Introduction to Optical Mineralogy

Introduction to Optical Mineralogy

Gabi (Gelu) Costin

- 2011 -

GLC 201 - Introduction to Optical Mineralogy

Contents INTRODUCTION _________________________________________________________________ 4 Recommended textbooks, websites; pracs, tests & exam info ______________________________ 5 Objectives of the course_____________________________________________________________ 5 1. WHAT IS LIGHT? ______________________________________________________________ 6 1.1. Light as a wave ........................................................................................................................ 6 1.2. Light as particle ....................................................................................................................... 7 1.3. Polarized light .......................................................................................................................... 7 2. ISOTROPIC AND ANISOTROPIC MATERIALS ____________________________________ 8 3. INTERACTION BETWEEN LIGHT AND MINERAL ________________________________ 9 3.1. Reflected light .......................................................................................................................... 9 3.2. Absorbed light ....................................................................................................................... 10 3.3. Refracted light ....................................................................................................................... 11 3.3.1. Refractive index ............................................................................................................................. 11 3.3.2. Important things to know about the refraction taking place in minerals ............................................ 12

3.4. Transmitted light ................................................................................................................... 14 3.4.1. Thin section for optical studies in transmitted light.......................................................................... 14

4. VECTORIAL AND CONTINUOUS CHARACTER OF REFRACTION ________________ 15 4.1. Indicatrix ............................................................................................................................... 15 4.2. Interference colours (IF); birefringence (δ) .......................................................................... 17 5. PETROGRAPHIC MICROSCOPE _______________________________________________ 21 6. MINERAL IDENTIFICATION USING THE PETROGRAPHIC MICROSCOPE ________ 23 6.1. Orthoscopic study .................................................................................................................. 23 6.1.1. Observations using plane polarized light (PPL) mode ______________________________ 23 a) Transparency ........................................................................................................................... 23 b) Shape, habit, size ...................................................................................................................... 23 c) Cleavage ................................................................................................................................... 25 d) Colour (absorption colour) ...................................................................................................... 28 e) Pleochroism .............................................................................................................................. 28 f) Relief ......................................................................................................................................... 28 Becke line; Becke method for estimating the relief .................................................................................... 29 Twinkling (relief changing) ...................................................................................................................... 30 Chagrin (roughness in appearance of the mineral surfaces)...................................................................... 30

g) Inclusions, alterations .............................................................................................................. 31 6.1.2. Observations using crossed polarized light (XPL) mode ____________________________ 32 a) Isotropy/anisotropy .................................................................................................................. 32 b) Extinction angle ....................................................................................................................... 32 Determination of the extinction angle ....................................................................................................... 32

c) Birefringence ............................................................................................................................ 34 Colour of interference (colours of birefringence) ...................................................................................... 34 Finding the value of birefringence (δ)....................................................................................................... 35

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GLC 201 - Introduction to Optical Mineralogy d) Twinning/zoning ...................................................................................................................... 36 Twinning ................................................................................................................................................. 36 Zoning (compositional zoning) ................................................................................................................. 37

e) Orientation of nγ and nα ........................................................................................................... 38 f) Optical elongation ..................................................................................................................... 40 6.2. Conoscopic mode ................................................................................................................... 40 6.2.1. Interference Figures........................................................................................................................ 41 Interference figure for uniaxial crystals .................................................................................................... 41 Interference figure for biaxial crystals ...................................................................................................... 41

Determination of the optic sign .................................................................................................... 42 Estimation of the 2V angle ........................................................................................................... 44 Useful charts for mineral identification: the Tröger Chart _______________________________ 46 27 Key minerals species ____________________________________________________________ 47 Key Characteristics of common minerals: Speeding up mineral identification_______________ 48 A few hints for the relation chemical composition - optical properties _____________________ 48 Tips for discriminate between different mineral groups _________________________________ 49 Mineral association: helpful in identifying minerals ____________________________________ 49 Mineral Identification – A Beginner’s Guide __________________________________________ 50 Identification Tables for Common Minerals in Thin Section _____________________________ 53 Tables for Common Minerals in Thin Section _________________________________________ 54

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INTRODUCTION Why study minerals/crystal optics? 1) They assist in the identification of minerals – study their optical properties under the microscope. • Minerals are inorganic chemical compounds having a certain lattice shape, size and symmetry, being a result of the geometrical arrangement of the constituents (chemical elements such as Si, Al, O, etc). • Lattice (symmetry) + chemistry (nature of the chemical elements of the lattice) combine to make a unique mineral phase. The lattice (internal symmetry) of the mineral is reflected not only in the symmetry of the external crystal shape but also in the symmetry of optical properties of the mineral; therefore, determining the optical properties of an unknown phase assists in identifying the mineral phase; • Mineral identification is needed in petrological studies, structural geology, mineral exploration etc… 2) Microscopic study is the cheapest and fastest method for identifying minerals; however, there are limitations to the optical method, such as constraints of very small size (submicroscopic) of minerals, or complex solid solutions, etc. 3) Microscopic study is required for textural (natural arrangements of minerals) analysis; it is useful in determining the rock type, the crystallization sequence, deformation history or observing frozen-in reactions, constraining pressure-temperature history, noting weathering/alteration, etc. 4) Because the principles of light refraction and reflection are also relevant to seismicity (geophysics and geological exploration), water behaviour (groundwater management), and even to real life!

Remember that minerals have an ordered internal lattice (with an internal symmetry) which is also reflected in the external shape of the crystals. Therefore, it is expected that the optical properties of minerals somehow demonstrate this internal symmetry. In order to “see” the symmetry of the optical properties, and to determine the symmetry of a mineral, we need to understand: a) What light is, and especially polarized light; b) The difference between isotropic and anisotropic media (optical and other properties of minerals can be isotropic and anisotropic); c) The concept of vectorial and continuous properties; d) The tool of studying the optical properties of minerals (the petrographic microscope); e) The use of specific charts of physical properties in order to identify unknown minerals; f) A few specific optical properties which can help in quick identification of the common rockforming minerals. This handout represents a compilation realized by Dr. Gelu Costin from different resources: previous versions of power-point presentations and notes: Dr. Steffen Bütner, Dr. Stephen Prevec. Dr. Emese Bordy, Prof. Goonie Marsh internet resources several text explanations and some figures were added by Dr. Gelu Costin

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Recommended textbooks, websites; pracs, tests & exam info A) Recommended TEXT BOOKS and WEBSITES 1) 2) 3)

Perkins, D. & Henke, K.R. (2004): Minerals in Thin Section. Prentice Hall. Deer, Howie & Zussman (1992): Introduction to rock forming minerals Heinrich (1965): Microscopic identification of minerals

On short loan: Bloss, F. D.: Optical crystallography 548.9 BLO Shelley, D.: Optical mineralogy 549.125 SHE Others: Gribble, C.D. & Hall, A.J.: Optical mineralogy: principles and practice Battey, M.H. & Pring, A.: Mineralogy for students B) Lectures & Pracs * All material presented in the lectures is relevant for the pracs. Polarisation Microscopy is a method used in: 1. 201 Mineralogy/Geochemistry 2. 201 Introductory Igneous Petrology 3. 202 Sedimentology 4. 202 Igneous Petrology 5. 301 Structural Geology 6. 301 Metamorphic Petrology 7. 302 Economic Geology 8. Almost all modules on Honours level 9. More or less all studies on Masters/PhD level and beyond 1. Optical properties of some common mineral species on the Web: http://www.brocku.ca/earthsciences/people/gfinn/minerals/database.htm http://funnel.sfsu.edu/courses/geol426/Handouts/mintable.pdf http://www.geolab.unc.edu/Petunia/IgMetAtlas/mainmenu.html http://sorrel.humboldt.edu/~jdl1/minerals.list.html http://geology.about.com/od/thinsections/Thin_Sections.htm 2. More or less everything about minerals: http://webmineral.com/determin.shtml 3. More thin section photos + optical properties http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/ 4. First aid for conoscopy problems http://users.skynet.be/jm-derochette/conoscopy.htm C) Tests & Exams  No formal 45 min theory test  Instead: daily quickies (5 minute tests)  Thin section microscopy work can be expected as main part of the GLG 201/202 Prac Exam

Objectives of the course understanding the behaviour of minerals under transmitted polarized light understanding and practicing the determination of optical properties of crystalline solids identification of unknown minerals using optical property determinations and catalogues of physical properties rapid identification of common minerals in thin section

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1. WHAT IS LIGHT? Light may be seen as electromagnetic waves and/or as particles (quantum theory). 1.1. Light as a wave A wave* (Fig. 1) can be characterized by four parameters**: wavelength, frequency, velocity and intensity. * any kind of wave (e.g. optical, mechanical, thermal, acoustic, seismic etc) can be characterized by these above-mentioned parameters **a parameter is a physical property which can be measured a) wavelength (λ - lambda): distance between two neighbouring points experiencing vibrations of the same amount and in the same direction. Such points are said to be in phase. The wavelength is important in optical mineralogy, since it is this that affects our perception of colour. (coherent light = in phase, incoherent = not in phase).

Figure1. Graphical representation of light. λ = wavelength. a= amplitude (related to ε = intensity or energy of the wave).

Visible (white) or polychromatic light (Fig. 2) with wavelengths between 390 and 780nm (nano meter = 10-9 m = 1 billionth of a meter) is a small part of the electromagnetic spectrum which includes gamma- and X-rays, ultraviolet as well as infrared light, radio- and micro-waves. Sunlight contains the entire visible spectrum plus ultraviolet light and infrared light as well. Visible light includes 7 monochromatic lights which correspond to the 7 primary colours of the rainbow (as recognised by Sir Isaac Newton): violet, indigo, blue, green, yellow, orange, red.

Figure 2. Colours of the visible spectrum with their corresponding wavelength (in black and white).

The wavelength range of the colors from the visible spectrum are: Violet: 390 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm 6

GLC 201 - Introduction to Optical Mineralogy b) Frequency (η - nu): number of wavelengths passing a fixed point in 1 second; “pulse rate” c) Velocity (c) is related to frequency (η) and wavelength (λ) by: c = ηλ The velocity of light in a vacuum is higher than in any other substance (2.99773 x 108 m/s); (Slowing down waves = shortening their wavelength) d) Intensity (ε = the amplitude of the wave). The amplitude of the wave is related to the energy (the “higher” wave has more energy). The wave energy of light is given by the moving photons and therefore, the amplitude (intensity) of the wave makes the connection between wave and particle nature of light.

1.2. Light as particle Light is interacting with the electric fields produced by the nuclei and electrons of atoms  it will slow down light passing through them  the more atoms and/or e- that are in a given volume  the more the light rays will decelerate. Density of atoms in the mineral lattice and number of e- per atom in the material are important (note that the number of e- per atom is directly dependent on the atomic number of the element -see the Periodic Table of the Elements). As the atomic number is higher, the mass of the element is higher, and consequently the mass of the compound made by the heavy elements will be higher. Since density = mass/volume, this also reduces to considering density as the main factor in slowing down the light speed within materials. 1.3. Polarized light Natural light vibrates (oscillates) in all the directions perpendicular to the direction of propagation (fig. 3). Therefore we can say that there is infinity of planes of vibrations (all possible planes that intersects/contain the direction of propagation. Direction of propagation

Figure 3: Propagation and vibration of natural light; note vibration in all directions perpendicular to the direction of propagation (all vibration directions are perpendicular on the propagation line).

Plane polarized light (PPL) has one single plane of vibration, in which the direction of vibration is always perpendicular to the direction of propagation (fig. 4). We can use this plane of vibration as a geometrical reference for the optical properties of mineral. Keeping this plane fixed and rotating (changing the orientation of) the mineral, all of the mineral‟s optical properties can be measured or related to such a plane. Note that we can polarize light with a special designed material, called a nicol or polarizer. The name “nicol” comes from Nicol (Nicol‟ prism), a French scientist who first built a kind of prism of calcite, made of two halves of the same calcite crystal, adjusting the angles of the prism to a convenient value in order to eliminate all other planes of vibration but one. More commonly, these days, materials called polaroids are used for manufacturing polarisers (microscopic oriented crystals of iodoquinine sulphate embedded in a nytrocelulose polymer film). Note that the polariser does not absorb light (or the absorption is negligible), so it does not affect the observed colour of the mineral (fig. 5a). See the difference between a polariser and a colour filter (fig. 5b).

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Direction of vibration

Plane of vibration

Direction of propagation

Figure 4: Polarized light (plan polarized light -PPL)

a)

Figure 5: a) Polariser: the light exiting from the polariser has one single plane of vibration; The intensity of the light (amplitude of wave) is not affected; b) colour filter: the intensity of polarised light entering the filter is attenuated (some energy of the light was absorbed and the out light will be coloured but still polarised). The amplitude of the wave will therefore decrease.

In order to relate the optical properties of a mineral to a particular symmetry, we need to find an external optical-geometrical element (such as a reference plane – e.g. plane of polarization of the incident light) and to relate to it all the optical properties that we want to consider for a mineral. 2. ISOTROPIC AND ANISOTROPIC MATERIALS Isotropic (in a general sense) means that any physical property of the material is the same at any point and in any direction through the material (it is independent of orientation). Concerning mineral optics, the word “isotropic” refers to the optical properties of the mineral, which are the same and independent of the orientation (e.g. isotropic minerals). However, if a mineral is isotropic, it means that ALL of its physical properties are the same at any point. Minerals that are isotropic are the minerals with cubic symmetry (remember the symmetry of minerals crystallized in the cubic system have a=b=c and α=β=γ=90°), and materials that do not have a geometrical arrangement of the atoms, so they do not have an internal lattice (e.g. non-crystalline materials), such as glass, liquids, and gasses. Accordingly, an isotropic mineral has the same refractive index, the same absorption of light (and the same for any other physical property) at any point and for any direction in the mineral. Anisotropic (in a general sense) means that the properties of the material are not the same at all points or directions, but may vary continuously with changing direction (orientation) of observation (all minerals other than cubic are anisotropic). Examples of anisotropic behaviour when changing orientation include different absorption of light, different refractive indexes, etc. Anisotropic crystals have variable refractive indices because light travelling through the crystals will do so at different speeds, depending on the direction of travel (the orientation of the crystal to the incident light).

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GLC 201 - Introduction to Optical Mineralogy All minerals, other than those belonging to the isometric system, are anisotropic. But some of them are “more anisotropic” than others, and the isotropy-anisotropy is related to the symmetry of crystals. For example, all minerals can be grouped based on their symmetry according to 7 systems of symmetry, and beyond that, we can subgroup the symmetry according to the presence or absence of high order fold axes (A3, A4, A6): -minerals with superior symmetry (cubic or isometric system: a=b=c and α=β=γ=90°); several high order fold axes are present: 3 A4 or 3Ai4 and 4 A3. -minerals with medium symmetry (trigonal, tetragonal and hexagonal systems); all of them have one main axis of symmetry, only: A3, A4 or A6, respectively. -minerals having inferior symmetry (orthorhombic, monoclinic and triclinic); no high order axis is present (no fold axis superior to A2); among these, the symmetry decreases as the number of A2 axes decreases: orthorhombic: maximum 3 A2; monoclinic: maximum 1 A2; triclinic has the lowest symmetry, with no A2 axis.

3. INTERACTION BETWEEN LIGHT AND MINERAL As light intersects an isotropic material (let‟s say glass or an isotropic mineral, such as garnet), the light suffers several optical phenomena, and is decomposed into several components. The intensity (or the energy) of the incident light splits up accordingly (Fig. 6): a) Some fraction of the incident light is reflected by the surface of the mineral. The intensity of the reflected light is (εrl) b) Another component of light entering the mineral is refracted (εr): this refracted light is plane polarized!! c) a variable component of the light that enters the mineral is absorbed (εa) d) The remaining light (intensity), if any, succeeds in escaping from/through the mineral grain. This light is called transmitted light (εt); the transmitted light is also polarized by the mineral (the mineral acts like a complex polarizer). Thinking in terms of energies (or intensities), the budget of the initial incident light is:

εi = εrl + εr + εa + εt 3.1. Reflected light The reflection depends on the surface properties of the mineral but also on its nature (some minerals reflects more light than others). The strongly reflective minerals are those which reflect all (or almost all) of the incident light and no other light component is able to cross through and exit the mineral (no transmitted light). This means that the mineral is opaque to light. We can define reflectivity (or reflectance) as the fraction of incident light (in terms of energy or intensity) which is reflected from a surface. Reflectivity is therefore proportional to the intensity of the light reflected by the mineral. The reflectivity index (R) is the ratio between reflected light intensity versus incident light intensity (R= εrl/εi), a ratio which is lower than 1. However,typically R is expressed in percentages; R= εrl/εi x 100 %. In order to study opaque minerals we need to analyze the light reflected by the mineral (we need therefore to polish one surface of the mineral as well as possible in order to get the best reflectivity). The opaque minerals are studied with the chalcographic (reflected

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GLC 201 - Introduction to Optical Mineralogy light) microscopes (you will learn to use chalcographic microscopes another time, not within this term). Common experience (such as mirror imaging) tells us that the angle of incidence is equal to the angle of reflection. However, at a certain incident angle, the incident ray is refracted at 90 ; this is termed total reflection. The incident angle at which total reflection occurs is called the critical angle (θi-cr).Total reflection is used to determine the refractive index of an unknown material:

θr = 90, nair ~ 1 ni sinθi-cr = nair sinθr  sin θi-cr= 1 / ni Plane perpendicular on the mineral surface (and on the boundary) between air and mineral)

incident light (εi)

reflected light (εrl) used by CHALCOGRAPHIC MICROSCOPE AIR

θi

nair nm absorbed light (εa)

εo εe

θr AIR

Isotropic MINERAL (or gass) refracted light (εr=εo+εe)

θi transmitted light (εt) used by PETROGRAPHIC MICROSCOPE

Figure 6: Light intensities splitting out at the interface of light with the mineral. Notice the difference between the incidence angle (θi) and the refraction angle (θr). When exiting the mineral, the (transmitted) light will resume propagation at the original θi angle to the surface.

3.2. Absorbed light One fraction of the light that enters the mineral is absorbed. This absorption is responsible for the colours of materials that we see around us. How does it work? Inspired by the colours of the rainbow, Newton decomposed the natural light into its components using an optical prism. Looking at figure 2, we see that several colours can be distinguished in the visible spectrum (wavelengths between ~400 nm (violet) to ~800 nm (red). All of them are the components of the yellow light. If all the coloured lights from the visible spectrum are combined, we get a wave with an approximate average value of wavelength λ~(400+800)/2~600 nm (the real value is 575 nm). This is the wavelength of the yellow light (or natural light from the sun). It means that the yellow light contains a combination of waves that include all the wavelengths from the visible spectrum. 10

GLC 201 - Introduction to Optical Mineralogy When an incident yellow light (natural light from the window or the light emitted by a lamp) enters a material, some of the wavelength components can be absorbed by the material (the electromagnetic components of certain wavelengths of the incident light are consumed/combined into the electromagnetic field produced by the atoms and molecules of the material or we can understand this as the energy of the incident photons which is transferred to the electrons of the material, making them moving faster; the result of this absorption of energy is heat). The interaction of the light with the discrete nature of material is more complicated. For example, the transfer of energy from incident photons to the electrons of the material can produce not only increasing vibration of the molecules, but, if intensity of the incident photons is high enough, they can displace some electrons from their position (moving one e- from an orbital to another). This happens with X-ray emission (other photons vibrating with wavelengths in the X-ray spectrum (see fig. 2). The combination of the remaining wavelength components which were not absorbed gives the colour of the material that we observe. In other words, the colours that we observe around us are produced by selective absorption of light by different objects, and the selectivity of absorption depends on the composition of the material. If a material absorbs all the (visible) wavelengths in (proportionally) the same amount, the material will be colourless. If the material absorbs more from the lower visible spectrum (violet, blue), the colour of the material would be a combination of the remaining wavelengths from yellow to red (the observable colour would then be orange). If a material does not absorb any components of light at all, it would be… invisible. Well, this is not yet possible since the electromagnetic radiation will interact with the atoms and electrons of the material, so at least some absorption has to take place. The wavelengths of the reflected light also affect the appearance of colour. Note that the thickness of the medium can affect the eyes‟ interpretation of colour. Hence, many minerals which we are accustomed to seeing as coloured are colourless in thin section (for example, the various coloured varieties of quartz, such as amethyst).

3.3. Refracted light A component of the non-reflected light is refracted into the mineral. Refraction is a fundamental optical property of any medium which transmits light.

3.3.1. Refractive index Refractive Index (R.I. or n) is a measure of refraction. The refractive index (n) is the ratio between the velocity of light in vacuum (cv) and the velocity of light in the material (cm):

n = cv / cm

In optical mineralogy we can‟t actually measure the speed of light, but we can utilise this ratio of the speed of light in a mineral related to the speed in a vacuum. Since the speed of light in a vacuum, cv, is the maximum possible speed of light, the refractive index will be always greater than 1. Sometimes R.I. is defined as the ratio of “the velocity of light in air / the velocity of light in a medium” (i.e., any physical material other than air, as distinct from “a person who talks to ghosts”), as there is little difference for purposes of optical mineralogy (cvacuum almost = cair  nvacuum= 1; nair = 1.0003; nwater = 1.33). As we can see even from the above example, c depends largely on the density of the material. The higher the density is, the more difficult it is for light to travel within the material, so it gets slowed down. Since the cm is at the denominator in the definition of n, it means that n is higher when cm is lower (therefore, when the density of the material is higher). Accordingly, common sense tells as that nsolid > nliquid > nair.

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GLC 201 - Introduction to Optical Mineralogy The direct optical effect of observing refraction is that, looking at an object through a non-opaque material (liquid or solid), the margin of the object is observed as “displaced or moved” if you look at it from the side (i.e., away from the axis perpendicular to the material surface). The apparent “displacement” is higher when the angle is higher and when the refractive index of the material (or rather, the contrast in refractive indices) is higher. For example, if you see a fish in the river and want to touch it, be sure that you are exactly above him (and not laterally positioned) because otherwise what you see is not actually there where you see it, it is a “displaced” image of the fish produced by the difference in the refractive indices of air and water. The “displaced” imaged is due to the refraction angle which is always different from the incidence angle (see fig. 6). If you see a fish while looking through your petrographic microscope, it‟s probably time to take a rest. The angle of refraction (θr = angle of deviation from the incident direction) always depends on the refractive index (n). As nm gets higher, the angle of refraction will also get higher (as the light is “deflected” inside of the material). Therefore, given that n is related to cm, instead of measuring the velocity of light in the material (which is not an easy task), we can measure the angle of refraction and find the cm and n. Using Snell’s Law we have:

nair sinθi = nm sinθr After measuring θi and θr, then:

nm = nair x sinθi / sin θr The same is proceed for any two environments with different refractive indexes, ni and nr. If ni < nr, light is going to be deflected towards the plane normal (┴) to the boundary on entering the refracting medium. If ni > nr, light is going to be deflected away from the plane normal (┴) to the boundary. Note: if two materials in contact with one another have identical refractive indices, the optical boundary (meaning the sharpness of the boundary, and not, for example, a colour difference) between them is not observable. As the difference between the two refractive indexes gets greater, the boundary between the two materials is sharper and appears to get “thicker”.

3.3.2. Important things to know about the refraction taking place in minerals 1. The light which enters the mineral is refracted (slowed down) according to the density of the mineral (so also therefore according to the refractive index). 2. Light entering an isotropic media (glass or cubic minerals) produces a double refraction, such that the incident light is separated into two components, or rays. Both of the rays are polarized. One ray continues in the direction of incidence, and it is called the ordinary ray (εo); the other ray is refracted, and it is called the extraordinary ray (εe). These rays display a special characteristic: the polarization plane of the ordinary ray is always perpendicular to the polarization plane of the extraordinary ray (fig. 7)! This is due to the nature of any electromagnetic wave, which has a magnetic vector perpendicular to its electric vector. Since the refractive index is the same in any direction in an isotropic material, the two rays travel with the same speed and when they exit the mineral, there will be no delay between them. Therefore we can say that there is no retardation (Δ). The term “retardation” comes from the French word “retarder” meaning “to delay”). Because the retardation is zero, the isotropic materials are called “monorefringent” (because the refractive index corresponding to the extraordinary ray is identical to the refractive index corresponding to the ordinary ray; i.e., there is only one R.I. involved).

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Figure 7: Two plane polarized rays: the polarization planes are perpendicular to each other

3. The minerals with medium symmetry will also produce a double refraction, where the incident light splits into an ordinary ray and an extraordinary ray, as in the isotropic media. However, since the refractive index varies with orientation in anisotropic minerals, the extraordinary ray will also be slowed down in comparison to the ordinary ray (Fig. 8b). In this case, the retardation (Δ) is different from (greater than) zero. We call these minerals birefringent. The value of (Δ) should be directly related to the difference between the refractive indices along the direction of the ordinary ray (with the lowest refractive index, called nα) and that of the extraordinary ray (representing the highest refractive index direction, called nγ). So, the retardation is therefore proportional to (nγ-nα), which is known as the birefringence. The minerals with medium symmetry are called uniaxial, where the main (A3, A4 or A6) symmetry axis of the lattice (known as the “c axis”) is always in the direction of (i.e., parallel to) either nγ or nα. 4. The minerals of inferior symmetry produce one ordinary ray and two extraordinary rays (Fig. 8c), all of them polarized (the three polarization planes being perpendicular to each other). Each of these three rays corresponds to three different refractive indexes: the lowest one is nα and it corresponds to the direction of the ordinary ray, the intermediate refractive index nβ corresponds to the least delayed extraordinary ray, and nγ corresponds to the most delayed extraordinary ray. The minerals with inferior symmetry are called biaxial (see explanations for the indicatrix and the optic axis). For the orthorhombic minerals, the c, b and a axes are parallel to nγ, nβ and nα. For monoclinic crystals a maximum of two of the crystallographic axis can be parallel to two of the nγ, nβ or nα directions. For triclinic crystals, a maximum of one of their crystallographic axes can be parallel (or not) to any of the nγ, nβ or nα directions (remember that for triclinic crystals the angles between the crystallographic axis are α≠β≠γ≠90°, but nγ, nβ and nα are always mutually perpendicular). nα nγ= nα

nα

nα nγ

nβ nγ

O

E

a

O

E

b

O

E1 E2

c

Figure 8: Double refraction in minerals: a) in isotropic minerals or materials (nγ=nα); b) in anisotropic uniaxial minerals (n γ>nα); c) in anisotropic biaxial minerals (n γ>nβ>nα)

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GLC 201 - Introduction to Optical Mineralogy 5. If the incident light is perpendicular to the surface of the mineral, according to Snell‟s Law, the ordinary ray should then be also perpendicular to the surface of the mineral.

3.4. Transmitted light The light that remains after some fractions of it have been reflected or absorbed then exits the mineral. This is called transmitted light, and it has always a lower intensity than did the original incident light. The ordinary and extraordinary ray(s) also recombine as they emerge from the crystal, and since these rays are polarized, their recombined product is therefore also polarized (as either two or three planes of polarization, perpendicular one to each other). Note that since light is slowed down when passing through a material due to the refractive index contrast, and also part of the light is absorbed, the thickness of the medium therefore affects the transmitted light. If the material is thick, more of the energy of the light will be absorbed, and less light will exit the material. For example, a thin glass is transparent to light but the same glass at 10 m thickness will probably not let light pass through it. If a material (such as a mineral) has a high refractive index compared to air, it is likely to be transparent to light only in thin section. When it is, such as in hand specimen, the mineral will generally not allow light to be transmitted through it (although some minerals can be translucent in hand specimen, allowing some light through). The transmitted light intensity is related to the absorption, so measuring the intensity (energy) of the transmitted light allows us to calculate the absorption (providing the principles of absorption spectroscopy, infrared spectroscopy, etc.). However, since the transmitted light intensity is also dependent upon the mineral thickness, slices of materials (known as thin sections) should be both thin (for enhanced light transmission) and consistently the same thickness (or thinness). By convention, mineral thin sections are made at a standard thickness of 30 microns.

3.4.1. Thin sections for optical studies in transmitted light Minerals are the constituents of rocks, and usually a rock is composed of several mineral species. In order to study minerals we need to cut a slice of the rock, grind and polish a flat surface of it down to 30 microns thick, and glue it, using a polymerized resin, onto a glass slide (fig. 9). The refractive index of the resin must be known, in order to estimate correctly the (unknown) refractive indexes of minerals in thin sections (usually resin is 1.542 if the resin is Canada Balsam, as was traditionally used, or around 1.54-1.55 if other resins are used, such as araldite). A cover slip is usually glued on top of the thin section (with the same resin) in order to protect the sample from „weathering‟ but also to have the same (known) refractive index below and above the sample. Cover glass (<1mm; n~1.55) Synthetic resin; a few µm; n~1.55)

Sample (~30 µm; n variable / unknown)

Synthetic resin; a few µm; n~1.55)

Glass slide (~2 mm; n~1.55)

Figure 9: Profile through a thin section

The optical methods normally used do not measure the intensity of the transmitted light, but instead use this light to provide information about the optical behaviour of minerals. The microscopes using transmitted light are called petrographic microscopes and they are used for studying the transparent minerals (remember that for study of the opaque minerals, which do not transmit light, we would use the chalcographic, or reflected light, microscopes).

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GLC 201 - Introduction to Optical Mineralogy 4. VECTORIAL AND CONTINUOUS CHARACTER OF REFRACTION Refraction is a vectorial, and continuous, property. A vectorial property is a property that varies with direction (a different n is expected in any anisotropic material for each different direction of the incident light coming through the mineral). A continuous property is one which varies continuously and gradually (from a minimum value to a maximum value) within the material (such as refractive index). By contrast, a non-continuous property would abruptly change from one point to another (such as the cleavage of a mineral). 4.1. Indicatrix We must imagine a geometrical figure which can depicts the continuous variation of a property with a continuous variation of direction (orientation). Let‟s take the refractive index (n) as the optical property that we want to graphically represent. First, we can attribute a vector direction to any possible direction within the mineral. Secondly, we can attribute to each vector a value (length) proportional to the refractive index on each direction. If we consider an infinite number of vectors radiating from a central point within a medium, where each vector length is proportional to n, we can imagine a geometrical figure given by the surface connecting the tips of the vectors. This geometrical figure is called the indicatrix, and it graphically represents the variation in refractive indices in a crystal. The indicatrix is a method of rationalising optical phenomena, and provides a framework whereby optical phenomena of transparent media may be interpreted, remembered and predicted. If n has the same value in any direction, it means that all of the vectors (radiating from a point in the mineral) would have the same length, and consequently will describe a sphere (fig. 10a). This is the case for the isotropic minerals: the crystal has only one RI, and is optically isotropic. This applies to the cubic minerals (garnet, spinel, sodalite etc), where all possible sections through a cubic crystal produce a circular indicatrix section. If n varies continuously from a minimum value (nα) to a maximum value (nγ) the indicatrix will have the shape of an ellipsoid (fig. 10b), where the long axis is nγ and the short axis is nα. Different types and shapes of ellipsoids (indicatrixes) can be imagined for the anisotropic minerals (fig. 10b,c). However, two specific sections of the indicatrix are important for making the connection to the symmetry of the mineral: a) the section that contains the maximum possible values of nγ and nα which is called section of maximum birefringence and b) the sections with a circular shape (called the isotropic section). The perpendicular direction on such sections is called optical axis (or direction of monorefringence). Three main types of indicatrixes are possible (see Fig. 10a,b,c):

15

GLC 201 - Introduction to Optical Mineralogy Figure 10 a)

Isotropic indicatrix (any section of the sphere is a circle; n=radius of the circle (fig. 10a).

Uniaxial indicatrix (revolution/rotation ellipsoid). the direction perpendicular to circle section is called the optic axis.  If the optic axis is parallel to (contains) the maximum R.I., nγ, then it is a positive uniaxial indicatrix (a rugby ball shape, positioned for a penalty kick).  If the optical axis is parallel to (contains) the minimum R.I., nα, then it is a negative uniaxial indicatrix (a rugby ball being passed?).

Figure 10 b)

Figure 10 c) Biaxial indicatrix  If the bisectrix of the 2V angle is parallel to nγ, then it is a positive biaxial indicatrix (imagine a flattened rugby ball;  an ellipsoid elongated in one direction (nγ) and flattened from a perpendicular direction (nα);  If the bisectrix of the 2V angle is parallel to nα, then it is a negative biaxial indicatrix (imagine a sphere flattened from one direction (nβ) and even more flattened from a perpendicular direction (nα).

Biaxial positive

Biaxial negative

Figure 10: a) indicatrix for an isotropic mineral (n γ=nα); b) indicatrix for an anisotropic mineral (n γ>nα) called uniaxial indicatrix; by convention, always the higher refractive index is written as n γ , the minimum refractive index is n α; c) indicatrix for anisotropic minerals (nγ> nβ >nα), called biaxial indicatrix; nβ is the intermediate refractive index, being the radius of the circular section (and always perpendicular to the optical axis).

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Optically positive? (slightly off-centred)

and negative

4.2. Interference colours (IF); birefringence (δ) Interference colours are produced when the mineral is placed between two polarisers, having the polarization planes orientated mutually perpendicular (i.e, perpendicular to one another). By convention, the polarizer closest to the light source is called “the polarizer”, and the other one is called “the analyzer”. -The polarizer has a E-W privileged direction producing E-W oscillating white light waves. -The analyser is consists of a polariser with a N-S privileged direction. -The sample (thin section of a mineral) is in-between the polarizer and analyzer and can be rotated to change its orientation (the nγ and nα orientation in relation to the polarization planes of polarizer and analyzer) in a petrographic microscope. Remember that: -Transparent minerals are, in effect, polarisers with TWO privileged directions -These privileged directions are ALWAYS mutually perpendicular -Their orientation depends upon crystal lattice properties -A polarised (E-W) light wave is split into two waves which can pass through the crystal along its privileged directions -The two waves pass through at different velocities, so that there is a faster wave with a lower RI (nα) and a slower one with a higher RI (nγ) Let us follow the behaviour of polarized light on its way from the polarizer through the sample and on to the analyzer: The EW-polarized white light leaves the polarizer with the normal speed of light in air (nair~1) and hits the sample. Here (Fig. 11) the light is refracted and one ordinary (fast) ray and one extraordinary (slow) ray (or two, if the crystal is biaxial) are created. The vibration planes of the rays produced will always be mutual perpendicular. These polarized rays will exit the sample with the speed of light in air and be recombined, but the extraordinary ray(s) will have been delayed by the sample; therefore there will now be a difference in the „phase‟ of their wavelengths, proportional to the retardation (the delay of the extraordinary –or slow) ray. This difference in phase (also called path difference or retardation, Δ or R) is manifested as a wavelength difference (in the range of microns to hundred of microns).

17

GLC 201 - Introduction to Optical Mineralogy The privileged directions nγ and nα of the crystal at 45° to polariser and analyser The waves are forced into N-S direction; because of Δ interference occurs interference colour! nγ and nα waves both propagate at the same velocity (n=1) and hit the analyser at diagonal angles

analyzer mineral

Waves leave the crystal with a path difference: the retardation Δ (or R) [nm] Both waves pass through the crystal at different velocity; nγ is getting delayed The E-W wave hits the crystal and gets split up into the faster nα wave and the slower nγ wave E-W oscillation white light leaves the polariser with normal speed of light (n=1)

polarizer Figure 11: Maximum interference colours obtained at 45° between nγ or/and nα and the N-S (and E-W) polarization planes.

When hitting the analyzer, the mutually perpendicular rays coming from the sample will arrive at the N-S “gate” of the analyzer. What will be the outcome? It will depend on the orientation of the sample and its crystal lattice (and hence the orientation of the mutual perpendicular rays coming from the sample). If nγ or nα comes out along the N-S plane (or the W-E plane, since nγ and nα are mutually perpendicular), the two rays will be eliminated by the analyzer, such that the nγ and nα of the sample will be compensated by the nγ and nα of the analyzer (Fig. 12, right). The result will be a dark image (black or dark gray). This situation (or orientation) is called extinction (as the light has become switched off; “extinct” comes from Latin extinct meaning „switched off, terminated, ended‟).

W3 =0

Amplification

Extinction

Figure 12: Amplification (giving the increase of the intensity,) and extinction (mutually compensation/annihilation of the intensity of the light).

When the stage is rotated from this position, the grain will start to increase its light intensity and become coloured. The colours are the result of interference (adding and/or subtracting wavelengths) between the nγ or nα rays of the sample, which are forced to pass through the N-S plane only. The

18

GLC 201 - Introduction to Optical Mineralogy interference colours will be at their maximum (Fig. 12, left) when nγ or nα of the sample are at exactly 45° to the N-S plane of the analyzer (the N-S diameter of the field of view in the microscope). In this position we observe the maximum intensity of the interference colours (IF), called the birefringence colours (Fig. 13 - Michel Levy chart). From the maximum interference position, continuing to rotate the stage in the same direction, the intensity of the colours gradually decreases til we return to total extinction. After rotating the stage for 45° from the maximum illumination position, another extinction position is obtained (i.e., the grain becomes dark again). When rotating the stage through 360°, all anisotropic minerals show 4 positions of extinction, (interference = 0) one at every 90°, alternating with 4 positions of maximum interference colours (interference = maximum) also at every 90° from one another. Between each position of extinction and the following position of maximum interference there are 45° of rotation. Note that: 1) There is no interference colour produced without the analyser! 2) The interference colour depends on the retardation Δ (i.e., the distance between nα and nγ when leaving the crystal). 3) Only waves propagating in the same plane can interfere! 4) The maximum brightness of the crystal in the microscope if nγ and nα are at 45° to polariser and analyser! At this position we observe the maximum birefringence. Birefringence (δ) is the difference between nγ and nα , so δ = nγ - nα nγ - nα = Retardation (Δ) x Thickness of the crystal (d) δ = Δ / d  and (Δ) correlates with the interference colour (IF) Graphically, δ is a straight line, in a chart (Michel-Levy) where Δ and d are the x and y axes, respectively. The line crosses the origin of graph (see the Interference Colour Chart, also known as the birefringence chart or Michel Lévy chart). The Michel-Levy table contains 4 orders of colours (each order has a total wavelength of 550 nm). The orders are separated by a violet colour and, as we can see in the chart (fig.13), as we go to higher retardation (Δ), the colours become more pale and mixed, sometimes difficult to describe.

19

, , d, IF colour: all on the Michel Lévy chart!

= 0.026

d

, IF 1st Order

2nd Order Figure 13: the Michel Lévy chart

3rd Order

GLC 201 - Introduction to Optical Mineralogy 5. PETROGRAPHIC MICROSCOPE The petrographic microscope is used to analyze the properties of the transparent minerals. The main components of petrographic microscopes are shown in Fig. 14. The light source (1) is on the bottom of the microscope, under the blue filter. The blue filter is needed for absorbing the strong yelloworange component of the light emitted by the electric bulb, in order to produce normal-looking white-coloured light (and therefore „normal‟ interference colours). The 2nd diaphragm is used for reducing the intensity of light (useful sometimes, for evaluating properties such as relief and chagrin). Similar effects can also be obtained by using the light intensity control dial (2). Let‟s once again follow the light on its way up to our eye (along the optical axis of the microscope); The white light coming up from the blue filter passes through a group of other diaphragms and apertures (13) also used for adjusting the light intensity and homogeneity. On its way up, the light passes through the polarizer (3), which is mounted so that the polarization plane is East-West in the image we see through the eye-piece, or ocular (fig. 15). Above the polarizer is mounted a mobile lens (convergent lens, 4). In normal use, this lens is kept out of the way of the light path. Above the convergent lens there is a rotating plate (11), which is the stage, and is graduated (360°) so that angular measurements can be made. In the middle of the plate there is a round hole where the polarized light goes through. Here we put the thin section (sample), so that the light from below can pass up through the sample. The polarized light will interact with the sample and the resulted light will continue upwards. To magnify the light transmitted through the sample, an objective (or a set of objectives) is normally used (5), having different powers of magnification (usually 2.5x, 6.3x, and 10x, 20x, 40x or more). Up to 4 objectives are mounted on a typical nosepiece (6). Above the objective, the analyzer (10) is mounted. It also polarizes light, and is mounted so that its plane of polarization is perpendicular to the polarization plane of the polarizer (i.e., the analyzer has the polarization plane mounted N-S -fig. 15). The analyzer is mobile, so it can be pushed in (or pulled out) so that observations can be made either with or without the analyzer. The final magnification of the image is provided by the ocular (9), which typically provides 10x additional magnification. The total power of magnification of the microscope is equal to the power of magnification of the particular objective in use, multiplied by the power of magnification of the ocular; these values are written on both the objective and the ocular. For some specific determinations, the lamda plate (λplate = gypsum plate, or λ/4-plate = muscovite plate; 7) and the Bertrand lens (8) can be used. In normal use, these pieces are all kept out of the light path. The focused image through the microscope is achieved by using the focus knobs (12) (one large, for coarse focusing, and one smaller, for fine focus). Looking through the microscope without any thin section present, and having all the mobile components (the convergent lens, analyser, lambda plate, and Bertrand lens) kept out of the light path, we should see a white field, homogenously lit (we see the white light, polarized by the polarizer). This microscope mode is known as plane polarized light = PPL. Introducing only the analyser, we get the microscope mode for crossed polarized light (CPL, or colloquially XPL). With no sample, the observed field in the microscope should now be dark (all light eliminated by the crossed polariser and analyser). Why? The analyser lets pass through only the light vibrating in the N-S plane (the analyzer polarization plane). However, it does not receive any vibrations in that plane since the incoming light from the polarizer is vibrating only in the W-E plane. This is how we confirm the 90° angle between the polarization planes of the two nicols, the polarizer and the analyzer (since the analyser can be rotated, this need not always be the case). Both the above modes (PPL and XPL/CPL) use plane polarized light which is transmitted through the mineral in mutual perpendicular planes. For this reason, the study of minerals using either of these modes, or setups, is called orthoscopic study.

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GLC 201 - Introduction to Optical Mineralogy In contrast, introducing the convergent lens and the Bertrand lens to the XPL mode, we get the conoscopic mode (for identifying the optical symmetry of minerals using convergent polarized light). The study of minerals using this mode is called conoscopic study. The λ-plate (gips), as well as the λ/4 plate (muscovite) are called compensators. They can be used for certain observations in both orthoscopic and/or conoscopic modes.

Petrographic microscope 9) Ocular (eyepiece)

8) Bertrand lens

10) Analyser 7) Lambda (λ-) plate (accessory plate) 6) Objective nosepiece

11) Rotating stage

5) Objective lens 4) Condenser lens

12) Focus

3) Polariser 2) Light intensity control dial 13) Diaphragm / aperture 1) Light source & filter 2nd diaphragm

Figure 14: Petrographic microscope: main components

N

W

E

S Figure 15: N-S and E-W direction of the polarization planes as seen at the microscope; polarizer has the polarization plane oriented E-W and the analyzer has the polarization plane mounted N-S.

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GLC 201 - Introduction to Optical Mineralogy 6. MINERAL IDENTIFICATION USING THE PETROGRAPHIC MICROSCOPE

6.1. Orthoscopic study -Condenser lens and the Bertrand lens are OUT!6.1.1. Observations using plane polarized light (PPL) mode -Analyser is OUT!The observations typically made in PPL are transparency, shape/habit/size, colour, pleochroism, cleavage, relief (Becke line, Chagrin), and inclusions/alterations. a) Transparency A mineral is opaque if it appears totally black and stays black regardless of the rotation of the stage). The light cannot pass through the mineral, at all. Since the petrographic microscope is designed for studying the transparent minerals only, we cannot get diagnostic reflected light information here. However, we can observe shape, habit, and transparent inclusions, where present. Usually the opaque minerals are either sulphides (e.g. pyrite, chalcopyrite, etc.), oxides (e.g. magnetite, hematite, or ilmenite), or graphite. If the mineral appears anything other than totally black (no matter what other colour is observed!) it means that the light passes through the mineral, so the mineral is transparent. b) Shape, habit, size Shape: euhedral (or, if metamorphic, we call it idiomorphic), subhedral (hypidiomorphic) or anhedral (xenomorphic); Habit: isometric, prismatic, tabular, sheeted, etc. Size: estimated in mm, based on the field of view determined from the magnification by the objective and ocular lenses. Looking at the mineral boundaries, we can see the shape of the analyzed grain. Remember that the mineral as seen in thin sections is just a section through the mineral, which can have different orientations related to the 3-dimensional (3-D) shape of the grain. In order to estimate the habit, several grains of the same mineral should be examined. The shape can be regular (geometrical features such as squares, rectangles, triangles, or combinations of these); different regular sections of grains seen in the same thin section suggest a euhedral grain (all grain boundaries are linear crystallographic faces with predictable interfacial angles). If the grain shows irregular boundaries only, the grain is anhedral (xenomorphic). If the grain has both regular and irregular boundaries, it is subhedral (hypidiomorphic) - see tables below. The shape and size of the grains are related to the conditions of growth (crystallization). When crystals grow, depending on how favourable the conditions are, they may develop all of their crystal faces, or none of them at all (no preferred faces, so crystal grows as a shapeless blob = anhedral growth), or anything in between.

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Crystal habits Degree of crystal development

grain has most/all well-developed crystal faces (i.e., linear grain boundaries whose orientations are controlled by the crystallography of the particular mineral) grain has some well-developed crystal faces

grain has no well-developed crystal faces (its boundaries are defined by the shapes of the adjacent crystals)

Igneous minerals (crystallised from a liquid)

Metamorphic minerals (crystallised by solid state diffusion)

euhedral (idiomorphic)

idioblastic

subhedral (subidiomorphic)

subidioblastic (hypidioblastic)

anhedral (allotriomorphic)

xenoblastic

shaded grains as examples

Straight, or linear, grain boundaries can occur by a variety of mechanisms: Well-developed crystal faces; Linear boundaries can be found in Recrystallisation (solid-state grain should show the same or interstitial grains adjacent to euhedral modification of grains to similar shape throughout the rock, or subhedral grains; the interstitial accommodate energy from heating and the same relationship to grain is anhedral, and its shape is or deformation) can result in linear cleavages (where present); the controlled by its neighbours (and is grain boundaries, but these will not shape is controlled by crystal therefore not consistent throughout reflect the crystal symmetry of the symmetry of the mineral. the rock, and not consistent with mineral, and will usually not respect to cleavages, etc.). produce consistent mineral shapes

euhedral

interstitial (anhedral)

recrystallised (anhedral)

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More common crystal/grain habits Name equant

Description equidimensional (i.e., a ~ b ~ c)

columnar

elongate in one direction, “blocky”, with other two dimensions similar (i.e., c > a = b)

tabular

rectangular, but flat (“table-like”) (i.e., c > a > b).

lath-shaped

Thin, narrow and flat (so a variant of tabular, but specifically a narrow type). (Actual laths are strips of wood).

fibrous

elongate in one direction, tapering

acicular

elongate and “pointy”, needle-like

prismatic

elongated, with pyramidal pointed terminations

sheaf

radiating collection of elongate grains

rosette

radiating collection of elongate grains

skeletal

the framework of a mineral; partially internally replaced

Shape

c) Cleavage Cleavages are planar surfaces of low cohesion produced by weaker atom bonds across them. They are visible when the cleavage is more or less vertical in the thin section. Cleavages seen in thin sections are linear expressions of the intersection of particular planes of crystal faces with the cut surface of the thin section; these faces have low surface energies and are therefore favoured to “express themselves” in the crystal as preferred planes of growth and preferred planes of splitting of the crystal. Not all faces have equal surface energies; some minerals may have three “good” cleavages (e.g., calcite), some have a “perfect” cleavage (e.g., micas), and some may have no cleavages at all (e.g., olivine, which therefore has no “preferred” planes of 25

GLC 201 - Introduction to Optical Mineralogy splitting, and gets fractured, instead). All cleavage planes of a mineral must match that mineral's symmetry. The same mineral will always have the same cleavage. Cleavage is said to be basal when it occurs perpendicular to the major axis of the mineral, and prismatic when it occurs parallel to the major axis. Multiple cleavages that produce geometric polygons are referred to using the name of the geometric polygon, such as octahedral cleavage in the mineral fluorite, cubic cleavage in the mineral halite, or rhombohedral cleavage in calcite. Cleavage, being related to structure, can be important in the correct identification of a mineral's symmetry. Remember, cleavage must obey the symmetry of the mineral and must be parallel to a possible crystal face. A mineral of the isometric symmetry class can either have no cleavage or at least three directions of identical cleavage that form a closed three-dimensional polygon. A mineral of a uniaxial class (trigonal, tetragonal or hexagonal) will potentially have a cleavage perpendicular to the dominant axis and/or prismatic cleavage of either 3, 4 or 6 directions respectively, running parallel to the axis. Other cleavage directions are possible, but will always be controlled by the symmetry of the crystal (Fig. 16). A biaxial mineral, those belonging to orthorhombic, monoclinic or triclinic classes, cannot have more than two identical cleavage directions.

Enstatite (Opx)

Biotite

Figure 16: Mineral cleavage: left: enstatite, with prismatic cleavage (parallel to the prismatic faces) and two basal cleavages. Right: biotite, with one perfect basal cleavage.

The cleavage (quality and number of different cleavage planes) is diagnostic of some mineral species. From the shape of the observed grain in thin section and the quality and orientation of the cleavage(s), we can have an idea of the orientation of the section cut through the 3-D grain morphology. In figure 17 we can see basal sections of amphibole (left) and pyroxene (right), displaying two characteristic sets of cleavages. a b d c

c d

b a

Figure 17: Basal face with basal cleavage (two intersecting cleavages). Left: amphibole, where the angle between the two cleavages is ~ 60° or 120°. Right: pyroxene, where the angle between the two cleavages is ~ 90°;

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GLC 201 - Introduction to Optical Mineralogy A crystal with one perfect When seen in a cross-section Although the mineral has 4 basal cleavage, such as a cut parallel to the c-axis, we sets of faces (labelled a to d), phyllosilicate, could be would see this system of only 2 of them form depicted as shown below: cleavages represented as a set prominent cleavages (b and of parallel lines of ~equal d). In thin section, we might spacing: planes or faces. see 2 cleavages at ~90° angles to one another, or we might see only one of them (with the other poorly developed, or absent), or none at all, depending on how the crystal has grown, and how it has been cut, relative to the orientations of these cleavage

a

basal cleav age

b

d c-axis

c

c

c-axis

d

b a

The quality of the cleavage is estimated observing the density, continuity and width of the cleavage lines (which are always parallel lines) in thin section (Fig. 18). Remember, this estimation should be done on grains cut almost perpendicular to the cleavages. The quality of cleavage is described as perfect, imperfect, good, distinct, indistinct, poor, or absent. The quality decreases from perfect (dense, almost continue and thin lines of cleavage) to weak cleavage (few, disperse segments of thicker lines) to absent (no cleavage, different curved and/or broken thick lines). For example: Perfect cleavage: micas, all phyllosilicates; Good cleavage: feldspars, pyroxenes, amphiboles; Weak cleavage: apatite, sodalite, olivine; Absent: quartz kfs px

grt Figure 18: Left: one good cleavage in K-feldspar (kfs) and absent cleavage in garnet (grt); Right: good cleavage (prismatic) in pyroxenes (note that the centre of the image shows a whole in the thin section).

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GLC 201 - Introduction to Optical Mineralogy d) Colour (absorption colour) The mineral is colourless if it appears white (we see the white light source!). If any other colour is observed, the mineral is coloured (and the colour can be described). The observed colour is the absorption colour (absorption of a part of the white spectrum). The observed colour should be described as colour, nuances and intensity. For example: pale yellowish brown, bluish light grey, etc. If when rotating the stage, the colour changes, then the mineral has pleochroism (see below) and the range of colours should be described, rather than a single colour. e) Pleochroism The term “pleochroism” comes from the Greek: pleos – many; chromos – colours. A mineral shows pleochroism when the absorption colour (colour or nuance, or/and intensity) changes when the stage is rotated. It means that absorption of specific light wavelengths depends on the crystal orientation. This happens when the mineral is anisotropic. All anisotropic coloured minerals have pleochroism. However, the intensity of pleochroism (the changing of colour) can be different (from strong to weak). Common examples shown below include strong pleochroism of biotite and hornblende (Fig. 19 and 20). We describe the pleochroism as ther strong, moderate or weak, and try to describe the colour variation from the lightest to the darkest colour/nuance (e.g. pleochroism from light yellowish green to dark bluish green).

Figure 19: Strong pleochroism of biotite, as stage is rotated 90°.

Figure 20: Strong pleochroism of hornblende, at 90° of rotation.

f) Relief Refractive index (RI, n) is a measure of the speed of light in material relatively to the speed of light in vacuum. The higher the RI, the slower the light propagation in the mineral. “Relief” refers to the relative difference in RI between neighbouring crystals. Examine the grain boundaries for the relief of a crystal (Fig. 21, 22):

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n=1.55 n=1.7

n=1.55

Cover glass (<1mm; n~1.55) Synthetic resin; a few µm; n~1.55) n=1.55 Sample (~30 µm; n variable) Resin Glass slide (~2 mm; n~1.55) (hole) n=1.55

Cpx n~1.7 Figure 21: Crystals with higher RI (n) seem embossed compared to low-RI minerals or resin; Here: Clinopyroxene (Cpx) has a high relief compared to the resin but does not have a high relief compared to other Cpx crystals.

Figure 22: Left: low relief of quartz (it can hardly be distinguished from the resin because the refractive indexes of quartz and resin are very similar). Right: High relief of garnet comparing to the resin. It boundary appears extremely distinct and thick.

Becke line; Becke method for estimating the relief The Becke line is a narrow bright line along grain boundaries caused by light refraction and scattering along the crystal surface. When lowering the rotating stage (using the fine focus), the Becke line migrates into the phase of higher RI (Fig. 23). This is called Becke method and it is a very sensitive method (determines n to ± ~0.02). The microscope mode for the Becke line test: • PPL setup mode • High power objective lens (40x or higher) • Close the diaphragm for better contrast

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Focused

Stage slightly lower

Qtz/Fsp Grt

Becke line moves into Grt

Figure 23: Becke line observed at the boundary between garnet (Grt) and quartz-feldpar (Qtz/Fsp) aggregate. The Becke line moves into the garnet (into the mineral with higher refractive index) when slightly defocusing the image as the stage is lowered. Note also fine bright lines (also Becke lines) between the quartz and feldspar grains.

Although relief is most useful as a comparative term (some minerals show higher relief than others), the relief can be positive or negative compared to a reference material of fixed and known RI. This reference standard is the resin, which has a known refractive index (n = 1.541.55). All minerals with relief higher than the resin have positive relief and all minerals with lower relief than the resin, have negative relief. In order to determine if the relief is positive or negative, we need therefore to directly compare the mineral with the resin (using the Becke method). We should therefore look in the thin section where the unknown mineral is in direct contact with the resin (usually look at the margin of the thin section, or look for holes in the thin sections, if any). Twinkling (relief changing) This property is specific (diagnostic) for carbonates (calcite, dolomite, magnesite, etc). When a mineral has nγ much higher than ~1.54 (nresin) and nα lower than ~1.54, it will show a changing of relief (from positive to negative) when rotating the stage (Becke line moves from one side to the other at the mineral boundaries, cleavage lines, or micro-fractures. This movement of the Becke line when rotating the stage (and NOT when lowering the stage!) produces a variation of the white light intensity (boundary and cleavages turn from fine to thicker lines) and the mineral appears to have “pleochroism” (from colourless to light gray). However, the phenomenon is not actually related to absorption, but to the high difference between nγ and nα. Chagrin (degree of rough appearance of the mineral surfaces) Chagrin is a rarely used but an often useful term! It is produced by light refraction between the mineral surface and the resin at the top (or bottom) of the thin section, as well as between the mineral and very small cracks in it which are filled with resins. Fine, irregular, dense Becke lines will form along micro-cracks, giving the image of a rough, irregular surface. The chagrin is most obvious in minerals with strong relief and with absent or weak cleavages (where microcracks are widespread in the volume of the grain). Olivine, apatite, sphene and garnet show characteristic chagrin (all have high relief and absent or weak cleavage!). However, the chagrin can be positive (olivine, apatite, sphene, garnet) or negative (sodalite, fluorite), depending on the relief of the mineral (Fig. 24, 25).

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Figure 24: Positive and negative chagrin ordered by relief (relief ~zero is albite ~ resin).

Cpx

Ol

Ol

Cpx Figure 25: Olivine (Ol): strong positive relief, weak or absent cleavage, strong positive chagrin (roughness surface); clinopyroxene (Cpx): positive relief, good cleavage, weaker chagrin. Is the grain in the middle of the image olivine or clinopyroxene? Why?

g) Inclusions, alterations Minerals can have inclusions, which can be solid (other finer-grained minerals) or fluid (liquid and/or gas) inclusions. Choose a higher magnification objective and describe the inclusions, if present (transparent or opaque, colourless or coloured, relief, etc.). If altered, other minerals (alteration minerals) can appear at the margin of the analyzed mineral, or along its cleavages or cracks. Describe the alteration mineral separately using a higher magnification objective.

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GLC 201 - Introduction to Optical Mineralogy 6.1.2. Observations using crossed polarized light (XPL) mode -Analyser is IN!The observations in crossed nicols are: isotropy/anisotropy, extinction angle, birefringence colour, twinning/zoning, finding the orientation of nγ and nα , optical elongation, and specific textures. a) Isotropy/anisotropy A transparent isotropic mineral is dark gray or black in crossed nicols, and the colour doesn‟t change during rotation of the stage (there are no interference colours, since nγ=nα). NOTE: do not confuse an isotropic mineral (or an isotropic section through an anisotropic mineral) with an opaque mineral! An opaque mineral is totally black whether the analyser is in OR out, while the transparent, isotropic mineral is not opaque! If the mineral is anisotropic, it shows 4 positions of extinction and 4 positions of maximum interference when rotating the stage. b) Extinction angle The extinction angle is the angle between one vibration direction of the mineral (n γ or nα) and the N-S polarization plane of the analyzer (the N-S direction of the microscopic view). The extinction can be parallel, symmetric or oblique. In order to measure the extinction angle, we need to identify crystallographic features of the mineral, such as cleavage planes, crystallographic faces or twinning planes (crystallographicallycontrolled orientation). These features serve as reference directions. We rotate the stage to set the crystallographic reference (e.g. an elongated face parallel to a cleavage, as shown in Fig. 26a) parallel to the N-S (vertical) or E-W (horizontal) direction of the microscopic field (ensuring that the cleavage lines are more or less either vertical or horizontal). Is the crystal in this position at maximum extinction? If yes, it means that the angle of extinction is zero (or 90°), and the extinction is called parallel extinction, meaning that the nγ or nα, (we don‟t know yet at this point which one is which) is parallel to the N-S polarization plane. If in the vertical position the crystal is not in maximum extinction but it shows an interference color, it means that the angle between nγ or nα is different from zero (or 90°). We should rotate (incline) the crystal in order to find its extinction position. It means that the crystal has an inclined extinction. a) Parallel extinction: the cleavage is in N-S or E-W orientation when the crystal is in extinction position

b) Oblique extinction: the cleavage is not // N-S or E-W when the crystal is in extinction position. The extinction angle is usually <45%

c) Symmetrical extinction: When the n γ and nγ are parallel to the bisectrix of two identical faces. -Requires ALWAYS two (2!) equivalent crystallographic surfaces (cleavages, crystal faces). -A crystal shows symmetrical extinction if both cleavages/crystal faces show the same angle to the crosshairs at extinction. Figure 26: a) Parallel extinction; b) Oblique extinction; c) symmetrical extinction.

It is as simple as that: what it is the position of the crystal when it is in extinction, compared to the N-S plane of the microscopic view? Is it parallel to the N-S plane? If yes, the extinction is parallel

32

GLC 201 - Introduction to Optical Mineralogy (angle of extinction is zero), if not, the extinction is oblique (Fig. 26b). The angle of extinction will be the angle we have to rotate the stage in order to get the nearest extinction position. Some specific sections in minerals (e.g. basal faces) show symmetrical extinction (Fig. 26c). Determination of the extinction angle Move the stage so that the crystallographic reference (e.g. the crystal face, cleavage, etc.) is aligned N-S (Fig.27, left). Record the position of the stage. Then rotate the stage (in the sense where the crystal arrives faster in the extinction position) until the crystal has its maximum extinction (remember that there are four positions of extinction at each 90°). In this second position, when the crystal is extinct, record the stage position again (Fig. 27, right). The difference between the two readings is the extinction angle.

Reading 1

Reading 2

Fig. 27: Extinction angle = angle between Reading 1 and Reading 2. The extinction angle is characteristic for each anisotropic mineral. However, it can differ even within the same mineral group (i.e., olivines, pyroxenes, etc.), because of compositional differences caused by solid solution substitutions that also influence the mineral‟s crystal structure. For example, see the variations in extinction angles in the pyroxene group (Fig. 28), below.

Figure 28: Variation of the extinction angle within the pyroxene group (Saggerson, p. 24).

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c) Birefringence In order to describe the birefringence we should be able to: 1) describe the colour of interference (also called colour of birefringence) and 2) find the value of the birefringence. Interference colour (birefringence colour) First, we need find the crystals of our particular unknown mineral with the highest interference colours, using both PPL and XPL settings to identify grains of the same mineral (same relief, absorption colour, pleochroism, cleavage, etc.). Then, we describe birefringence by comparing the interference colours observed with the microscope with those within the Michel-Levy chart (e.g. first order yellow). But how do we know it is first order yellow (as distinct from the other order yellows)? Colours in the Michel-Levy chart are repeated in each order. ****. Remember that although the colours usually repeat in each order, they are pale and diffuse as the order gets higher (e.g. compare the yellow from each order). We have two methods to establish the order of the interference colour: a) using the isochromatic lines (called isochromates, or isochromes) and b) using the λ-plate. a) using isochromates The margin of grains are usually oblique to the light path, and because of this, the polarized light is dispersed (as in the dispersion in Newton‟s prism; Fig. 29). A white light ray entering an optically denser medium and leaving by a plane inclined to that of entry will have its colours separated, analyzed, spread out. This is because each colour has a different wavelength and so is differently slowed down (refracted) by the medium. Red (longest wavelength) is slowed the least and violet (shortest wavelength) the most.

Figure 29: Dispersion of light at an oblique boundary of a refringent material.

Observing the dispersion of the interference colour of the mineral at oblique grain boundaries allows us to estimate the order of birefringence by counting the number of violet colour bands and adding 1. For instance, in Fig. 30a we observe a plagioclase with a gray colour of birefringence (no violet isochromates at its boundary). The order of colour is therefore 0+1=1. In Fig. 30b we have a muscovite. The bluish green colour of birefringence is in the second order (we observe one violet isochromate, so the order is 1+1=2). In Fig. 30c we see calcite with a diffuse, white-greyish colour of birefringence. At its rim, we notice 3 violet isochromates, so the order of birefringence is 3+1=4.

34

GLC 201 - Introduction to Optical Mineralogy a) 1 order IF colours b) Isochromates indicate c) High order white Green 2nd order in calcite st

a

b

c

Figure 30a-c: Identifying the order of the interference colour using the isochromates (see text). Finding the value of birefringence (δ) Knowing the thickness of the thin section “d” (which is standard, 30 microns) and observing the birefringence colours in Michel-Levy chart, we can graphically obtain the value of birefringence (values written at the top and right of the Michel-Levy chart) by intersecting the band of the observed colours with the “d” value horizontal line (Fig. 31). From that point, going up right on the chart following the line δ=Δ / d, we get the value of δ (birefringence). For example, the maximum interference colour of quartz is first order white. We look for the intersection of the first order white band in the Michel-Levy table with the horizontal (d) line corresponding to 30 microns thickness. From that point, going up (interpolating between the radiating lines), we get a birefringence value of 0.009, as written at the top of the chart. Looking in the same sample, we will also find grains of quartz with lower birefringence (gray) which means that their orientation is different (the section is not cut parallel to the optic axis, and therefore, our view is not completely perpendicular to the optic axis). If we want to know what maximum colour of birefringence to expect from a particular mineral (knowing the value of δ from mineral tables), we go down from the value on the δ line until we intersect the d line. At this intersection we see the colour of birefringence that corresponds to a particular δ value. IMPORTANT: -The standard thickness of thin sections is 30 microns (the “d” horizontal line of interest is at 30 microns!) - The Michel-Levy chart is made for maximum birefringence of minerals, only!! Do not try to memorize a mineral using a specific unique birefringence colour. It is pointless and wrong! Always remember that different colours can be possible for differently-cut orientations. What does this mean? Some specific cuts of anisotropic minerals (sections perpendicular to the optic axis, called sections of monorefringence – circular sections of the optical indicatrix) behave isotropically. For example, apatite (calcium phosphate) has a prismatic habit, crystallized in the hexagonal system (with medium symmetry, so it has a uniaxial indicatrix). If the section is cut parallel to the prism faces, it will show maximum interference colours (section parallel with the optical axis). If the cut is perpendicular to the prism, the section will be isotropic. We can understand from here that the interference colours of one mineral species depends on the orientation of the section cut, so that the interference colours can

35

GLC 201 - Introduction to Optical Mineralogy vary from isotropic (black) when the cut is perpendicular to the optic axis, up to the maximum interference colours when the cut is parallel to the optic axis.

Figure 31: Sketch of the Michel-Levy table showing the graphical method to identify the birefringence value

(from Saggerson (1983)

d) Twinning/zoning Twinning A twin is a symmetrical growth of two or more crystals of the same mineral. The common plane of the twinned crystals (which is called the twinning plane) is a symmetry plane, seen in thin section as a straight line separating two identical crystals (e.g. crystal (1) and crystal (2)) which have a symmetrical optical orientation to the twinning plane, i.e., the indicatrices of the two twinned crystals are symmetrical to the twinning plane. This is observable by rotating the stage (Fig. 32); when crystal (1) is in extinction, its twin crystal (2) shows interference colours. Continuing the rotation of the stage, crystal 1 shows interference colours and crystal (2) will enter into a position of extinction. If the section is cut perpendicular to the twinning plane, the extinction angles of crystal (1) and crystal (2) should be identical (if the crystallographic reference for measuring the extinction is the twinning plane!). If the section cut is not perpendicular to the twinning plane, the extinction angles will be different. If the section is cut parallel to the twinning plane, the twin cannot be observed at all, in the plane of the thin section.

Figure 32: Polysynthetic twins in plagioclase; section ~ perpendicular to the twinning plane (useful for measuring the anorthite content in plagioclase)

36

GLC 201 - Introduction to Optical Mineralogy If more than two crystals are twinned, having parallel twinning planes, the twinning is called polysynthetic (sometimes also called lamellar twinning). Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane. Such twinning is one of the most diagnostic features of plagioclase (Fig. 33). Several laws of twinning are possible and they can be recognized using the microscope (e.g. Polysynthetic twins after the “albite” and “pericline” laws in plagioclase, Carlsbad twins in plagioclase or orthoclase, cyclic twins in leucite, etc.).

Two ~ perpendicular polysynthetic twin sets (albite-law twins perpendicular to the pericline-law twins)

Orthoclase (K-feldspar)

Carlsbad twinning in plagioclase (also common in orthoclase)

Plagioclase (Na-Ca-feldspar)

Polysynthetic albite-type twins in (Tartan twinning, typical of plagioclase microcline – K-feldspar) Figure 33: Examples of twinning in feldspars.

Zoning (compositional zoning) Compositional variation within a crystal can be shown by different interference colours (Fig. 34). Zoning is possible in minerals which consist of solid solutions, where the compositional differences reflect variation of major element ratios: e.g. Mg/Fe, Na/Ca, etc.). Note that the difference in composition is visible as differences in the interference colours under the petrographic microscope only if the symmetry of the crystal is low (it is visible for inferior symmetry minerals, rarely visible for those with medium symmetry, and never visible in high symmetry – isometric crystals, such as garnet, which is commonly zoned, but this is not evident in thin section).

Figure 34: Compositional zoning in amphibole (a) and twinned plagioclase (b)

37

GLC 201 - Introduction to Optical Mineralogy e) Orientation of nγ and nα What we know from maximum brightness and total extinction of a mineral under the petrographic microscope in XPL mode is that, at complete extinction the privileged directions nα and nγ are N-S and E-W, and at maximum brightness nα and nγ are NW-SE and NE-SW oriented. However, we don‟t know which is which! Why do we need to find the nα and nγ of a crystal? Because it is the only way to establish the optical elongation, which is a characteristic of any anisotropic crystal! How do we find out which is nα and which nγ? The microscope is designed so that the input of the λ-plate is always at 45° to the polarizer and analyzer. Looking at the λ-plate, we can read the orientation of nγ of the plate (Fig. 35). Figure 35: λ-plate, showing the orientation of nγ of the plate. In this case, n γ is perpendicular to the direction of input, meaning that it is at 45° to the N-S and E-W polarization planes.

n slow

We rotate the crystal into its position of maximum brightness. Now, either nα and nγ is oriented NE-SW, meaning parallel to nγ of the λ-plate. Note that: - The wavelength of the λ-plate is 550 nm (~575 nm is the extent of one order in the Michel-Levy table!); - When introduced, nα and nγ of the λ-plate will interfere with the nα and nγ of the mineral, modifying the retardation (Δ) - The wave can combine their wavelengths by addition (increasing retardation) or subtraction (decreasing retardation). The addition or subtraction will only be by 550 nm, meaning by 1 λ. - Adding retardation is when nα and nγ of the plate are parallel with nα and nγ of the mineral. - Subtracting retardation is when nγ of the plate is parallel to n α of the mineral (and nα of the plate is parallel to nγ of the mineral). - Increasing or decreasing the retardation can be seen by the changes in the interference colours in the Michel-Levy table (to the left with 550 nm or to the right with 550 nm starting to the interference colours observed for the mineral). Let us take an example (Fig 36a-c): 1) First, bring the mineral to total extinction (this example is for a crystal with parallel extinction). Here, we know that the two directions of vibration of the mineral are vertical, and also horizontal (but we don‟t know which is nα and nγ)(Fig. 36a) 2)

Rotate the stage exactly 45° (preferably bringing the longest faces of the crystal parallel with the direction of the introduction of the plate (meaning NW-SE). In this position the two directions of vibration of the mineral will be at 45° to the N-S and E-W lines and the mineral will show its maximum interference colours (Fig. 36b).

3)

Observe and locate the interference colour in the Michel-Levy table (pay attention to the order of the colour!)

4)

Introduce the plate (λ-plate or λ/4-plate)(Fig. 36c) 38

5)

GLC 201 - Introduction to Optical Mineralogy Observe the changed colour and locate this new colour in the Michel-Levy table

If the new colour is situated at the right of the first observed one, it means that the wavelengths of the mineral and the plate were added (the retardation increased in the Michel-Levy table). This means that nγ of the mineral is parallel with the nγ of the plate (Fig. 36c). Knowing the nα is perpendicular to nγ we have determined now the position of both nα and nγ of the mineral. Note that if we rotate the crystal another 90°, having already introduced the plate, the nγ of the mineral will now be parallel to the nα of the plate, and so the wavelength will subtract one from the other and the retardation will decrease (the interference colour will appear further to the left, by 550 nm).

nγ

n slow

Figure 37: a) Bring first the mineral to total extinction (this example is for a crystal with parallel extinction); in this position nγ and nα will ALWAYS be vertical for one, and the other horizontal (at this stage, however, we don‟t know which direction is nγ or nα); b) rotate the stage by 45° to arrive at maximum interference and identify the colour of interference in the MichelLevy chart; in this position, the nγ and nα will ALWAYS be positioned at 45° relative to N-S polarisation planes of the nicols (therefore, one of the two will be EXACTLY along the direction in which we introduce the λ-plate); c) introduce the plate and identify the modified colour using the Michel-Levy chart: check if the retardation (Δ) has increased or decreased. If Δ increased (colour moved to the right in the Michel-Levy chart), then we know that nγ of the mineral is parallel to nγ of the plate.

Variable birefringence in anisotropic minerals: the problem of crystal orientation in thin sections Figure 38: left: XPL image of plagioclase crystal showing cleavage (centre of the image); middle: same image after introducing λ-pate colour moved to the right in the Michel-Levy chart (from grey to yellow), so we know that in this position, the interference of λ-plate with the crystal produced an addition of (Δ)  therefore, the nγ of the crystal is parallel to the NE-SW direction of the microscope view (meaning parallel to the orientation of nγ shown on the λ-plate). Relating this observation to the reference direction (cleavage) of the crystal, we can say that nγ of the crystal is parallel to the cleavage; right: crystal rotated by 90°. Note that the interference colours changed; in this position we get subtraction of Δ (colour moved to the left in the Michel-Levy chart)  nγ of the crystal is parallel to n α of the λ-plate. Don‟t be confused… No matter how we rotate the stage, even colour changes, the orientation of nγ and nα of the crystal in relation to crystal shape or reference (cleavage) always stays the same (in this example nγ of the crystal is always parallel to the cleavage, no matter how the stage is rotated).

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GLC 201 - Introduction to Optical Mineralogy See the example in Fig. 38, for the grain in the centre of the microscope view (left: no plate; middle: plate introduced; right: crystal rotated by 90°). f) Optical elongation Since we have determined the orientation of nα and nγ in the crystal, is easy now to derive the optical elongation. Remember that nγ is higher than nα. The two directions of vibration represent the two main axes of the indicatrix (ellipsoid). We now relate the determined ellipsoid to the mineral form! We know that the exterior form of crystals is directly related to its symmetry (therefore to its a,b and c crystallographic axes). Usually (but not always), the longer faces are parallel (or displaced by the extinction angle value) to the c axis. How are nα and nγ placed in relation to the direction in which the crystal is more developed (longer), in relation to the c axis? If nγ is parallel (or at the extinction value angle) with the direction of the longer faces, we say that the crystal has a positive optical elongation (the elongation of the ellipsoid corresponds to the direction of the c axis). This crystal can also be described as “length-fast” (the long axis of the crystal is parallel to the direction of the lowest R.I.). If nα is parallel to the c axis, then the optical elongation is negative (Fig. 39), and the crystal is “length slow”. c- axis

c- axis

c- axis

c- axis

a)

b)

c)

d)

Figure 39: a) parallel extinction, positive optical elongation; b) parallel extinction, negative optical elongation; c) oblique extinction, positive optical elongation; d) oblique extinction, negative optical elongation.

Question: what is the optical elongation of the plagioclase shown in figure 38?

6.2. Conoscopic mode Microscope setup: XPL (crossed nicols), high magnification (powerful objective) Convergent lens in! Bertrand Lens in! Chose a coarser grain and make certain that the rotating stage and the objective are centred (by rotating the stage, the crystal should remain at or near the crosshairs in the centre of the microscopic field). Conoscopic study helps us to see if a crystal is uniaxial or biaxial. (Note that a 2-D section through a 3-D uniaxial (circular ellipsoid) or biaxial indicatrix (flattened ellipsoid) would have the same (elliptical) form). In conoscopic mode, light penetrates the crystal in a conical shape, not as parallel rays as in orthoscopic mode. This gives a kind of three-dimensional impression of light propagation in the crystal. Details can be found in the literature; here we will only discuss the practical side of thin section conoscopy. We can derive the real shape of the indicatrix by using the interference figures and determining the optical sign.

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6.2.1. Interference Figures The conical (convergent) shape of light interferes with the sample and the shape of the interference result is an image (interference figure) that forms somewhere between the analyzer and ocular. In order to bring the interference image closer to the ocular, the Bertrand Lens is used. An interference figure consists of two dark diffuse intersecting lines (or curves) called isogyres (regions of zero path difference) and circular coloured rings called isochromates, representing regions of identical path difference; The higher the (Δ), the more isochromates there are (e.g. Fig. 40). Interference figures for uniaxial crystals Uniaxial interference figures ideally look like this: Isochromates

Isogyres (regions of zero path difference) Rotation of the stage does not change the image Figure 40: Interference figure for calcite (section perpendicular to the optic axis).

When the section is perpendicular to the optical axis of an uniaxial crystal (e.g. calcite, which is trigonal), the interference figure is a cross (Fig. 40) centred in the middle of the view. Rotating the stage, the cross, as well the isochromates, do not move. At the intersection of the isogyres is the optic axis (which corresponds to the A3 fold axis of symmetry of the calcite) If the section is oblique to the optic axis, the cross will be out of view and we have to rotate the stage. By rotating the stage, we will observe one vertical “arm” of the cross, moving horizontally as we rotate the stage, and when it disappears, a horizontal arm will show up, moving vertically (fig. 41). Optic axis

Figure 41: Interference figure for a uniaxial crystal, section oblique to the optic axis. The optic axis is outside the interference figure but the vertical and horizontal black lines move horizontally and vertically, respectively, as the stage is rotated.

Interference figure for biaxial crystals If the section is perpendicular to one of the optic axes, the interference figure will appear as a curved, dark, diffuse arm that rotates within the image view. If the curvature is high, it means that the angle between the two optical axes (the 2V angle) is small, and if the curvature is small, then the 2V angle is high. (Fig. 42).

41


Figure 42: Interference figure for a biaxial crystal, section perpendicular to one of the optic axes.

The most useful biaxial interference figures are those for sections perpendicular to the 2V bisectrix (the bisectrix of the acute angle between the two optic axes). In these figures we can see both optic axes (Fig. 43a).

Shelley 1993

Figure 43a: Position of the optic axes

Figure 43b: Movement of isogyres during stage rotation. From top to bottom, the 2V angle increases.

How do we find the section cut most closely perpendicular to the acute bisectrix? Trial and error! Start with the lowest interference colour section you can find and, in the Conoscopic Mode, work your way up until you find the right (i.e., most useful) interference figure. Determination of the optic sign -using the λ-plateThe optic sign can be either positive or negative, and this tells us if nγ or nα, respectively, is parallel to the 2V bisectrix. After finding the interference figure, we introduce the λ-plate. The wavelength introduced by the plate can produce either addition or substraction in the retardation of the isochromates (Fig. 44)! What does the optic sign mean? It shows the shape of indicatrix in relation to the optic axis of the crystal. For uniaxial crystals: is the optical axis parallel or not with the nγ or nα? For biaxial crystals: is the bisectrix of the acute 2V angle parallel with nγ or nα? See the shape of biaxial indicatrices in Fig. 45.

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Figure 44: Determination of the optic sign Observe the upper right or lower left quadrant: Add the λ-plate -Constructive (addition) colours indicate a positive optic sign; -destructive (subtraction) colours indicate a negative optic sign.

Figure 45: Positive and negative optical sign a) Crystal biaxial positive : Acute bisectrix: nγ nα and nβ T acute bisectr, δ > 0 b) Crystal biaxial negative Acute bisectrix: nα nβ and nγ T acute bisectr. δ > 0

A quick reference for the determination of the optic sign of minerals with low birefringence (IF 1st order grey-white) is provided in figure 46.

43


a)

Uniaxial

to optic axis

b) Biaxial

to acute bisectrix

c) Biaxial

to optic axis

Figure 46: Determination of the optic sign for crystals with low birefringence.

Estimation of the 2V angle This estimation is an approximation using the interference figures. The best sections are those perpendicular to the acute bisectrix (Fig. 47a). The estimate is done by comparison with images from Fig. 47. a) 2V=90: one straight line rotating in the opposite direction compared to the rotation of the stage; b) There is a moderate 2V angle if the isogyres are moderately curved; c) There is a low 2V angle if the two wings of the cross meet and break slightly as we rotate the stage. The two wings do not leave the interference figure when rotating the stage only if the section is ~ perpendicular to the acute bisectrix (i.e. a uniaxial-like interference figure).

Figure 47: Estimation of the 2V angle (see text). After Shelley (1993).

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Is the interference figure good enough for seeing the optical character and determining the optical sign? Interference figures: The Good, the Bad and the Ugly (Fig. 48):

The Good: Section

to the acute bisectrix

The Bad: Section oblique to the acute bisectrix

Another Good one: Section ± to OA The Ugly: “Flash Figure” Section to the obtuse bisectrix (biaxial minerals) or parallel to the OA (uniaxial minerals): confusion guaranteed. Figure 48: Types of possible sections obtained for biaxial crystals. Which one is good?

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Useful charts for mineral identification: the Tröger Chart A (sometimes dangerous) shortcut to identify minerals with the petrographic microscope involves using the Tröger Chart, which has the refractive index on the x axis and birefringence values on the y axis. The zero value of birefringence (isotropic crystals) positioned at the middle of the chart, so that the birefringence values increase from zero up but also decrease from zero down in the chart. In the upper part are found minerals with positive optical sign, while in the lower part of the chart are minerals with negative optical sign. To make a distinction between uniaxial and biaxial crystals, the uniaxial are represented with bold circles. The steps to take are: a) Check refractive index (n)/chagrin (low, medium, high); for n(RI)<1.65 use part one of the chart (Fig. 49a) and for n>1.65 use part two of the chart (Fig. 49b) b) Check the maximum birefringence (birefringence colour; then estimate the value of birefringence using the Michel-Levy chart) c) Determine the optic character (uni- or biaxial) and the optic sign d) Find the region on the Tröger Chart corresponding to these determined values e) Check the optical characteristics of minerals occurring in that region f) Check the likelihood of the determined mineral occurring in the rock type investigated g) Don‟t forget: there are more mineral species than shown on the charts!

Figure 49a: Tröger Chart part 1 (refractive indexes from 1.45 to 1.65)

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Figure 49b: Tröger Chart part 1 (refractive indexes from 1.65 to 2.80)

27 Key mineral species It is useful to know the key optical characteristics for the minerals listed below (the common rock-forming minerals). 1. Quartz 3. K-feldspar* 5. Biotite* 7. Amphibole* 9. Talc 11. Garnet* 13. Staurolite 15. Chloritoid 17. Titanite 19. Olivine* 21. Opx* 23. Apatite 25. Sillimanite 27. Nepheline

2. 4. 6. 8. 10. 12. 14. 16. 18. 20. 22. 24. 26.

Plagioclase* Cordierite* Tourmaline* Muscovite Chlorite* Spinel* Rutile Calcite/Dolomite Zircon/Monazite Cpx* Epidote* Kyanite Andalusite

* Solid solutions with variable optical properties Italics: learn formula; the rest: learn the general composition

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GLC 201 - Introduction to Optical Mineralogy Key Characteristics of common minerals: Speeding up mineral identification Many common mineral phases have unique characteristics (or combinations of two or three) which make them unmistakable. Examples Quartz: low RI (~like the resin); low birefringence (1st order IF colour); uniaxial positive; no (visible) twins. Plagioclase: low RI and birefringence (~like Qtz); lamellar twinning; biaxial positive or negative. Staurolite: pale yellow pleochroism; high RI; frequently idiomorphic. Carbonates: very high birefringence, relief changes when you turn the stage; uniaxial negative. Identify the key characteristics and note them in your mineral catalogue. A few hints for the relationship between chemical composition - optical properties Some cations from the Transition Elements in the Periodic Table (including Fe, Cr, V, Ti, etc.) which have several possible valence states in rocks, produce more intense but variable absorption of light, and are called chromophores. The result is that minerals rich in these elements will be more strongly coloured in thin sections (in PPL mode): Fe2+ gives gray, yellow to greenish colours, depending on its concentration and on the absorption produced by other cations (e.g. in olivine, pyroxene, amphibole, chlorite). Fe3+ gives brown colours (in oxydated hornblende = brown hornblende) or green in oxydized biotite. Cr3+ gives pale green colours (e.g. in spinels, Cr-diopside, fuxite (Cr-mica), Cr-staurolite, Cr-cordierite); Ti produces reddish-brown colours (such as in Ti-rich biotite). In addition to the strong selective absorption, the presence of these cations also increases the refractive indices of the mineral, causing higher relief. This is useful in composition estimations for minerals that are part of solid solutions. For example, in olivine or pyroxene, Fe2+ shares a structural position with Mg2+ (so Fe2+ can substitute for Mg in any proportion). The Mg-rich end member of the solution will be colourless, but the solid solution becomes more coloured and the refractive index increases as it has more Fe2+ instead of Mg. The Fe2+ end members will be green with higher relief. When dealing with silicates (as we usually are in rocks), coloured minerals as seen in PPL can be expected to have a positive relief (have refractive indices superior to the resin). There are few exceptions: the bluish hauyine and nosean from the sodalite group have negative relief, but the bluish colour is given by the absorption produced by small amounts of the [SO4] molecule. The silicates with Al, Ca, Mg, or with Ca, Na, K (note the absence of chromophores) are typically colourless (e.g. all feldspars, feldspathoids, white mica). The substitution of Ca for Na in plagioclase solid solutions produces no colour change, but does induce an increase in the refractive index (relief) and the extinction angle. Michel-Levy proposed a method to estimate the Ca-end member (Anorthite CaAl2Si2O8) in a plagioclase, based on the extinction angle. The carbonates always show twinkling (a modification of relief from positive to negative) when the stage is rotated. Together with the high order birefringence (4th order), the twinkling is diagnostic for carbonates.

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GLC 201 - Introduction to Optical Mineralogy Tips for discriminate between different mineral groups All cubic minerals are isotropic. All orthorhombic minerals, as well as all uniaxial minerals (medium symmetry: trigonal, tetragonal, hexagonal), have parallel extinction (except for basal sections, which have symmetrical extinction). All monoclinic and triclinic minerals have oblique extinction (except for basal sections which have symmetrical extinction). All phyllosilicates have parallel extinction and perfect basal cleavage; the extinction is not total (smooth) but „rough‟ (small bright coloured spots are present across its entire surface). All orthosilicates have relatively high refractive indices (relief) All tectosilicates have low or medium-low refractive indices (relief) Sulphates (e.g. gypsum) have usually negative refractive indices (relief) Heavy elements (down periods in the periodic table) produce high relief (Ba, U, REE etc) in their host mineral Sulphides are all opaque (as some of the oxides: magnetite, hematite, ilmenite); yellowishbrown alterations on fissures (no pleochroism, no birefringence) are usually Fe-hydroxides (goethite, lepidocrocite etc) or hematite (dark reddish).

Mineral associations: helpful in identifying minerals Not all minerals can be naturally associated in a rock. Most rocks have 2-5 abundant minerals and a few other minerals as possible accessories or alteration. The natural association of minerals in rocks is controlled by their stability, which mainly depends on chemistry, pressure (including water pressure), and temperature. -olivine and quartz are never found together in equilibrium in the same thin section (one is undersaturated in SiO2, the other is super-saturated in SiO2, respectively). -feldspathoids (nepheline, sodalite, cancrinite, etc.) are never found together with quartz (same explanation as above); -if olivine has been recognised (medium-high relief, no cleavage, strong chagrin, high birefringence), it is frequently associated with pyroxenes (no chagrin, good cleavage, similar relief, parallel extinction = orthopyroxene; oblique extinction (30-45°) = clinopyroxene) and/or amphiboles (longer prisms, stronger pleochroism, typical basal sections with 120° angle between cleavages, medium relief, lower extinction angle), and/or plagioclase (colourless, low birefringence first order, polysynthetic twinning) -grid twinning is typical for microcline (K-feldspar), and is commonly associated with quartz and (sodic) plagioclase -perthitic textures - fine lamellae of albite (relief zero or negative) in a host of K-feldspar (stronger negative relief than albite); perthites are typical for K-feldspar (orthoclase, microcline, rare in sanidine).

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GLC 201 - Introduction to Optical Mineralogy Mineral Identification – A Beginner’s Guide to Identifying the Common Rock-Forming Minerals using Transmitted Light Microscopy

Is your mineral?: COLOURLESS? low relief? hole in slide, or basal section of ISOTROPIC? a non-isotropic mineral high relief? garnet NON-ISOTROPIC? UNIAXIAL? LOW RELIEF? positive? quartz negative?

nepheline, scapolite MODERATE RELIEF? positive, usually as laths muscovite, talc HIGH RELIEF? Positive-negative (“relief pleochroisme”, with distinct cleavages BIAXIAL? LOW RELIEF? may have polysynthetic twinning HIGH RELIEF? conchoidal fracture? idio- to subidioblastic, in a metamorphic rock? granular, with anomalous 1st order colours? up to 2 distinct cleavage directions fine-grained, granular, with very high δ

calcite

feldspars, cordierite

olivine Al2SiO5: andalusite, sillimanite, kyanite clinozoisite, epidote opx, cpx, wollastonite, all amphiboles other than hornblende zircon, monazite

ISOTROPIC?

green?

COLOURED? spinel

black? NON-ISOTROPIC? UNIAXIAL? (relief masked by mineral colour) pleochroic brown, green, orange (pseudouniaxial or very low 2V)? pleochroic pale brown to colourless? BIAXIAL? very pale green to colourless, very weakly pleochroic? pleochroic pale pink to pale green to colourless? pleochroic yellow-brown to colourless?

“opaque” (oxides, sulphides)

pleochroic distinctly green, brown, bluegreen?

hornblende, tourmaline

pleochroic brown to colourless, often euhedral, very high δ? reddish-brown needles, very high δ? pleochroic blue, purple? AMORPHOUS (no optic sign)

titanite

opaque interior, brown at thin edges

chromite

red

hematite

biotite phlogopite cpx, chlorite, chloritoid, muscovite, serpentine hypersthene staurolite

rutile riebeckite, glaucophane

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A Birefringence Primer Interference colours (birefringence) produced when the polariser and analyser are both “in” (crossed nicols, or crossed polars). For mineral diagnostic purposes, the colours refer to “maximum birefringence”, produced only when mineral grains are aligned perpendicular to their c-axis (i.e., many grains will show interference colours below the maximum, but the „average” or typical colour seen in a thin section is usually close enough. In strongly-coloured minerals, interference colours may be masked by the mineral colour; if the apparent interference colour looks “odd”, compare it with the actual mineral colour in plane-polarised light, to avoid confusion. Birefringence (nγ - nα = δ) “Order”

0 to 0.005 Lower 1st Order

black, grey, white + anomalous colours: “Berlin” or Prussian bluegrey, green-grey quartz, Common plagioclase minerals feldspar, microcline, cordierite, chlorite, clinozoisite, andalusite, nepheline, scapolite Some common problems: Interference Colours

0.005 to 0.01

0.01 to 0.015

0.015 to 0.018

Upper 1st Order “straw yellow”, red

Lower 2nd Order purple to blue, green

Upper 2nd Order yellow, orange/red

quartz, orthoclase (yellow or lower); sillimanite, opx, wollastonite

kyanite, amphiboles, cpx, biotite

muscovite, olivine

0.018 to 0.028

0.028 to 0.08

0.08 to 0.2

3rd Order

4th Order

and beyond

blue, green, yellow, red

pale green, pink

“bright brown”

talc

zircon

titanite, calcite, rutile

How can I tell if I’m looking at “1st order” red, or 2nd or 3rd order colours? Look at the edges of the grains, or along fractures, where they are thinnest; you should see fine rings of the lower interference colours (e.g., so if it‟s 1st order red, there will be no blue-indigo edges) Look at conoscopic figure; isochromes correspond to the same colour bands (usually these are subtle, but it works well in come cases, like calcite & biotite, for example) Lower order colours are “deeper”, higher order “brighter”, higher orders are pale, mixed, diffuse. 51

GLC 201 - Introduction to Optical Mineralogy If a mineral is black, does that mean it is automatically “1st order Black”? Not necessarily; it could also be o an opaque mineral (light is not transmitted through it), so it is also black under planepolarised light (i.e., with analyser “out”) o an isotropic mineral is always black under cross-polars; it has no birefringence and is therefore not “1st order” per se o basal-orientated sections (looking directly down the c-axis, in general) can be 1st Order black, but this is not the maximum birefringence for that particular mineral. o a hole in the slide, often the result of “plucking” of certain minerals, or where there are void spaces (not uncommon in volcanic rocks and sediments); it will have “very low relief” and no crystal shape or other properties. Got it narrowed down yet? Yes? – Good! Now go look up the detailed properties of the possible minerals, and match them to the observed properties & associated minerals and textures. No? – Is it similar to anything? (probably); There may be some common “similar” minerals not listed here in related mineral groups, other solid solution end-members, etc., so start with the mineral(s) it looks the most similar to, and work from there. Still stumped? Follow the identification Table for Common Minererals in Thin Sections. If still stumped....Ask a petrologist…

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Identification Tables for Common Minerals in Thin Section These tables provide a concise summary of the properties of a range of common minerals. Within the tables, minerals are arranged by colour so as to help with identification. If a mineral commonly has a range of colours, it will appear once for each colour. To identify an unknown mineral, start by answering the following questions: (1) What colour is the mineral? (2) What is the relief of the mineral? (3) Do you think you are looking at an igneous, metamorphic or sedimentary rock? Go to the chart, and scan the properties. Within each colour group, minerals are arranged in order of increasing refractive index (which more or less corresponds to relief). This should at once limit you to only a few minerals. By looking at the chart, see which properties might help you distinguish between the possibilities. Then, look at the mineral again, and check these further details. Notes (refer to notations and observations in the tables below): (i) Name: names listed here may be strict mineral names (e.g., andalusite), or group names (e.g., chlorite), or distinctive variety names (e.g., titanian augite). These tables contain a selection of some of the more common minerals. Remember that there are more than 4000 minerals, although 95% of these are rare or very rare. The minerals in here probably make up 95% of medium and coarse-grained rocks in the crust. (ii) IMS: this gives a simple assessment of whether the mineral is common in igneous (I), metamorphic (M) or sedimentary (S) rocks. These are not infallible guides - in particular many igneous and metamorphic minerals can occur occasionally in sediments. Bear this in mind, even if minerals are not marked as being common in sediments. (iii) Colour in thin sections (TS): the range of colours for each mineral is given, together with a description of any pleochroism. Note that these are colours seen in thin-section, not handspecimen. The latter will always be much darker and more intense than thin section colours. (iv) RI: the total range of refractive index shown by the mineral with this coulour is shown: This covers any range due to compositional variation by solid solution, as well as the two or three refractive indices of anisotropic minerals. (v) Relief : is described verbally, followed by a sign indicating whether the relief is positive or negative (ie greater or less than the mounting medium of the thin-section - 1.54). Minerals with refractive indices close to 1.54 have low relief, those with much higher or lower refractive indexes will have high relief. (vi) Extinction: angles are only given where minerals usually show a linear feature such as a cleavage and/or long crystal faces. For plagioclase feldspars (stippled) the extinction angles given are those determined by the Michel-Levy method (see a textbook for details). (vi) Int. Figure: this gives details of the interference figure. Any numbers given refer to the value of 2V (normally a range is given), followed by the optic sign. For uniaxial minerals the word "Uni" is given, followed by the sign. Your course may or may not have covered interference figures. If not, ignore this section! (vii) Birefr: Birefringence is described verbally. In some cases the maximum is given as a colour, in other cases you will need to cross-refer to an interference colour chart. (viii) Twinning etc.: a few notes about twinning, or other internal features of crystals may be given. If no twinning is mentioned, then the phenomenon is not common in thin section, but this does not mean that it NEVER occurs. (ix) Notes: general tips on appearance, occurrence and distinguishing features. May include indication of whether the mineral is length fast or slow - again a feature not covered in all courses - but a useful and easily-determined property. 53


Tables for Common Minerals in Thin Section

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bisectrix

dominatrix

optical dominatrix

optical indicatrix

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Introduction to Optical Mineralogy

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