secondary resinite% clearly has been mobili*ed at some stage, and occurs as veins or cleat fillings and in pods or seen in ordinary white light $"ig. 1.7b%. 'lginite has a high cavities such as fusinite cell lumens. Crelling and 2utcher intensity of greenish#white or yellow fluorescence in low $()0?% showed that secondary resinite can have quite rank materials, but is darker in fluorescence colour at different fluorescence properties to either primary resinite or greater levels of organic maturation. wo of the genera of algae that may be identified in exudatinite $see below% in the same coal. ' third and coals -Pila and .einschia' have been related to the living common mode of occurrence for resinite, however, is as bodies which have been weathered out of the other plant species /otryococcus #raunii0 '.C. Cook $personal tissues and incorporated as transported particles within communication% has suggested that much of the material in cannel coals that has been described attrital coal layers $"ig. 1.1f, 1.5f%.
"ig. 1.5 Deflected#light photomicrographs of coal $under oil immersion%, $a% &porinite $dark% and macrinite $light% in durain from a Carboniferous high volatile bituminous coal, :entucky $x50?%. $b% @icrinite $white% as lenses in vitrinite $grey% and as a thick layer $x50?%. $"rom 2avis et al ()=/.% $c% hin layers of dark grey cutinite enclose light grey vitrinite derived from leaf mesophyll. Vurassic @aghara seam, >gypt $x6?7%. $d% hick cutinite in leaf coal from eping Country, Viang Wi 8rovince, China $x50?%. $e% hin layers of dark suberinite in a 8alaeocene sub#bituminous coal from 4yoming. he thick black lines in the bedding plane are desiccation cracks $x50?%. $f% enses of resinite. Aote the small vertical cracks !oining resinite occurrences these represent secondary mobili*ation of resinite or exudatinite $x50?%.
as bituminite $&ection 1.1.5% may very well be the material described by Gutton et al $()0?% as ‘alginite /( a lamellar alginite with affinities to the genus Pediastru10 $f% iptodetrinite iptodetrinite, the member of the liptinite group equivalent to inertodetrinite, is composed of fragments of the liptinite macPrais sporinite, cutinite, resinite and alginite $"ig. 1.7a%.
material has a strong yellow fluorescence when irradiated with blue light. "luorinite has a maximum fluorescent intensity at a lower wavelength than other liptinite group macerals at the same level of rank. Consequently, the redMgreen quotient, that is, the ratio of the relative intensity at /5? nm to the relative intensity at 7?? nm, is always lower than those of the other macerals. "luorinite is believed to originate from plant oils and fats. t is a ubiquitous maceral of >uropean coals $eichmtiller, personal communication%, but is rarer in the coals of the eastern U.&.'.
between those of vitrinite and sporinite. 9ituminite may even form the groundmass of some durains and sapropelic coals. 8reviously, it had often been identified as XXXXXX uXXXXXXXXXXXXXXXXXXXXXXXXY... liptodetrinite, the fragmented form of liptinite, but i ne iipimuc liiaeciaid ui cuai nave me piupcny m displaying a fluorescence when viewed under blue light eichmuller $()=5a% notes that bituminite has a fluorescence irradiation. 2escriptions of the apparatus employed for property that clearly distinguishes it from all other macerals, studies of these autofluorescence characteristics are given namely a fluorescence intensity that increases by as much as by van Ei!*el $()=(%, ;tten!ahn et al $()=5% and .C.C.8. 6??F after a 1? min period of irradiation. he usual $()=/%. Gigh# pressure mercury or xenon lamps are used for fluorescence colours are orange to brown, and the maximum illumination in qualitative fluorescence microscopy, with a fluorescence of bituminite occurs at a longer wavelength blue or ultraviolet excitation filter to remove much of the than does that of other macerals. eichmuller has suggested that bituminite represents the visible light. ' blue filter, for example, with a maximum transmission wavelength of about 5?? nm may be used in decomposition products of algae, bacterial lipids and animal con!unction with a red suppression filter to achieve the proteins. Gowever, Gutton et al $()0?% believe that some of desired results. n some microscopes, the vertical the material that has been called bituminite is really alginite illuminator contains a dichroic beam#splitting mirror which 9 $&ection 1.1.1.e%. eichmuller $()=5a% has also suggested reflects light of below 7(? nm. ' barrier filter removes the that the generation of some micrinite in coals results from : XXXXXXXZr u + . . XXXXXXXX reflected excitation rays and protects the eyes from U ( C U i a g C l l C @ f c U l U l l U l l l l i l l l C . exposure. "or the blue light assemblage !ust described, a barrier filter with peak transmittance at 71? nm is generally $c% >xudatinite most suitable. 's a result of fluorescence studies, eichmtiller he mode of occurrence of exudatinite indicates that it is a 2222222222222 t i r \ n A - i 2 (222222i / - \ \ \ 222222 secondary maceral which has been soft and mobile at some anu iciiiiiiunci aim wun ( i y M M nave named a number of materials present in coal which had not stage during the coalification process. >xudatinite appears been previously identified or distinguished from other coal black under reflected light in oil immersion, and it is only constituents. he new maceral names proposed include by the use of a dry ob!ective or fluorescence illumination fluorinite, bituminite and exudatinite. he optical properties that what appeared to be empty cracks and cavities are of these macerals, summari*ed in able 1.7, reveal that these sometimes seen to be filled with a material that typically has are quite different substances from the better known liptinite an orange to yellow fluorescence in blue light irradiation. he cell lumens of fusinite or semifusinite and the macerals, namely sporinite, cutinite, resinite and alginite. chambers of sclerotinite frequently provide the cavities in which exudatinite may occur. &ome cracks containing $a% "luorinite exudatinite may also be !oined to primary 3.3. The +l(orescence o+ the liptinite )acerals
A
A
C HH
U
9ecause of its black appearance, sometimes with internal reflections, in reflected white light with oil immersion, this pure organic substance could be mistaken for lenses or layers of clay minerals in the coal. Gowever, eichmtiller $()=5% has noted that this $b% 9ituminite 9ituminite is the most frequently occurring of the three new liptinite macerals. t is seen as irregularly# shaped shreds, wisps and layers of a material with reflectance intermediate
'9> 1.7 ;rigin and properties of new liptinite group macerals. $@odified after eichmtiller ()=5a.% 'ppearance in reflected light $oil% @acPrai fluorinite
bituminite
exudatinite
lenses
"orm
streaks and groundmass
cavity fillings
9lack, occasional internal reflections as
Deflectance intermediate between vitrinite N sporinite black
ntensity strong
weak
variable
Colour brilliant yellow
7(?#7=? nm
ca. ?.7
weak, even negative
ca. 6./
very strongly positive #
mostly yellow to ca. /17 N orange and red# 70? nm brown
n addition to the presence of the three new macerals, fluorescence microscopy has revealed certain phenomena which eichmiiller $()=5b% has associated with the generation of mobile ‘bitumen’. hese include ‘oil exudations’, ‘smear films’ or the darkening of vitrinite as a result of irradiation, and fluorescent vitrinite, presumably due to the incorporation of lipoid substances. he fluorescent vitrinite is usually that which occurs in attrital bands rather than as bands of telocollinite. $e% uantitative fluorescence photometry
DedMgreen quotient
orange to brown ca. /17 nm
lipinite macerals such as cutinite and resinite. he reflectance and fluorescence intensity of the exudatinite, however, are respectively higher and lower than those of the primary liptinite occurrences. eichmtiller $()=5a% has observed that the maximum fluorescence of exudatinite occurs at a significantly different wavelength to that of sporinite, regardless of the rank of the coal. ;ther distinguishing features of exudatinite are the broad maximum in its fluorescence spectrum, and a tendency to display an initial increase in fluorescence intensity, followed by a decrease as the time of exposure is extended $able 1.7%. $d% ;ther fluorescent materials
'lteration of fluorescence intensity with time
@aximum fluoresence intensity $W%
$%
ca. 6.6
often an in# crease then decrease
n quantitative spectral fluorescence photometry, the recommended optical apparatus is somewhat different to that described above for qualitative work $;tten!ahn et al ()=5 .C.C.8. ()=/ van Ei!*el ()=)%. ' mercury lamp is used with an ultraviolet filter orfilters to produce excitation mainly by the mercury band at 1/7.7 nm, since a full fluorescence spectrum cannot be obtained with the blue light filter combination. &pectral analysis of the fluorescence emitted by the ob!ect in the range 5??#=?? nm is made by a motor#driven continuous interference filter or grating monochrometer synchroni*ed with a chart recorder. he barrier filter is withdrawn in this case while the measurements are taken. he photomultiplier used should also have an adequate response through the relevant spectral range. t is the shape of the fluorescence spectra that is currently used in coalification studies, not the absolute intensities involved. he parameters measured include the peak wavelength, the redMgreen quotient, and the ‘alteration’, which is an increase or decrease $fading% in intensity after specified periods of irradiation. ;tten!ahn et al $()=5% have shown that the peak wavelength and the redMgreen quotient obtained on sporinite increase with increasing rank up to medium volatile bituminous coal, which is the highest rank in which the fluorescence phenomenon is normally encountered. 3.3./
Macerals in lo0 ran coals
ignites and sub#bituminous coals have physical and chemical properties which seem to set them apart from coals of higher rank. ikewise, when they are examined under the microscope, they appear more complex, showing greater variability in the macPrai materials. @any petrographers
therefore believe they can better characteri*e these low rank coals with a
special macPrai classification rather than constrain $b% Gumodetrinite themselves with the system described above that is he humodetrinite sub#group contains the macPrais attrinite traditionally used for hard coals. and densinite. 9oth of these consist of fine, microscopically he macPrais derived by humification of ligno# discernable fragments, mostly less than (? -i1 $"ig. 1.7d%. cellulosic tissues show the greatest changes as rank 2ensinite differs from attrinite in gPlification, the particles progresses. t is therefore in the terminology for these macPrais that the greatest differences between the two that it appears to have undergone a greater extent of classifications are found. his group of materials in low having been cemented together with some loss of detail of the particulate structure. rank coals is referred to in this chapter as the ‘huminite’ macPrai group, and is regarded as equivalent to and the precursor of the vitrinite macPrais found in higher rank $c% Gumocollinite coals. able 1./ summari*es the classification of huminite macPrais, and gives details of their supposed origin and he third sub#group, humocollinite, includes the two equivalents in the hard#coal classification system. he [ macerals gelinite and corpohuminite. Eelinite consists of amorphous humic gels $"ig. 1.7c% and corpohuminite group contains six different macPrais, which are consists principally of the coalified products of from disrincmished from each other on the basis of ffrain —
------------
—0----------------------
- -. . .
%_&
si*e and degree of gPlification. hey are organi*ed into three sub#groups, namely humotelinite, humodetrinite and humocollinite, which represent a series of coarse#grained, fine#grained and colloidal# si*e particles of humic materials, respectively.
piiiuuapiitiiv3a, nmui cue piiiiicuy een ww.iv.uuuo uuivcu
tannins. Corpohuminite typically has an elliptical or a rod# like form imposed by the surrounding cell walls. t is resistant to weathering, and isolated individual bodies or groups of bodies may become concentrated as a result of destruction of the cell tissues. Corpohuminite often has a $a% Gumotelinite reflectance higher than that of other huminite macerals $"ig. 1.7e%. t is especially abundant in corky tissues and in he humotelinite sub#group contains the macPrais textinite lignites derived from conifers. and ulminite, both of which have maintained a recogni*able he .C.C.8. !nternational "and#oo$ of Coal plant cell structure. n the case of textinite, the cell outlines Petrography $()=(% gives many other details of these appear sharp because they are ungelified. Ulminite may macerals, and also documents many of their technological exist in various stages of gPlification, but, however properties in processes including briquetting, low# and high# indistinct, a cell structure is still discernable $"ig. 1.7c%. temperature carboni*ation and extraction, and their 'lthough textinite is a common component of soft behaviour in weathering processes. brown coals, such as those mined in the @iocene deposits of 4est Eermany, it has been observed only rarely in the higher rank lignites of Aorth 'merica. 8resumably it has 3.3. The che)istry o+ coal )acerals been transformed into ulminite as a result of geochemical "igure 1./ shows the differences in elemental chemistry of gPlification. extinite is preferentially formed from the cell some important macerals in the same coal seams. t can be walls of resistant plant tissues, notably those of conifers. seen from this that the overall chemical composition of a coal sample to some extent '9> 1./ Classification of the huminite macerals of low rank coals. $@odified from .C.C.8. ()=(.% >quivalent in hard coals @acPrai group huminite
[ @acPrai subgroup humotelinite
humodetrinite
humocollinite
@acPrai textinite ulminite
attrinite densinite gelinite corpohuminite
&ource ungelified cell wall material gelified plant tissues with recogni*able cell structure humic detritus
formless humic gels secondary colloidal cell excretions, and primary cell infillings $tannins%
teliniteMtelocollinite teliniteMtelocollinite
< desmocollinite
gelocollinite corpocollinite
"ig. 1.7 Deflected#light photomicrographs of coal $under oil immersion%, $a% 2ark oval alginite with crenulated margin in centre of field. &porinite and liptodetrinite appear somewhat lighter. Carboniferous boghead#cannel coal from 4est -irginia $x50?%. $b% 'lginite derived from colonies of .einschia0 9oghead coal, :entucky, blue#light illumination $xl1??%. $c% Ulminite consisting of highly gelified cell walls and with cell infillings of granular gelinite and more highly reflecting corpohuminite. 8alaeocene sub# bituminous coal from 4yoming $x50?%. $d% he groundmass of this layer, from the same coal as "ig. 1.7$c%, consists mostly of humodetrinite particles. he white fragments are inertodetrinite and the dark bodies are mainly sporinite $x50?%. $e% he cell fillings of vesiculated corpohuminite are higher in reflectance than the ulminite which encloses them. 8alaeocene lignite, @ontana $x50?%. $f% 8article of weathered medium volatile bituminous coal
"ig. 1./ Carbon and hydrogen contents of macerals. $"rom @urchison ()/5.% he smaller hatched lines connect points of the same rank, o Desinites > exinites - vitrinites @ micrinites.
reflects the mixture of macerals that it contains. he liptinite macerals at a given rank are richer in hydrogen than the corresponding vitrinite, and this in turn has higher values than the corresponding inertinite components. he liptinite macerals contain the most strongly aliphatic organic components, whereas the inertinite macerals contain the most aromatics. his has been shown by infrared spectroscopy, W#ray diffraction, physical constitution analysis and broadline
3. T8E NTURE ND ""ERNCE O* MICRO#IT8OT$"E!
4hile a knowledge of the macerals present in a coal is essential in most applications of coal petrology, there are also a number of areas where the manner , of distribution of these macerals through the coal and the way in which the different macPrai groups are associated with each other may be highly significant. ' coal in which the vitrinite occurs predominantly as relatively thick bands, for example, is likely to have different breakage characteristics, and probably different carboni*ation properties, to one in which the same amount of vitrinite is finely disseminated throughout the seam. he vegetation and the original swamp environments that gave rise to the two types of materials may also have been quite different in each case. 'ssociations of macerals, as determined microscopically, are called ‘microlithotypes’. Vust as the macerals themselves are often regarded as an equivalent to the minerals in other rocks, the microlithotypes may be considered as equivalents to the discrete beds, lenticles or laminae, made up of different mineral combinations, that 3.3.7 The )icroscopic appearance o+ coal are also fundamental components of many clastic and non# )inerals clastic sediments. he three macPrai groups, vitrinite, liptinite and he minerals occurring in coal are discussed in &ection 6.), and the scope of this chapter does not include further inertinite, can be associated as shown in able 1.= to form coverage of these or other inorganic constituents. a total of seven possible combinations. hree of these combinations are made up of one single macPrai group Aevertheless, the identification and $monomaceral microlithotypes%, three contain members of characteristi*ation of mineral species under the two macPrai groups $bimaceral microlithotypes% and the microscope is an important aspect of coal petrology. last contains a representative of all three groups :eme*ys and aylor $()/5%, and @ackowsky $()/0a% $trimaceral microlithotypes or trimacerites%. describe, in some detail, the appearance of the principal 'ccording to established convention $e.g. .C.C.D ()=(%, coal minerals in microscopic studies. the association must have a minimum band . width of 7? Ton before it can be classed as a microlithotype. n addition,
constituents that make up less than 7F of the association are normally disregarded. hus, a band of vitrinite with a small amount of $say% liptinite would not be classed as a clarite unless the liptinite was present in greater abundance than
7F, while a trimacerite must contain at least 7F of each of the three macPrai groups. ow concentrations of mineral matter are usually ignored in the determination of microlithotypes. f
'9> 1.= @icrolithotypes( and carbominerites. @icrolithotypes itrite clarite trimacerite
f duroclarite vitrinertoliptite 3 clarodurite all on - and all on and > all on all on >
all on vitrinite $-% all on - and exinite $>% on -s inertinite $% and >, but with - I and > on -, and >, but with > I - and on -, and >, but with I - and > vitrinertite durite inertite liptite
(attritus’. n the same manner, fusain bands less than 1= fi1 thick are arbitrarily assigned as a constituent of ‘opaque attritus’.'nother problem, and one of greater practical importance, is the fact that the system does not lend itself nearly as well to studies of all ranks of coal as does the &topes#Geerlen system, simply because of the difficulty, and in some cases the impossibility, of preparing thin sections of high rank coals. 'lso, the C l i t # ci# nM3= nrdic-l8fl surfaces has become 'L((' V/*
a quantifiable technique through reflectance measurement, while thin section examinations retain the problem of variation in optical properties with section thickness. he hiessen#9ureau of @ines system is now obsolete in practice, but as a large amount of descriptive work on U.&. coal was done by hiessen and his colleagues, it is still widely used for review purposes. 3./.2 The genetic classi+ication o+ the U.!.!.R. cade)y o+ !ciences
he lithotypes and microlithotypes of the &topes# Geerlen system are recogni*ed on the basis of their physical appearance and macPrai composition, respectively, and no systematic palaeo#environmental
#
6?#/?F $by volume% clay mineral remainder maceral 6?S/?F $by volume% carbonate mineral, remainder maceral 7S6?F $by volume% pyrite, remainder maceral 6?#/?F $by volume% quart*, remainder maceral 7#/?F $by volume% of various minerals I /?F clay, quart*, carbonate, I 6?F pyrite
'ssociations of microlithotypes with mineral impurities $carbominerites% carbargilite carbankerite carbopyrite carbosilicite carbopolyminerite ‘dirt’, pyrite
the amount of mineral matter is significant, but the relative density of the microlithotype is less than (.7, the abundance and type of mineral matter can be described by a qualifying ad!ective, using terms such as [argillaceous durite7 $&tach et al ()=7%. Gowever, where the mineral matter is more abundant, and the relative density of the association lies between (.7 and 6.?, the material is referred to as a ‘carbominerite’. he types of carbominerites normally recogni*ed, and the volumetric percentages of mineral species that correspond to the required density range, are also given in able 1.=. he names of both microlitho# types and carbominerites both have the suffix ending Hite’, as for example, in ‘vitrite’. he methods of microlithotype analysis are discussed more fully
3./ OT8ER C#!!I*I CTION !$!TEM! *OR CO# MICROCO M"ONENT !
he &topes# Geerlen system for identification and nomenclature of coal constituents, as described in &ection 1.1, is the principal system of classification used throughout the world at the present time. 9ecause it is based on three maceral groups, vitrinite, liptinite and inertinite, analytical results can be plotted readily in simple representations such as triangular diagrams, yet where greater detail is required, data can be readily extended to encompass the individual macerals or sub# macerals of each group. n this section, however, some other systems used to classify the microcomponents of coal are considered.
3./.1 T he Thiessen &(rea( o+ Mines syste) o+ coal classi+icat ion
"ollowing very comprehensive studies of coals in thin section at the U.&. 9ureau of @ines, Deinhardt hiessen $hiessen ()6? 8arks N ;’2onnell ()7/ .C.C.8. ()/1% developed a system of description for the microscopically recogni*able ingredients of coal. he three ma!or ‘components’ of banded bituminous coal in this classification can be identified at either the macroscopic or microscopic level. hese are ‘anthraxylon’, equivalent to the bright vitrain bands of coal, ‘fusain’, which is much the same as defined in the &topes#Geerlen system, and ‘attritus’, which is represented by those bands of coal with a dull, prarmlar armearance and consisting of a micro#
c<########## Sr H################ ############ ##################### o # ######### #
fragmental mixture of varied entities. @icroscopic examination of thin sections enables the ‘constituents’ of attritus to be distinguished as either translucent attritus or opaque attritus. ranslucent attritus includes spores, cuticles, resins etc., and opaque attritus includes granular opaque matter $micrinite%, sclerotia etc. able 1.0 summari*es the hiessen#9ureau of @ines nomenclature and classification, and correlates the terms used with those of the &topes#Geerlen system. ' feature of the hiessen# 9ureau of @ines system is that arbitrary thickness limits were set for some of the components and constituents. 'nthraxylon, for example, includes only those vitrain bands greater than (5 4in thick, and any vitrinite with a lesser band thickness would be described as ‘translucent humic degradation matter’, a constituent of ‘translucent
Co al Pet rol ogy and Pet rog rap hic An aly sis '9> 1.0 Correlation of the hiessen#9ureau of @ines and &topes#Geerlen classifications. $@odified Deflected light &topes# Geerlen &ystem
ransmitted light hiessen#9ureau of @ines &ystem 9anded components Constituents of attritus 'nthraxylon $translucent%
ranslucent attritus
'ttritus
-
;paque attritus
@acPrai group
vitrinite more than (5 fi1 in width
-itrinite
translucent humic matter
vitrinite less than (5 Tun in width
spores, pollen, cuticles, algae cuticles, algae resinous and waxy substances
sporinite, cutinite, alginite resinite
iptinite
brown matter $semitranslucent%
weakly reflecting semifusinite weakly reflecting macrinite
nertinite
rp+l9rtirta :c;otiniie
granular opaque matter
)ai+, ii*** ------------------------------------—
micrinite amorphous $massive% opaque matter, finely divided fusain, sclerotia
"usain $opaque% after .C.C.8. ()/1.% fusinite less than 1= TTm in width strongly reflecting macrinite strongly reflecting sclerotinite fusinite and semifusinite more than 1= fm in width
interpretation is implied. ndeed, as can be seen from &ection 1.0, there is often no adequate consensus of opinion among
@acPrais
coal petrologists regarding the en# vironmental conditions that gave rise to many of the ma!or coal lithotypes. Gowever, at the
nstitute of Eeology, 'cademy of &ciences of the U.&.&.D., @oscow a genetic classification of microcomponents of humic coals was developed following detailed study of the ma!ority of coal deposits and basins in the U.&.&.D., representing a wide range of tectonic and environmental settings $imoveev N 9ogoliubova ()/7 .C.C.8. ()=(%. 4ithin this system, coals are classified according to the material composition of the coal $class and subclass%, and the degree of structural preservation or degradation $group%. he hori*ontal rows in able 1.) represent
the six ‘classes’ of materials. n thin section, the classes gelinitic, semigelinitic, semi# gelifusinitic, gelifusinitic, quasigelifusinitic, and fusinitic contain materials which vary progressively from red, through brown to black. his progression reflects increasingly aerobic conditions in the peat bog, due in turn to the degree of flooding and water movement. he processes by which the original ligno# cellulosic plant tissues were transformed into the microcomponents characteristic of these coal classes are seen from the table to be ‘gPlification’ for gelinitic
and semigelinitic coals, ‘fusini*ation’ for fusinitic coals, and a two#stage process of gPlification followed by fusini*ation for the other three classes. ' characteristic feature of quasigelifusinitic coals is that they contain large amounts of detrital quart* and clays, as a result of deposition under flooded, running# water conditions. he vertical columns in able 1.) are the genetic ‘groups’ of coal, namely telinitic, posttelinitic, precollinitic, collinitic and leiptinitic. hese five groups represent progressively greater physical and biochemical degradation that developed in response to increasing tectonic stability of the area of peat accumulation. n a tectonically stable area of peat accumulation, for example, extensive decomposition of vegetal material would have led to the formation of the collinitic group of coals. 4ithin a tectonically mobile area, by contrast, a more rapid rate of subsidence would have provided greater opportunity for the preservation of plant materials, giving rise to the peats from which the telinitic group of coals would form. herefore, in order to classify a coal using this system it is necessary to characteri*e both the type of substance $class%, and the structure $group% of the microcomponents present. able 1.) shows how each group is sub#divided into sub#groups based on the class of microcomponents, each of which represents a definable environmental setting. hus, gelinite# posttelinitic coal, for example, is genetic sub#group representing the product of a heavily flooded, stagnant peat bog within a relatively tectonically mobile area. 'lthough some of the terms are similar to those in the &topes#Geerlen system, they are not used in the same way. "or example, the term ‘fusinite#telinitic coal’ does not imply the presence of the vitrinite group macPrai telinite. Dather it is a coal containing fusinite with a distinct cellular structure. Aot only is the U.&.&.D. 'cademy of &ciences nomenclature used for strictly genetic purposes, but also for the industrial evaluation of coals in that country. he above description provides only a summary of some of the principal features of the Dussian genetic classification, since each of the sub#groups may be further differentiated into several genetic types. he system has not been used extensively outside the U.&.&.D., possibly because of its complexity, while western petrographers are also deterred by the descriptions being based on thin#section examination. 'nother significant drawback at the present time is that correlations between the genetic coal types and original coal swamp facies have not been confirmed for coal basins outside the U.&.&.D. Aevertheless, the approach and findings of the Dussian petrologists should be given careful con#
# I q
q
\a a
0 c ] 1
. V « 11 % 23
; .1 %, 4
_. is II N (5 3
a
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\a (< !i
= y
I i !
$ &
a g. `iV
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u N
B7 #` 1 89 2: ’.
It
#s’ B6 E U1 4
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t,i ¡A
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1Ka G
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a J ``` O 6 +"
t8 s i 1 6 9 \0 g: a #i
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the conditions of illumination. his property is related to the aromaticity of the organic compounds in the coal concerned, and increases progressively for all macerals as the rank of coal increases. 3./.3 !pac)an ter)inology and 8recision measurement of the reflectance of classi+ication individual macerals, particularly vitrinite, is widely &packman $()70% noted that each conventional used as an index of coal rank. he technique has maceral term really represents a suite of materials the advantage over chemical parameters, such as with greatly varying physical and chemical those outlined in &ection 6.(?, that it is applied to a properties. Ge therefore considers that is is single selected petrographic constituent, and is important to designate macerals by the extent of therefore not the metamorphic changes which have occurred. he ending 3inoid 7 in, for example, vitrinoid, according to this classification implies a maceral of the ‘-itrinite &uite, 7 but one with distinctive properties rather than a range such as exists for sidPration in any interpretations environments of coal formation.
(
of
XXXXXXX XXXX n;8
flip
2e various nntunuiua cum luoiiivi3o nuiuu Y.x.w nertinite &uite 7, and exinoids and resinoids within their own respective suites. &ubsequent usage of the &packman system seems to have diverged somewhat from this original concept. &chapiro and Eray $()/?% established 66 vitrinoids, each with a designation derived from the vitrinite reflectance distribution. hus, vitrinoid -66 is an vitrinite. ikewise, he proposed that there anthracitic vitrinoid with a maximum reflectance in the range 6.6?#6.6)F. &uch categories are sometimes referred to as vitrinoid types or ‘-# types7 $&ection 1.=./%. 'lthough it may be useful in aspects of coal utili*ation $&ection 5.1./%, this is a somewhat unfortunate development because it means that vitrinites are not being sub#divided on the basis of the kinds of material involved, but on quite arbitrary divisions of the normal distribution of vitrinite reflectance readings. hese divisions may group different materials together, or unnecessarily subdivide portions of the same basic material into different -#types. 3. RE*#ECTNCE O* >ITRINITE ND OT8ER MCER#!
he ‘reflectance 7 of a maceral or other particle in a coal is the proportion of directly incident light, usually expressed as a percentage, that is reflected from a plane polished surface under specified
influenced by the relative proportions of the different macerals within the coal. Considered in con!unction with an index of coal type, such as the actual percentage of vitrinite present, the rank of the coal determined in this way provides an ideal basis for petrographic classification $e.g. 9ennett N aylor ()=?%. 3..1 Theoretical 'asis o+ re+lectance )eas(re)ent
Deflectance measurements made with the precision required for coal rank determination are generally carried out by comparing the amount of light reflected from the maceral concerned to the amount of light reflected from a standard substance under the same illumination conditions. &uch measurements can be carried out with dry ob!ectives or with water# or oil# immersion lenses, but most studies employ an oil#immersion technique, using an oil with a refractive index of (.7(0 at 61C and an incident light wavelength of 75/ nm. he amount of light reflected from the surface of the maceral is determined from the electrical output of a photomultiplier system $&ection 1.=./%, and the reflectance can be calculated from the following formula+
A
s
where . L reflectance of the coal maceral . s = reflectance of a calibration standard A L deflection of galvanometer, chart recorder etc. of photomultiplier system for the maceral and A s L deflection of galvanometer, chart recorder etc. for the calibration standard. n most cases, however, the petrographer sets the reflectance of the standard as the galvonometer reading, so that the reflectance is read directly. he relationship between reflectance -.' and the other optical properties of both the reflecting material and the immersion medium can be expressed by 9eer’s equation as follows+ -n 8 n 4 f > ?@$ @ ) -n > n 4 '@ > n@ $ e
where n L refractive index of reflecting material k L absorption index of reflecting material and n 4 L refractive index of medium in which the measurement is made. he refractive index of immersion oil changes with temperature, and this in turn
influences the value obtained in reflectance measurement. n order to
obtain a value of the relectance in oil -.J corrected to a standard temperature $61 or 67 C%, it is necessary, in theory, to measure the reflectance in two media $air is convenient as the second% and calculate n and k from the following equations+
-itrinite in coal often displays a three#
n L 1 /..
8
i a . ' % - i # . ' and k 6 L !g, - n l% 6 # - n # ( f 6 $( # . ' where i^ L measured reflectance in oil . = o
y a
measured reflectance in air and n + L refractive index of oil at temperature of measurement. &ubstituting the calculated values for n and k and the known value of n 4 at 61 C $usually (.7(0% into 9eer’s equation above allows the calculation of . o $61 C%. Aote that the reflectances in these equations are expressed to unit base, rather than as percentages. n practice, such computations are seldom per# formed. nstead of correcting for the actual oil temperatures, the . o of glass calibration standards is calculated from their refractive indices using the "resnel equation, with a value of (.7(0 assumed for r $s # W (?? $6 Y( f
DL &
where . s is the reflectance of the standard expressed as a percentage, n @ is the refractive index of the glass, and n + is that of the immersion medium. ;bviously, if the reflectances of the maceral being measured and the standard used for calibration are very close, any variation in refractive index of the oil due to a change in temperature from 61 C will cause both to vary by similar amounts. Gowever, if the coal and the standard have widely differing reflectances, any variation in refractive index of the oil will introduce significant errors. Consequently, it is customary to calibrate reflectometer systems with standards whose reflectances have been calculated using n? L (.7(0, and which are close to those of the macerals under study. 3..2
Optical anisotropy o+ re+lectance
"ig. 1.= he anisotropic character of coal. =^and . are the maximum and minimum reflectances of vitrinite, respectively. . is an apparent minimum reflectance, intermediate between . and .
dimensional variation in reflectance that is similar to the variation in optical properties exhibited by uniaxial negativesubstances. he optic axis of the indicatrix in these circumstances is approximately normal to the bedding plane. he vertical axis is shorter than the two hori*ontal axes, mainly in response to the vertical stresses imposed by the weight of superincumbent strata $"ig. 1.=%. 4hen viewed under plane polari*ed light, all planes or sections through a given layer of vitrinite, according to this model, should display the maximum reflectance of the material in at least one orientation on the microscope stage. ' section cut perpendicular to bedding should display the maximum reflectance when the plane of polari*ation of the light is along the bedding trace and the minimum reflectance of the material when it is at right angles to this direction. ' section cut parallel to bedding, on the other hand, theoretically should exhibit the maximum reflectance in any orientation, while one that is oblique to bedding should display the maximum reflectance when the light is polari*ed along the bedding trace and a value intermediate between the minimum and the maximum $i.e. an apparent minimum% when it is at right angles to the bedding. n a grain mount made up of crushed fragments of the coal embedded with random orientation in a plastic binder, every vitrinite particle examined should exhibit the maximum reflectance in at least
one position during rotation of the microscope Aot all coals display uniaxial reflectance stage when the field is illuminated with vertically characteristics. Gevia and -irgos $()==%, Cook et incident al $)0la% and evine and 2avis $()05% have all plane polari*ed light. he value of this maximum reported the existence of coals that demonstrate a reflectance can be recorded for a large number of biaxial anisotropy. 'n important implication of this particles and the ‘mean maximum reflectance 7 observation is that not all coal sections need -.1a4 ' calculated to give a widely used rank necessarily display the maximum reflectance parameter. value. @ost of the above reports describe biaxial f the reflectance is measured in non#polari*ed negativecoals, but evine and 2avis $()05% have light, on the other hand, the reflections from all also reported a sample of low volatile bituminous directions on the vitrinite surface will be integrated coal from 8ennsylvania that had the characteristics to give a ‘random reflectance 7. "igure 1.0 of a biaxial positive material. compares the influence of the reflectance anisotropy of a surface upon reflectances obtained 3..3 >itrinite re+lectance and coal ran in polari*ed and non#polari*ed light. ' statistical mean of random $non#polari*ed% readings taken on he reflectance of vitrinite, as well as that of other many vitrinite particles in a polished grain mount macerals, particularly those of the liptinite group of a coal theoretically gives the same value for the $see below%, increases progressively with the rank random reflectance as would be obtained if the of the coal in which it occurs. "igure 1.) illustrates random readings were taken in polari*ed light the relation between the maximum reflectance of $without stage rotation%. Gowever, the range of vitrinite and the carbon content of the same theoretically possible values from a sample is material $another rank indicator%, arid able 1.(? much less for non#polari*ed iigbt .1 t? - .1a4 . gives an indication ofthe ranges of mean maximum min%M6 ( than for polari*ed light $ .1a4 to .1in '8 he vitrinite reflectance $H max% that correspond to the statistical relationship between random, maximum principal '.&..@. rank designations, based on other parameters as outlined in &ection 6.(?.6. he and minimum reflectances $Gevia N -irgos ()==% rate of change of reflectance is not uniform with is also shown in "ig. 1.0. 2avis $()=0c% has $iyM*M, Tnone anu 3 ;;: nuwer anu i3avis summari*ed the relative merits of maximum and /1
. .. .
..'
random reflectance petrographic studies.
1_
/1 ; r,;< TT_____________________ _________I T=<_________* _
determinations
in
coal respect to many of these other indices, and, in fact, reflectance is a sensitive indicator of rank change in higher, rather than lower rank deposits. "igure 1.) also shows the variation in minimum reflectance, and hence, from the difference between maximum and minimum reflectance, the ‘bi#reflectance7 of the vitrinite. his latter property also increases significantly with rank, although, as discussed further in uiv.iv, io uvn nv,vv,ooai relation between rank and anisotropy in some individual coalfields.
n iimi [ ........
a uiiuui in
3..
5.?(-
Re+lectance o+ other )acerals
? I Y K K XXXXXXXXX XX K Y Y Y X XXXXXXXXXXX( XXXXXXXXXX l ( X =?
Carbon $F%
=7
.0?
07
)?
l
i
t i l l
)7
"ig. 1.) he relationship between carbon content and maximum $\% and minimum $o% reflectances of vitrinite. $"rom 2avis ()=0.%
'9> 1.(? -itrinite reflectance limits $in oil% and '.&..@. coal rank classes. Dank @aximum reflectance $F% sub#bituminous high volatile bituminous C high volatile bituminous 9 high volatile bituminous ' medium volatile bituminous
B?.5= ?.5=#?.7= ?.7=#?.=( ?.=(#(.(? (.(?#(.7?
low volatile bituminous semianthracite anthracite
(.7?#6.?7 6.?7#1.?? $approx.% I1.?? $approx.%
semifusinite, micrinite and sporinite relative to that of vitrinite in a number of coals. he reflectance of fusinite approaches that of vitrinite as rank increases, and in the case of a peranthracite $'lpern N emos de &ousa ()=?% the fusinite may actually have a lower reflectance than the vitrinite in the same coal.
he coalification track of sporinite displays a sharp increase in reflectance, known as the ‘coalification !ump’ $&tach N @ichels ()77%, at a rank corresponding to a vitrinite reflectance of about (.6F. he reflectances of these two macerals then merge at a vitrinite reflectance of about (.7F.
he reflectance of resi# nite is similar to that of sporinite in the same coal, while that of cutinite is somewhat higher. Cutinite also exhibits a strong reflectance anisotropy. Gower $()=0% has described liptinite macerals $sporinite, resi# nite "igure 1.(? shows the random reflectances of fusinite, ,'-
$a%
"ig. 1.0 Dandom reflectance in polari*ed and non#polari*ed light. n polari*ed light, the random reflectance obtained on a single particle would vary between D and D[ # , the apparent minimum reflectance, depending upon the orientation of the particle. n non#polari*ed light, the reflectance in all directions will be integrated into the random reflectance reading, regardless of orientation. he relationships shown are from Gevia and -irgos $()==%. $a% 8olari*ed light, $b% Aon#polari*ed light. X X D3rcpraaite the average of a number of readings. D nild 3 L D rand L $6D max D min%M1 ran n?n-?l a @ maA B min
"ig. 1.(? Deflectances of different macerals through a range of coal rank. $"rom Goover N 2avis ()0?.% \ -itrinite sporinite ' fusinite ' semifusinite o micrinite.
and cutinite% in an anthracite sample, all with similar random reflectances of about 5.)F. he bireflectances of the liptinite macerals, especially 3../ Re+lectance at di++erent 0a4elengths the cutinite, in this case were greater than that of "igure 1.(( shows that, for bituminous coals, the the vitrinite. reflectances in air and oil, and the refractive index of vitrinite, decrease with increasing wavelength of the incident light. 4ith increasing rank, the dispersion curves for air reflectance and refractive index tend to flatten, and, in the case of anthracites, there is a reversal in slope, so that values increase towards the red end of the spectrum. "rom the visible region of the spectrum into the ultraviolet, the reflectances and refractive indices of vitrinites have been observed to decrease. he absorption coefficients, however, tend to peak within the ultraviolet region near 60? nm, extending into the visible range in the case of anthracites $Eilbert ()/6 @cCartney et al ()/7%. his strong absorption in the ultraviolet has been interpreted as being due to the presence of significant proportions of aromatic structures. he different slopes of the dispersion curves for vitrinites of varying rank, relative to the slope for a calibration standard, provide a compelling reason why reflectance measurements should be made in monochromatic light.
(.=?(---------------- (------------------ (-----------------(----------------* /?? 77? 7?? 57? 5?? )a4l4nDtE Fnm5
"ig. 1.(( 2ispersion of the optical properties of vitrinites $"rom 2avis ()=0.% $a% 'ir reflectance, $b% ;il reflectance. $c% Defractive index, o 06./#01.5F carbon '05.?F carbon '0=.=S00.(F carbon K )(.5F carbon )6.0F carbon \ )6.0FS)1.1F carbon.
@urchison and Vones $()/5% and Gevia $()=5% have shown that the shapes of the dispersion curves for liptinite macerals are fairly similar to those of vitrinite from the same coal, although, of course, displaced to lower values. he curves for inertinite macerals are less steep than those of the vitrinite those of higher reflecting inertinite have very low slopes, and some display minima in the range 50?#77? nm $Gevia ()=5%. i ! 282nnu '8uir 'A' -Y: ; ' 1
M.
111V« lll (llil/l(
@icropetrographic study of coal may be used either as a technique of quality evaluation in the testing of a coal’s economic value, or as an aid to understanding the geologic history of the material. hough the selection of samples to be studied in each case may be somewhat different, both types of study generally involve measuring the relative proportions of the various macerals andMor microlithotypes that may be present, and assessing the rank of the coal by means of such properties as vitrinite reflectance. he actual techniques involved in such studies are similar in many ways to petrographic methods used in other branches of geology, although a number of refinements have been introduced to deal specifically with coal and related materials. 's with other aspects of coal analysis, several international and national standards are available that describe the most appropriate procedures in
some degree of detail. n other cases, such as with has set. ;nce set, the moulded resin and coal pellet the introduction of automated microscopy to coal is ground and petrography, much of the necessary information can only be found in the discussions of various research investigations. 3.7.1
!a)ple preparation
's indicated in &ection 1.1, almost all petrographic studies of coals are now carried out by means of polished section techniques. 8olished sections of coal may be prepared from single lumps, broken pr sawn from a hand specimen, a section of drill core or an exposed coal face, or they may be prepared from a representative sample of the seam or seam sub#section, crushed to a granular or coarse powder form. ndividual lump specimens have the advantage that they can preserve the geometric relationships of the various bands and other masses in the coal to a greater extent than crushed coal samples, but have the disadvantage that they only represent a limited sub#section of the seam in question. ' relatively small amount of a well# prepared, representative crushed sample, on the other hand, can provide data on the abundance and optical characteristics of the macerals or microlithotypes present in a much larger mass of in situ or mine product material. ump specimens for polished blocks are usually air#dried and impregnated with an appropriate resin if necessary to provide strength. he specimen is then trimmed to expose the. face to be studied $usually one perpendicular to the bedding planes%, and mounted in an appropriate mould with a cold#setting resin material. ;nce set, the embedded specimen can be ground or cut along the desired plane, and polished with successively finer abrasive powders. Crushed coal specimens for petrographic study are usually prepared to a coarser particle si*e than are samples for many other analyses. "or most purposes, a maximum particle si*e of ( mm is required, compared to 6?? fi1 for $say% proximate analysis, but it is also necessary to produce a minimum amount of fine material in the crushing process. hese requirements should be taken into account when designing the sample preparation sequence for a coal analysis programme in which petrographic studies are to be included at some stage. he crushed material is well mixed with a cold# setting resin to form a paste or slurry, and the mixture poured into a mould of appropriate shape to harden. Care should taken to avoid the solid particles settling and segregating before the binder
polished for petrographic examination. f any segregation of panicles has occurred, the pellet should be cut in the vertical plane and one half polished. 'n alternative procedure is to employ a hydraulic press to prepare cylindrical moulds in which close packing prevents segregation. Erinding and polishing are carried out with a senes of abrasives on wet laps of low#nap cloth andMor silk. ' high quality of relieffree polish is required for reflectance determinations. he techniques of polished section preparation are described in some detail by the .C.C.8. $()/1%, &tach et al $()=7%, '.&..@. $()0(a% and the nternational ;rgani*ation for &tandardi*ation $.&.;.% $in preparation, a%. 'lthough much less widely used, the methods of preparing coal thin sections for microscopic examination are given by hiessen $()6?%, van :revelen $()/(% and "rancis $()/(%. 3.7.2
maceral and also activate the stage
Maceral analysis
he relative proportions of the various macerals in a coal, at least on a volumetric basis, can be determined from either polished blocks of lump coal or grain mounts by the techniques of modal analysis. n the past, this has included ‘lineal @g. 1.(6 @icroscope equipped with &wift automatic point counter. analysis’ by means of the integrating stage $van :revelen ()/( Ealehouse ()=(% advance mechanism each time a counter is uy a0 depressed, moving it on to the next grid point. vRiuvix iiiuv3u iiv linage Mv.ixc#aii otiltd 'ccording to Gilliard and Cahn $()/(%, a two# of micrometer#graduated screw spindles. >ach spindle was moved only when a particular maceral dimensional grid is the most efficient method of was under the cross#hair in the field of view. 't the conclusion of the examination, the amount of each maceral present was determined from the contribution that each spindle had made to the total traverse length. oday, however, modal analysis is more commonly based on the technique of point counting $Elagolev ()15 Chayes ()5), ()7/%, in which the microscope stage is moved in a series of fixed increments and the identity of the maceral (M6 B *5 = @ falling beneath the cross#hairs after each advance is recorded. he volumetric abundance of each maceral is determined from the total number of points at which it was encountered in relation to the total number of points recorded in the traverse network. &tage movement can be accomplished manually with a mechanical stage equipped with check stops, or even by visually ad!usting the stage vernier scale. Gowever, there are automatic point counters such as that displayed in "ig. 1.(6, which keep a tally of the number of counts made on each m r < T- t - T - i c it*> nrr? :a+an-( t(a atTamaan :.r n nam/i
analysing volume proportions, with a grid spacing such that the ma!ority of structural features should be intersected by no more than one grid point. he .&.;. $in preparation, b% recommends that maceral analyses should be performed on minus ( mm coal particles, and, with the .C.C.8. $()/1%, that interpoint and inter#line distances should be approximately ?.7 mm. he probable error, at the )7F confidence level, involved in counting individual components $ B *5 ' is given by the equation
P $loo # p ' where p is the percentage of the individual component, and n is the total number of points counted. "or components present in proportions of (?, 7? and )?F, the probable errors involved in a maceral analysis of 7?? points are j 6.=, 5.7 and 6.=F, respectively, assuming of course that no errors in identification are made during the analysis. he repeatabilities of such results $6"M-6% are 1.0, /.5 and 1.0, respectively. he reproducibility, that is the difference between results of two different operators
analysing different sub#samples of the same coal, "luorescence microscopy can enable distinctions to has been found to be about (.7#6 times the be made between liptinite macerals which would theoretical repeatability $.&.;., in preparation, b%. otherwise appear uniformly dark in reflected white hus, maceral percentages should be reported to light. 'lso, in petrographic analyses performed in the nearest integer and not, as is frequently done, blue light, the percentages of the fluorescent to the first decimal place. liptinite macerals often appear greater than when he total number of points counted in maceral conventional white light is used. dentification is analysis can vary according to the standard made more readily by observing the fluorescence procedure followed, the purpose of the analysis, colours on a dark background than by observing and the accuracy desired. Ealehouse $()=(% the dark reflections on a medium background. provides useful nomograms and tables for able 1.(( provides a comparison of selected determining quickly the number of points that must analyses carried out both in white and combined be counted in order to achieve desired levels of white and blue light, showing the larger accuracy. he .C.C.8. $()/1% recommends proportions of liptinite macerals which were counting 7?? or (??? points to give an accuracy of recorded in the combination analysis $2avis ()=7%. about j 6#1F. .&.;. $in preparation, b% requires n the combination analysis, the percentages of that at least 7?? points be counted. he '.&..@. liptinite macerals were determined in blue light, $()0(b% standard deals with precision in maceral and the balance, due to nonfluorescing vitrinite and analysis by requiring that two separate analyses of inertinite group macerals, apportioned according to (??? points be performed on each of two grain the results of the white#light analysis. mounts. "or the values to be acceptable, the mean difference between maceral percentages of the two 3.7. &ases +or reporting )aceral analyses grain mounts should not exceed 6F. he magnification employed for maceral 8etrographic analyses are reported on either a analysis should be such as to permit resolution of mineralfree or mineral#containing basis. "or the most maceral occurrences, i.e. at least x6?? latter, the amount of mineral matter may be overall. Gowever, '.&..@. $()0(b% recommends determined together with macerals during point that it should be greater than W5??. ;il immersion counting, or calculated using an empirical formula. ob!ectives are normally used to provide best 2irect determination using techniques such as identification conditions. radio#frequency oxidation is also possible $&ection &election of the macerals or maceral groups 6.).6%. and, if desired, the mineral species to be counted &ome difficulties are associated with the depends upon the purpose for which the analysis is determination of mineral percentages by modal intended and the predilections of the petrographer. analysis. Certain minerals, including quart* and the &ome laboratories count only the three maceral volumetrically important clay minerals, often groups, whereas the '.&..@. standard specifies appear dark, almost black, under oil immersion. t the six macerals that should be counted. is therefore possible to mistake them for voids or -arious conventions can be adopted to help the for the resin binder, esneciallv where there is no netrwranher maintain consistency in those surrounding matrix of instances ##r########## J # # # # # # # # # # # # ! 8888 88C 8 8+ 8888 8 888888888888888 8888 888888 88
- J
-
- - -
- -
# where the cross#hair lies over the perimeter of ############# (_j # higher reflecting coal macerals to provide an particles or on boundaries between macerals $&tach optical contrast. ;ther minerals, including pyrite et al ()=7 .&.;., in preparation, b%. grains, may be plucked out during polishing, so that their proportions may tend to be 3.7.3 Co)'ined 0hite and 'l(e underestimated. n some instances, the minerals light petrographic analysis are so finely divided that it is impossible to resolve them under the microscope, while much of the inorganic content of lower rank coals also exists as ion#exchangeable cations within the organic molecular structure rather than as discrete mineral species. n spite of these difficulties, it is often useful to include ‘total minerals’ as a category in the point# count analysis, or even to count individual classes of minerals, such as clays, carbonates, pyrite,
quart* etc. &everal formulae have been developed to calculate the volume percentage, rather than mass percentage, of mineral matter in coals. he '.&..@. $()0(b% equation below is based on the 8arr formula $&ection 6.).6%, and average densities of 6.0? and (.17 have been assumed for the mineral and organic contents, respectively, to convert the mass of mineral matter to a volume percentage. X XXXXXXXXXXXXXXXX(?? $(.?0' .77&%M6.0 XXXXXXXXXXXXXXXXXX $-;l% (?? # $(.?0' ?.77&%M(.17 $(.?0' # ?.77&%M6.0
when ' and & are the ash and sulphur values of the coal, respectively, expressed on a dry basis.
'9> 1.(( Comparison of white#light and combined white# and blue#light maceral analyses $8&;C#(61 lithotype of Ao. 7. 9lock &eam, 4est -irginia D o L ?.=(F%. 'nalysis 4hite light $F% Combination $whiteMblue% $F%
-itrinite
&porinite
1=
65
6)
11
Cutinite
Desinite
'lginite
?
6
?
(
/
1
"usinite
&emifusinite
@acrinite
@icrinite
(7
5
7
(1
1
5
((
he .&.;. $in preparation, b% has simplified this to+ 'lthough the technique is not as widely used as maceral analysis, the relative proportions of the various microlithotypes and, if necessary, of \ @@, L @@2 ,4ol- ?????????????????????????p????????????? carbominerites $coal#mineral associations% in a 6.= # $?.?(( @@ p% coal sample can also be determined by modal where @@ p, the weight percentage mineral matter, analysis methods. n the most common of the two is derived through the 8arr equation and equals analysis techniques, the microscope eyepiece is fitted with a 6? point cross#line reticule $"ig. 1.(1%. (.?0' ?.77&. 'n empirical equation has also been given by he magnification characteristics of the microscope are matched to the actual si*e of the .&.;. $in preparation, b% as follows+ grid to delineate a 5p1 squarein the field of view, normally with a x67 ob!ective being used. >ach @@, n L ?./(' # ?.6( band or maceral association falling within this ,4olsquare can be evaluated in terms of si*e $i.e. if it is 9enedict et al $()/0% have used the equation greater than 7? p1 in thickness% and also with below as a simple method for estimating the respect to the relative proportion of the various volumetric mineral content of coals+ constituents. "or example, if all 6? points on the reticule fall on vitrinite, the microlithotype has less @@$vo% L ?./ $' &% than 7F of any other maceral group and is therefore classed as a vitrite. f one or more of the where ' and & are as defined above. 6? points coincide with inertinite, however, the ;ccasionally, the results of maceral analysis material would be classed as a vitrinertite $able are reported on a weight percentage basis. o do 1.=%. this one must assume a value for the relative >ach observation on a group of 6? grid line density of the macerals at the particular rank level intersections is regarded as a point, even though of the sub!ect coal. he .C.C.8. $()/1% lists the area covered may in fact cross the boundary densities for five macerals at () rank levels, based between two different naturally occurring on the percentage of total carbon. able 1.(6 gives microlithotype bands. 't least 7?? points should be a comparison of a single maceral analysis reported counted to achieve a satisfactory result, with care on a volume percentage mineral#free basis, a being taken to ensure that the whole of the volume percentage mineral# containing basis, and a specimen’s surface is covered to avoid any bias weight percentage mineral# containing basis. due to particle segregation. he .C.C.8. $()/1% and &tach et al $()=7% describe conventions that should be followed when the reticule does not fully 3.7./ Microlithotype analysis cover the image of a coal particle, and lies partly on the binding material of the crushed coal specimen. he second method of performing microlithotype analysis is termed ‘selon la ligne’ $according to the line%. ' simple eyepiece measuring reticule is used, with divisions spaced equally such that at least 6? fall within 7? p10 n the case of particulate samples, W
(?
\
'9> 1.(6 @aceral analysis reported to different bases high volatile 9 bituminous coal $8&;C#/0%. -itrinite "usinite &emifusinite @acrinite @icrinite -olume $F%, mineral#free -olume $F%, mineral#containing 4eight $F%, mineral#containing
0( =) =7
) 0 )
5 5 5
( (
( (
(
(
&porinite Desinite 1 1 1
@ineral
( (
1
(
/
"ig. 1.(1 wenty point eyepiece reticule for microlithotype analysis.
the reticule is rotated so that the line lies perpendicular to the bedding displayed in any particle. @icrolithotype identification is made by considering the ‘natural band 7 of 7? p1 in width containing the mid#point of the reticule. he 7F limit is determined by estimating the proportion of macerals or minerals across the width of the band under consideration. 4here stratification is uncertain, the reticule should t__
_______ ' G —
_______ _i ___________________ ___________ *.$
uc maiiuanicu m me vcmuai piu@iiuii aiiu uic
estimation made on a circle of radius 7? p10 he ‘selon la ligne 7 method is also particularly suitable for obtaining microlithotype profiles of column samples of coal. ' traverse can be made down the entire column of coal, a reticule length at a time. 'nalysis by either method on the same coal will give different results, because the 6? point ocular method, which does not recogni*e ‘natural bands 7 in the sample, tends to produce more bimaceral and trimaceral categories. V.4. Gunt $personal communication% has, in comparing the two methods on the same sample, measured the amount of vitrite duroclarite clarodurite by the ‘selon la ligne 7 method at 1?F, compared to 57F by the 6? point ocular method. 3.7.
Re+lectance )eas(re)ent
he light source must be stabili*ed to ensure that no variations in photomultiplier output arise due to minor fluctuations in light intensity with surgesin the power supply. 9ecause the amount of light reflected from the coal is rather small $usually less than 6F% the power rating and intensity of the light source should be high to increase the signal#to#noise ratio of the system. ' (?? 4 quart*#halogen lamp is most commonly used for this purpose. f ‘maximum 7 rather than ‘random 7 reflectance is to be measured, a( polari*er is placed into the B5 Y Y e Y Y
UCcU ll. U
VXtlgl ll lli3
d
specimen surface by means of a vertical illuminator. n microscopes that use a 9erek prism as the vertical illuminator, the polari*er should be set in the 57 position $9erek ()1= 9roadbent N &haw ()77%. he reflected light from the surface of the coal passes back through the ob!ective to the vertical illuminator. 9etween the vertical illuminator and the photomultiplier head are a filter and a limiting aperture. he filter is usually a band interference filter of 75/ nm, which is the wavelength of the mercury line within the green region of light. he aperture, which may be of fixed or variable dimensions, limits the area of the field on which the reflectance measurement is actually taken. ' square of about 5#7 3m on the side is most commonly used. n many modem photometric systems, an image of the aperture can be seen superimposed upon the actual area of measurement within the field of view. he measuring device used in most reflectance systems is a photomultiplier tube, selected to give
' number of methods, both direct and indirect, have been employed to measure the reflectance of coal macerals, and these are reviewed by 2avis $()=0c%. ;f these techniques, however, the single#beam comparative method is now used by most, if not all, coal petrologists throughout the world. he single#beam comparative method is based on the $Eray odd N 2rexler ()=)% and photodiodes $;tte N use of a reflected#light microscope photometer, calibrated 8fisterer, in preparation% have also been used as satis# factory alternatives for this by means of glass, mineral or synthetic standards, which purpose. ' stabli*ed power are then replaced by the vitrinite in the coal sample for a supply is required for the series of reflectance readings. 2etailed descriptions of the photometer. ' chart, meter equipment and procedures used in the process are given by I the .C.C.8. $()=(%, Ealopin and Genry $()=6%, 8iller $()==%, '.&..@. $()0(c% and .&.; $in press, c%. he components of a reflectance photometer are illustrated in Qart Eal?D4n "ig. 1.(5. n
mnnAnna nt
Ct illgll itopuiiov U,
n m T^irritvll nVlAtAn AA11 n tpl*c
(((((. l#[lgl,ai 3G-,4ll --&@lll4
'
lam
PE?t?mltili4r catE?4 Int4r4r4nc4 Hlt4r— 4a +aDniHcati?n sJst4m * itE limitinD a4rtr4 P?lari4r
2l[ild V4rtical illminat?r OK,4cti4--- #4cim4n *
rr77
-rr
"ig. 1.(5 ;ptical components of a microscope photometer.
Coal Petrology and Petrographic Analysis
3*
or computer is needed to observe andMor record the sample is a blend of medium, high and low volatile reflectance readings, and a data processor is often an bituminous components. he use of such data in integral part of the reflectometer system. establishing the coking characteristics of a coal or blend is he scale on which the reflectance measurements are discussed further in &ection 1.).1. read is established from *ero to the reflectance of the primary standard, preferably one which has a reflectance 3.7.7 (to)ated )icroscopy o+ coal !ust slightly above that of the sub!ect vitrinite $Vuckes ()=1%. he reflectance of glass standards -.' can be ' number of methods are under investigation to provide calculated from the "resnel equation $&ection 1./.(%, but, automatic microscopic measurement of the petrographic because there is some indication that the surface properties properties of coal. he principal goal of research with these of materials may also influence their measured optical systems is to develop the capacity of performing rapid and properties $8iller ()==%, they may be calibrated by the accurate maceral analyses, based on the characteristic supplier. t is possible to obtain or prepare single mounts differences in reflectance of the various components in a single coal. hese techniques have the potential to provide which contain a number of such standards if required. ;nce the upper limit of the scale has been calibrated the improved reproducibility due to the removal of operator# *ero should be corrected for any photomultiplier dark induced sources of variation, and also enable a greatly current and back reflectance $primary glare% from the back increased number of sample points to be investigated ner lens of the ob!ective $:otter ()/? Vones ()/6 Ealopin N unit time. Genry ()=6 8iller ()==%. 't least one additional standard ---*------C- *= 1* M* 2avis $()=0c% has reviewed many of the early attempts should be checked against the scale and the calculated $or to automate coal petrographic analysis, including the calibrated% and determined values should not differ by clockwork stage and recording device of -endl $()15% and more than about j ?.?6F actual reflectance. &uitable vitrinite particles are selected for measurement the '@>2', which incorporated ten electronic counters from a grain mount which has been carefully polished to and a belt#drive stage mechanism $9omberger N 2uel give a level, scratch#free and relief#free surface $'.&..@. ()/5 9ayer et al ()/0%. @ost of these, however, were ()0(c%. ' recommended practice is to select the particles limited by the data#handling capacity of the instruments from a grid pattern covering most of the grain mount. 'll used. ' of the various vitrinite macerals and sub#macerals might be included for applications where coal properties are being related to technological behaviour, such as carboni*ation. Gowever, for precise measurements of the degree of metamorphism it may be best to restrict the measurements to bands of homogeneous telocollinite, which tends to be slightly more highly reflecting, rather than the more heterogeneous desmocollinite. n the case of lignites, the preferred vitrinitic, or huminite maceral for reflectance measurement is ulminite. he number of readings that must be averaged to obtain the mean reflectance may vary, depending on the application, between about 6? and (??. ;btaining a reflectance distribution chart on a blend of two or more coals, however, may require 7?? or more readings. "ig. 1.(7 (M6 -#type reflectance histogram for a blend of Checking and ad!usting the calibration of the high, medium and low volatile bituminous coals. &le+ coal blend. i^ max L (.657F. microphotometer should be done at intervals during the analysis, and also at its conclusion. he standard deviation of the mean for (?? readings on a single seam should be about ?.?(#?.?6F $.C.C.8. ()=( .&.;., in press, c%. he reproducibility between two different operators analysing sub#samples of the same coal might be ?.?0F. n addition to reporting mean reflectance, it is also common for the reflectance distribution to be reported as H -#types’ $vitrinoid types%, or -#types’, which represent ranges ?.( and ?.?7F, respectively. "or example, -#type = covers the reflectance range ?.=?#?.=)F, and F -#type =.67 covers the range ?.=?#?.=5F. "igure 1.(7 is a histogram plot of a l h -#type distribution. Deference to able 1.(? $&ection 1./.1% indicates that this particular -
_
_
Coal Petrology and Petrographic Analysis
+ system developed by the U.&. 9ureau of @ines 4ith specirnen#plane scanners, on the other hand,the $@cCartney N >rgun ()/) @cCartney et al ()=(% also light source and detection equipment are aligned along the employed continuous scanning, and recorded reflectance optic axis of the microscope, and the sample is moved data on magnetic tape for subsequent computer processing. across the field in a plane perpendicular to that axis. he hree generations of automated microscopes, using both detector is usually a photomultiplier tube, and is capable of continuous and stepping stage movement with data resolving reflectance values more precisely than the processing in real time, are described by 2avis and -astola sensors in image#plane systems. ' single reflectance $()==%, :uehn and 2avis $()=)% and Goover and 2avis reading is taken over a small area $usually about (#5 Mim $()0?%. across% after each increment of stage movement. 'lthough the instruments used in modern automated &pecimen#plane analysis is performed on a linear or microscopy vary considerably, they can be classified into raster pattern across the exposed polished surface with the two types, image#plane or specimen# plane scanners, on the individual reflectance readings either immediately ad!acent basis of the type of scanning technique employed $4eibel to each other or separated by an interval of no more than et al ()=6 :uehn N 2avis ()=)%. n image#plane scanners, several tens of micrometres. n addition to accumulating the scanning is performed by a sensor on the video some thousands of individual data points, the analysis can pro!ection of the microscopic image, and for this reason, also, if ad!acent readings are taken, provide information on techniques of this type are known as image#analysis particle si*es based on intercept or chord#length systems. @any thousands of points are analysed for each distributions. 'n instrument of this type, developed at the field, but reproducibility of results is also a function of the 8ennsylvania &tate University, is illustrated in "ig. 1.(/. number of fields analysed during a scan. &uch systems he principal output of most automated microscope have the advantage of being able to analyse the si*e and systems is a reflectogram or histogram of the frequency of shape of the particles under investigation. -arious readings at all reflectance levels $"ig. 1.(=%. @ost applications of the technique to coal and coke petrography reflectograms have two prominent peaks, one at the low are described by 9.C.D.' $()=7%, Garris $()==%, Garris et reflectance end representing the binder used to pelleti*e the coal particles, and the other representing the contribution al( $()==%, eiss $()=)% and Chao et al $()=)%. for the preponderant coal component, vitrinite. 9ecause the systems use non#
"ig. 1.(/ 'utomated reflectance microscope consisting of eit* ;rthoplan microscope equipped with @8- 6 photometer system and stepping stage, and 2igital >quipment @AC#(( computer. $"rom 2avis et al ()01.%
'ttempts to derive actual maceral analyses from such polari*ed light, the reflectance of the vitrinite mode is the random reflectance. Goover and 2avis $()0?% have shown data have involved curve#stripping techniques to overcome how random reflectance can be converted to maximum the problems of overlapping reflectance distributions of macerals and of boundaries $@cCartney et al ()=( :o!ima reflectance by the following equation+ et al ()=5 Goover N 2avis ()0? :o!ima N &akui ()0?%. (.?/( . where . and . , are 'lthough themodels employed tend to oversimplify the . )a6 rand the complex interactions of particle composition, si*e and mean maximum texture, some agreement with visually#derived analyses has )a6 ran reflectance and the mean random reflectance respectively. been obtained. ' ma!or problem of automated techniques in he data supplied to the computer in automated analysis with visual microscopy at present is are only a series of reflectance readings, and interpretation comparison f'n ( mriil 8 ( 1 M r U f n n f of which macerals these represent is usually made by sole means of discrimination between coal constituents interpreting the shape of the reflec# togram. he three since, in visual analysis, the shape of entities, and even reflectograms in "ig. 1.(= for example, are of their modes of association with other materials may also be petrographically very dissimilar lithotypes from a single taken into account. 'nother problem has been the difficulty seam section. he profiles clearly reflect the maceral group in interpreting ‘edge effects’, the spurious readings compositions as determined by conventional visual obtained on the edges of particles and the boundaries analysis and depicted in the accompanying bar graphs. between macerals. 'utomated reflectance microscopy is an ideal tool for quality control evaluation of coals in industrial situations $2avis N -astola ()==%. he reflectogram profile is sensitive to changes in the composition of coal feedstocks, especially in blends, and can be used to monitor quality in various areas of coal utili*ation. "igure 1.(0 gives an illustration of the kind of advantage this approach has over chemical analysis. he coal represented was marketed as a midvolatile material, but the reflectogram shows it to be mainly a blend of high and low volatile bituminous components with only a minor amount of medium "IV&
'
SJll Ll ll & I t V L l SJL littlt ilDilL
Lilt
'
"ig. 1.(= Comparison of automated microscopy reflectograms with visual petrographic analyses. $"rom 2avis N -astola ()==.% <-ertical scale based on the most frequent reflectance reading, with a xl; factor for reflectances greater than ?.(6F. >x., exinite -it., vitrinite n., inertinite.
Coal Petrology and Petrographic Analysis
3
the kind of detail that can be reproduced in this way.
;4raD4 ran?m r4f4ctanc4 N5
"ig. 1.(0 'utomated microscopy reflectogram of a blend of high, low and medium volatile bituminous coals. $"rom Goover N 2avis ()0?.%
component, with only a minor amount of medium 1.0 ;DEA ;" @'C>D'& 'A2 C;' G;R8>& volatile coal. he factors that influence the abundance and structure of -iitr-i rf< nvritp haY also aided the J the constituents in coals of varying type are so interrelated automatic measurement of the amount and si*e that it is difficult to identify a completely independent set distribution of this mineral in coals $9ayer et al ()/0 of controls. "actors that can have a bearing on the @cCartney N >rgun ()/) 2avis N -astola ()== :uehn constitution of a peat deposit include the following+ N 2avis, ()=)%. :uehn et al $()0?%, for example, have $a% he nature of the plant community. he type of used a specimen#scanning method to monitor changes in plants and the relative abundance of each form is an pyrite content and petrographic composition with particle obvious control, depending in turn on the geologic age of si*e and density in laboratory beneficiation experiments. the deposit, the physiographic setting, the ' new application for automated reflectance rlimafp anG tbp snnnlv nf nutrients+ ?0E8888888888888888888888888888888888888 r r G 888888888888888888888888888 & < microscopy $2avis et al ()01% has been prompted by a $b% he climate prevailing in and around the need to map the petrographic variation across coal depositional site. he temperature and humidity, as well as surfaces. Deflectance values are stored on disc in a any seasonal fluctuations in these factors, influence the matrix that enables the proper spatial arrangement of nature of the plant community, the extent of peat build#up values to be recreated as a map by an image processor and decay, and the rate and products of weathering in the and colour camera. he maps depict lithotype variation, swamp hinterland and the distribution of mineral occurrences, cracks and $c% he extent of plant decomposition. his in turn cleat in the coal. hey can be matched by computer depends on the nature of the plants themselves, on the against images derived by other techniques. "igure 1.() climate and on the >h and pG conditions in the swamp compares the reflectance map with a conventional waters photograph, and illustrates $d% he tectonic setting of the deposit. his is a ma!or ( U- lilgli W -ll--tUil-- -< ### n
< '
factor in controlling subsidence rates and in determining the physiographic setting, the rate of nutrient supply and the extent of plant decay
$e% he physiographic setting, palaeogeography or depositional milieu of the deposit. hese represent a combination of factors including the depth and movement of waters, the shape and areal extent of the peat swamp and the chemistry, including >h and pG, of the swamp waters.
"ig. 1.() Comparison of image derived from automated microscope reflectance mapping $b% with photograph $a% of a polished coal surface. $"rom 2avis et al ()01.%
[email protected] Origin o+ 4itrinite and 'right coal lithotypes
Coal Petrology and Petrographic Analysis
tissues. Cook $personal communciation% has suggested 5 that any chemical differences between band vitrinite and vitrinite in attrital layers may have been imposed by differences in the chemistry of the original plant materials, with the tissues that gave rise to the band vitrinite being hydrogen deficient and therefore oxygen rich. &uch material would have decomposed more rapidly than the hydrogen#rich tissues, and thus band vitrinite would have been preserved only under rapid burial conditions.
-itrinite has long been regarded as the result of deposition of ligno#cellulosic tissues $wood, bark etc.% in stagnant, highly toxic waters that protected the organic material from extensive biochemical decay $4hite ()11 asch ()/?%. Daistrick and @arshall $()1)% have noted that the great ma!ority of vitrain sheets in Carboniferous coals, which are usually about / mm thick, represent the bark shells of lycopods, although the wood of gymnosperms and cycads was another important source of this type of material. [email protected] Origin o+ inertinite and d(ll coal ;n the basis of detailed petrographic and palynologic lithotypes profiles through a number of coal seams, &mith $()/0% suggests that the vitrinite#rich layers in humic coals were he inertinite macerals are generally regarded as having most likely to have been deposited in areas that underwent been derived from the same types of plant debris as greater subsidence than the surrounding regions. n studies vitrinite, but have undergone chemical changes due to of 'ustralian coal measures, &hibaoka and &myth $()=7% processes such as charring, oxidation, mouldering and have shown that coals deposited in the thicker sections of fungal attack at an early stage of their depositional history. troughs $i.e. areas of greater relative subsidence% are rich in "usinite, for example, is thought to have originated in bright lithotypes, and that these are more likely to be many cases as plant fragments that have suffered partial associated with mineral partings than the duller, vitrinite# combustion in forest fires. &uch material, known as poor seams of the more stable shelf areas. &hibaoka and ‘pyrofusinite’ $eichmuller ()7?%, usually has a high &myth $()=7% and Cook $()=7% noted that the lower seams reflectance, appearing yellow or white under oil of the 'ustralian Aewcastle Coal @easures and the riassic immersion, and with distinct cell wall margins. 'nother pswich Coal @easures, which are ‘deep basin type’ form, called ‘degradofusinite’, is characteri*ed by deposits, are rich in vitrinite, whereas the llawarra Coal indistinct cell walls and a lower, semifusinitic reflectance. @easures, deposited with steady, slow subsidence, his material is thought by eichmuller to result from contained seams typically very poor in vitrinite. ;ther dehydration, oxidation and biochemical alteration of plant ‘shallow# basin type’ coals deposited on stable shelves or debris. Gowever, Eiven $in Eiven et al ()0?% has basement rocks, like the 9lair 'thol, eigh Creek and challenged the possibility that degradofusinite might be the product of biochemical decay. t is #a characteristically Callide coals, also are vitrinite poor. he fine#grained type of vitrinite, desmocollinite, such abundant constituent of the 8ermian coals of as is commonly found in clarain, is generally assumed to Eondwanaland. ;ther fusinite may be of primary origin, have originated either from smaller plant organs, such as derived directly from certain pigmented plant tissues. &emifiisinite, in general, is regarded as representing leaves, or from finer fragments of larger tissues that have been partly degraded. 2esmocollinite usually has a lower ligno#cellulosic plant debris that has suffered a lesser reflectance than the telocollinite $i.e. coarse vitrinite% in the degree of degradation than fusinite. "ragments of both same coal, and this has been attributed to either admixture fusinite and semifusinite, often forming discrete with fine liptinitic detritus $aylor ()//% or to the adsorp# inertodetrinite particles, can also be transported some tion of ‘bitumen’, possibly lipoid substances derived in part distance from their place of formation, accumulating in from liptinite, into the maceral’s structure $eichmuller quiet, sub#aquatic conditions with other particles such as ()=5b &packman et al ()=/%. &ome desmocollinite also spores or algae and possibly fine mineral matter. &clerotioids $secretion sclerotinite or carboni*ed resin exhibits fluorescence characteristics, probably also reflecting the presence of ‘bitumen’ impregnations. ' bodies% are also thought to result from oxidi*ing conditions similar explanation would account for the unusual in the peat swarnp, although such materials may also be fluorescence, reflectance and chemical characteristics of transported from an oxidi*ing environment nearby. rue some ‘!et’, vitrain lenses derived from coniferous wood and sclerotinite represents fungal material and is also formed in found in marine sediments with a high lipid content relatively dry oxygenated peat layers $&tach et al ()=7%. X @icrinite may possibly be formed in two ways. t may $raverse N :olvoord ()/0 Eiven et al ()=7 &packman represent a by#product of the coalification of liptinite et al ()=/ 2avis ()=0b%. 'nother hypothesis for the higher reflectance of macerals, including bituminite $eichmiiUer ()=5b% and telocollinite $or vitrinite ', ‘pseudo#vitrinite’, band exudatinite $&hibaoka ()=0%. he suggestion is supported vitrinite% is that the plant material has undergone oxidation, by its frequent association with spo# rinite. 'lthough probably during the early stages of its development eichmuller indicates that micrinite is not normally found $9enedict et al ()/0%. :och $()=?%, however, suggests that in peats or brown coals, others $e.g. &packman N it is due to the nature of the original plant material 9arghoorn ()//% have reported it in such materials. involved, noting that, in the Vurassic coals of 'fghanistan, ogether with the fact that it frequently occurs as cell this maceral type had the characteristics of coniferous
fillings in higher rank coal, this suggests that some algal colonies to flourish $@oore ()/0%. 'fter death, the micrinite may also be formed by the breakdown of cell remains of these colonies accumulated in euxinic bottom wall substances. oo*es, eventually to become masses of alginite or he origin of macrinite, a relatively rare component of bituminite. most coals, is not well understood. 't least some of it may, however, result from the oxidation $or fusini*ation% of 3.@. Te)poral and cli)atic in+l(ences on organic gels formed during peat accumulation. peat acc()(lation 9ecause they are made up mainly of fine inertinite debris and have a relative abundance of spore material, the he coals of different geologic periods were derived black durains of many humic coals are classically regarded from different plant assemblages, and under differing as sub#aquatic oo*e deposits, forming in deeper water climatic conditions. hese variations have clearly environments than the bright, vitrain# rich lithotypes produced some diversity in the petrographic character of $4hite ()11 asch ()/?%. Gacquebard et al $()/=% have coals of different ages. generally indicated support for these concepts, interpreting he plants of the Carboniferous, including such groups the high telinite $bright coal% contents of some Canadian as the seed ferns, lycopods and sphenopsids, were seams as representing forest moor environments, typical of commonly prolific producers of thick#walled megaspores the central part of a flood plain. he duller intervals of and microspores. 'fter the Carboniferous, however, the those seams, with relatively high liptinite and mineral importance of these plants declined, with many becoming contents, are thought to be the product of an open moor extinct, and the gymnosperms . $which include the seed environment with a generally higher water table. 's an ferns% became the dominant members of the plant alternative, however, &mith $()/0% has suggested that black community. hese plants produced smaller, thinner#walled durains, rich in macrinite and thick#walled crassispores, palynomorphs than did the Carboniferous plants, and were formed in a raised, well#drained peat swamp consequently contributed less sporopollenin to the resulting environment. enuidurains, in which thin# walled spore peats than the larger, thicker#walled spores common in the types are typically associated with semifiisinite and a high Carboniferous. @ore importantly the number of proportion of mineral matter, are interpreted by &tach et al megaspores produced was reduced to four, with three of $()=7% as representing a hori*on of peat oxidation above these aborting and then being retained in the the water table. &mith $()/0% on the other hand interprets megasporangium without developing an exine of liptinitic them as representing a sub#aquatic ‘incursion phase’ due to source material. flooding of the peat swamp. he angiosperms have been important members of he presence of a large amount of mineral matter swamp and marsh communities from the ertiary to the in a coal may also reflect the development of deep#water present. 'll of these plants retained the megaspore in the conditions. Gowever, some workers suggest that such ovule with no production of sporopollenin, and with a concentrations could also result from sub#aerial ablation reduction in total pollen production compared to many of peat, or from excessive peat decay in swamp waters of gymnosperms. here has therefore been a progressive pG greater than 5.7 $Cecil et al ()=)%. decrease in the contribution of spores and pollen to the coal#forming swamps from the Carboniferous to the ertiary, and a change from large, thick#walled [email protected] Origin o+ liptinite )acerals and palynomorphs to small, thin#walled palynomorphs in the sapropelic coals liptinites of the coals produced through time $4addell et al he organic components that give rise to the various ()=0%. liptinite macerals are described in some detail in &ection Eondwana coals, formed in the southern hemisphere 1.1.1. Unlike the ligno#cellulosic materials that make up mainly during the 8ermian 8eriod, were derived from the vitrinite, the liptinite precursors are largely resistant to well#known Hlossopteris flora. his assemblage was degradation and to the gPlification or similar processes dominated by gymnospermous plants and more restricted generally associated with peat formation. hey do, in species than the flora which inhabited the Carboniferous however, exhibit a number of significant changes during forests. he climate for these deposits is believed to have coalification or rank advance, and these are discussed been mainly cool temperate, in contrast to the sub#tropical more fully in &ection 1./.5. conditions that probably existed during much of the Cannel coals are believed to be derived from the more >uropean Carboniferous. resistant components of decayed plants, especially spore &ome of the characteristics of the Eondwana coals remains, that were washed into ponds, lakes and lagoons include a much lower sporinite content than is found in the to accumulate as fine grained, anaerobic bottom muds. Carboniferous deposits, and a higher inertinite content, 9oghead coals or torbanites, on the other hand, are made up mainly of semifusinite that formed under thought to represent circumstances in which clear, oxidi*ing conditions. Cook $()=7% has suggested that a dry aerated surface waters, free of humic matter, allowed climate may have been responsible for the abundance of
inertinite#rich coals in the late 8ermian coal#bearing sequences of 'ustralia. 's stated in &ection 1.1.1, thick cuticles may be indicative of a dry, warm climate. Cutinite is rare in 'ustralian 8ermian coals, but is quite common in the riassic pswich Coal @easures and Vurassic 4alloon Coal @easures of ueensland, especially in the Vurassic Dosewood and 2arling 2owns deposits. Desinite is found in larger quantities in Cretaceous and ertiary coals than in those of the Carboniferous because of contributions from the resins of conifers in the later#formed materials. &uberinite is a component of @eso*oic and, particularly, ertiary coals. rue fungal sclerotinite is also a ubiquitous component of ertiary coals, but is only rare in Carbo# niferous deposits. 3.A ""#ICTION! O* CO# "ETRO#OG$
Coal petrology represents one of the main avenues for gaining an understanding of the origin of coal deposits, and also provides invaluable information in establishing the most appropriate means of using the various coals formed by these processes. 8etrologic data, collected on either a megascopic or a microscopic scale, can be used to help solve problems with coal seam correlation, and may be considered in con!unction with other geologic factors to assist in interpreting the location, extent and quality of economic coal resources. &ystematic data on coal rank, compiled from features such as vitrinite reflectance, can also be used to establish the tectonic history of the region, and, as a by#product, assess the likelihood that petroleum hydrocarbons might have been generated at some stage of basin development as well.
Coal Petrology and Petrographic Analysis
7 3.A.1 !ea) correlation and other (ses o+ petrographic pro+iles
Correlations may be drawn from petrologic data wherever a coal seam laterally maintains a unique characteristic, such as a particular vertical sequence of lithotype variation. Gacquebard $()76% has described how the distinct and uniform petrographic composition and seam profile of the racy &eam of the &ydney coalfield, Aova &cotia could be used for correlation over distances of at least (6 km, while &myth $()/=% has suggested some correlations between seams of the Aewcastle and llawarra Coal @easures of Aew &outh 4ales on the basis of their petrographic profiles. 2avis $()/0% has also noted that two vertically ad!acent seams at @oura, ueensland, had such distinct profiles $"ig. 1.6?% that, even from megascopic data, it was possible to identify which of the two seams was present in a situation where the stratigraphic sequence had been complicated by faulting. n some instances, although the profile itself may vary, the presence of persistent hori*ons within the seam can be a considerable aid in correlation. Gacquebard et al $()/7% and Cameron $()=(% were able to trace individual durain layers in the Garbour seam of Aova &cotia across the width of the &ydney coalfield, a distance of over 16 km, even though the durains themselves changed laterally. ' fusain layer in the Ao. (6 seam of 4estern :entucky also extends over an area greater than (1? km 6 $'ustin N 2avis ()=)%. norganic bands and partings within seams are commonly useful marker hori*ons as well. he prominent ‘blue band’, a claystone parting of the Gerrin $Ao. /% seam of the llinois 9asin, has been traced over thousands of square kilometres in llinois $4illman et al ()=7% and western :entucky, while kaolinite clayrocks or tonsteins $&ection 7.1.(% have been employed as markers both within and between individual coalfields. &cheere $()7/% has demonstrated the correlation of such hori*ons from "rance, through
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3LLLL3LLLLLLLi cm
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.
.
V4rtical scal4
"ig. 1.6? 8etrographic profiles of two coal seams from @oura, 9owen 9asin, ueensland. $"rom 2avis ()/0.% 3 -itrinite & semi#inertinite J inertinite exinite K mineral.
9elgium and Golland and into Eermany, covering a 's indicated in &ection 1./.1, the principal use of distance of about 5?? km, and in ueensland, 'ustralia, reflectance data is as a measure of coal rank. t has long such hori*ons have been traced in individual seams been established that rank increases with depthin $2avis ()=6, ()=1 Carr N 2avis ()=1 9eeston ()=5%. stratigraphic successions or borehole profiles $a &ix carbonaceous shale partings also occur over an area phenomonen described traditionally by ‘Gilts aw’%, and greater than (7?? km 6 within the Ao. (6 seam of western this is thought to reflect the effects of rising temperature as the thickness of overburden increases. :entucky referred to above $'ustin N 2avis ()=)%. he vertical succession of lithotypes in seams has been used to study the variation in environmental $a% Deflectance profiles conditions during coal formation. "or example, Gacquebard et al $()/=% noted the lack of petrographic Deflectance profiles in boreholes have been used to study variability in coals of limnic sequences in contrast to the the tectonic histories of coal#bearing sequences. 2ow greater variability of those in paralic coal measures. he $()==% has shown the effects of such processes as loss of predominant seam profile in 'ustralian coals is one in cover, the development of unconformities, igneous which the abundance of vitrinite decreases from floor to intrusion and faulting on the profiles of organic roof, and this is interpreted by &myth $()=?% as maturation in sedimentary sequences. he temperature representing a rise in the depositional surface as a result and time required for coalification have been related to of the development of progressively drier conditions in the rank or level of metamorphism by authors such as the peat swamp. :arweil $()7/%, opatin and 9ostick $()=1%, Good et al $()=7% and 9ostick et al $()=)%. "igure 1.6( is a nomograph, modified after Good et al $()=7%, that 3.A.2 pplications o+ re+lectance )eas(re)ents enables palaeotemperatures to be derived from reflectances, provided the effective heating time can be estimated. he effective heating time is the time during which the sediments were exposed to temperatures within (7 C of the maximum attained. n con!unction with the depth of the seams in question, such data can be used to calculate palaeogeothermal gradients and hence allow the
"ig. 1.6( Delation of vitrinite reflectance to maximum temperature $ max% and effective heating time. $@odified from Good et al ()=7.%
thermal and tectonic histories of various coal#bearing of coals, mainly of 'llegheny age, in western sedimentary basins to be progressively reconstructed. 8ennsylvania, while &enftle and 2avis $()06% have Dank patterns and changes in geothermal gradient have mapped the reflectance of the ower :ittanning seam in been related to depth of burial, depth of basement $Cook the same area. "acer et al $()0?% have produced a third N :anstler ()0?%, basement lithology $:oppe N degree trend surface map of the lateral variation in 'nderson ()=5 'nderson N :oppe ()=/% and the reflectance for seams in the llawarra Coal @easures of thermal properties of the rocks in question $eichmuller Aew &outh 4ales. soreflectance maps can be used in the N eichmuller ()/0 "acer et al ()0?%. hey have also planning of mining operations to optimi*e the supply of been related to thermal events such as the initiation of coal blend components for metallurgical coke production crustal rifting $Cook N :anstler ()0?% and to differences $9enedict N hompson ()=0%. in heat flow between ad!acent crustal blocks $Gower N $c% nterpretation of stress patterns 2avis ()0(b%. n another type of study, the orientation of the principal reflectance directions in coal specimens has been used to $b% soreflectance maps interpret the directions of tectonic stress in several ateral variations in the rank of a particular seam or coalfield areas. Gower and 2avis $()0(a% and evine and reference hori*on across an area may be expressed by 2avis $()05%, for example, found that the maximum contoured ‘isoreflectance maps’ $&ection /.=.6%. retlectance of tightly folded biaxial coals of low volatile 9artenstein et al $()=(%, for example, have mapped bituminous and anthracitic rank in 8ennsylvania was changes in rank due to thermal metamorphism of oriented sub#parallel to the fold axes, while &tone and Carboniferous and Cretaceous sediments across the Cook $()=)% observed that the biaxial reflectance @iddle Cretaceous pluton of the 9ramsche @assif in anisotropy of samples from the proximity of normal north#western Eermany, and 9eeston $()==% has mapped faults in the &outhern Coalfield of Aew &outh 4ales vitrinite reflectance trends along the =7 km outcrop belt could be related to probable local stress patterns of the Eerman Creek seam in ueensland, 'ustralia. n associated with the faulting. &tudies of this type can be the latter case the rank of the coal varies from sub# applied to a number of problems with the stability of bituminous to semi#anthracite, and it was possible, by underground mining operations, such as those discussed consideration of the palaeogeothermal gradient, to in &ection =./. calculate the thickness of overburden that had been 'lthough reflectance anisotropy generally increases removed from the area of highest rank material. with increasing rank $&ection 1./.6%, values of Gower and 2avis $()0(a% have published a quadratic bireflectance obtained from vitrinite in the 9owen 9asin trend surface map showing the reflectance of ueensland show fairly uniform values
throughout much of the rank range in that area $2avis n the ()/?s, the work of 'mmosov et al $()7=% and ()=(%. Gower and 2avis $()0(b% found that another &chapiro et al $()/(% gave a new stimulus to thepractice index of anisotropy, of coal petrology through the development of procedures whereby the quantitative strength characteristics of R R metallurgical cokes from single or blended coals could be )a6 rand predicted. &uch procedures are described more fully in remained more or less constant along traverses from west &ection 5.1./. 'lthough these empirical methods have to east in western 8ennsylvania, whereas vitrinite generally been successful, some of the concepts reflectance generally showed marked increases. hey incorporated in the original mathematical models are not interpreted this to indicate that the past depth of burial accepted universally, and some care should be taken in their use. was also uniform across the area. 2epending on the rank of the coal, the models are based on the assumption that vitrinite, liptinite and $d% Eeneration of petroleum hydrocarbons possibly some of the semifusinite react in the process of n ()(7, 4hite observed that the presence, amount and coke formation and bind the other inertinite components type of petroleum in sedimentary rocks could be related and mineral matter into a porous fused solid. he model to the rank of any associated coals in the sequence. Dank devised by &chapiro et al $()/(% predicts coke strength at that time was defined solely by chemical parameters, on the basis of the relative abundance of the reactive and and the resulting theory has therefore become known as non#reactive constituents, and also on the abundance of the ‘carbon ratio theory<. t has been investigated in vitrinite in reflectance classes that correspond to the many parts of the world, and several of the findings of vitrinoid types $-#types% discussed in &ection 1.=./. these studies have been compiled by 9ostick and Gowever, the model makes an arbitrary subdivision of the 2amberger $()=(%. semifusinite, regarding one#third of that material as 9riefly, the theory suggests that a certain level of reactive and two#thirds as non#reactive, a procedure that maturation $i.e. rank% is necessary for organic matter in is not in accord with observations of aylor et al $()/=%, the source rocks to generate hydrocarbon liquids. 's the which suggest that this maceral does not become plastic rank increases, however, the liquids become lighter and during coking. t is also not always desirable to treat ultimately only natural gas is produced under the liptinite in the coal _ _ _ _ _ _ _ ^ 1_ _ . . prevailing temperature conditions. "igure 1.66 gives a as a reactive summary of correlations between coal rank $vitrinite maceral in the same sense as the viimiic, since me upimie reflectance% and the *ones of petroleum generation, and is mosiiy uevoiaim*eu in the carboni*ation process and also shows the ‘death line 7, where the rank becomes too contributes little to the final product. he breakdown of high for liquid hydrocarbons to form. vitrinite reflectance data into -#types, although it 's well as the rank, however, the composition of the represents a practical way of dealing with the coking organic matter $kerogen% in the sediments is also properties of the vitri# nites of different rank from the important in determining the type and amount of various coals in a blend, also makes arbitrary and petroleum that might have been generated. :erogen unproved distinctions between and within the vitrinite derived from algae and marine organic matter will types that might be present in a single coal. generate oil and gas in relatively large quantities, whereas ;ther methods of coke strength prediction include humic kerogen, mostly derived from terrestrial plants, is those of 9enedict et al $()/0%, which allows for the thought by some authors to generate only minor amounts poorer caking characteristics of ‘pseudovitrinite 7 of gases $2ow ()== issot N 4elte ()=0%. n addition, compared to ‘reactive7 vitrinite, and @ackowsky and the source rocks must contain sufficient kerogen $at least &imonis $()/)%, which incorporates parameters of coking about ?.7F organic matter% to provide for the possible conditions into the prediction. Eray et al $()=)% have accumulation of petroleum in the first place. described how results using the method of &chapiro et al $()/(% may be modified to allow for the coking conditions. 3.A.3 The petrographic prediction o+ coe 8etrographic methods also have been established to properties predict the pressures and expansion or contraction developed during carboni*ation, and the reactivities of cokes $9enedict N hompson ()=/ hompson N 9enedict ()=/ Eray Eoscinsky N &hoenberger ()=)%. 4
R !
J
3.A. The )icroscopic detection o+ coal o6idation
he oxidation of coals, caused by weathering in
"ig. 1.66
Correlation of coal rank and vitrinite reflectance with the *ones of petroleum generation and destruction. he relative importance of the *one of petroleum generation depends upon the composition of the original kerogen. $@odified from
outcrop or stockpile, can result in a deterioration of the coking properties of bituminous coals, and in detrimental effects on the specific energy and preparation characteristics of coals $&ection 5./%. Consequently, the detection of oxidation is an important aspect of the evaluation of coals for industrial usage. here are certain microscopic features of weathered coals which are easily recogni*ed. hese are the discolouration of particles $especially around the peripheries%, the presence of microfractures and unusual relief within the weathered macerals
$9enedict N 9erry ()/5 Crelling et al ()=)%. "igure 1.7f is a photomicrograph of weathered coal which displays these features. Eray et al $()=/% have described a technique for staining coals with an alkaline solution of the red stain &afranin ?. ;xidi*ed particles appear green under the microscope. he test, as described, is not reliable for coals higher in rank than high volatile bituminous however, a modification which overcomes this limitation has recently been developed $Eray, personal communication%.
