GEOL 4500 - Sedimentary Geology

1/23/2018

GEOL 4500 - Sedimentary Geology

S P Grains Compared with igneous and metamorphic rocks, sandstones contain a much more limited suite of grains. Quartz is by far the most common grain, sometimes comprising all or nearly all of the grains in a sandstone. Feldspars can also be common, especially plagioclase and the potassium feldspars, orthoclase and microcline. Several types of lithic grains may also be present, including fragments of igneous, metamorphic, and sedimentary rocks. Sandstones may also contain accessory minerals, although they are not considered in classification. Although a great number of accessory minerals could be present in the right circumstances, only certain types are relatively common, such as micas (biotite, muscovite, and chlorite), heavy minerals (hornblende, zircon, tourmaline, apatite, and others), and opaque minerals (hematite, ilmenite, and magnetite). Quartz Quartz displays low first-order interference colors (gray and white), low relief, and a lack of cleavage and twinning. Some quartz grains display sweeping extinction. Quartz grains can be composed of single crystals (monocrystalline) or multiple crystals (polycrystalline), (polycrystalline), which are fragments of quartzite or quartz veins. Although polycrystalline quartz is a rock fragment, it is commonly treated as quartz (and not as a lithic) in rock classification, owing to its chemical stability stability..

Quartz grains, showing low first-order interference colors.

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Grains of polycrystalline quartz, mixed with monocrystalline quartz grains.

Feldspar Like quartz, feldspar has low relief and low first-order interference colors (white to gray), and it can be easily misidentified as quartz where it lacks twins. Because so many feldspar grains are untwinned, it is common to have doubly-stained thin sections, in which the red stain is picked up by calcium-bearing grains (plagioclase) and the yellow stain is picked by potassium-bearing grains (microcline and orthoclase).

Pink-stained plagioclase and brown-stained potassium feldspar, plane-polarized.


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Same thin section as above; crossed-polars. Note calcite cement.

In this example, the potassium feldspar is not as intensely stained.

Same as previous thin section; crossed polars. Note brightly colored illite cement.


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Twinning is generally diagnostic of the type of feldspar, and for distinguishing it from quartz, which is not twinned. Unlike quartz, which has no cleavage, feldspar has cleavage, which can be manifested as planar and parallel sides, with planar fractures or parallel planes of inclusions. Whereas quartz tends to be clear in plane-polarized light, feldspar is often cloudy or light brown, owing to its greater tendency to be altered. In cross-polarized light, this alteration is often apparent by replacement with felted masses of sericite or illite, which have brighter high first-order interference colors, similar to muscovite.

Sericitization, with masses of small crystals with high first-order colors.

Plagioclase is distinguished from other feldspars by a single direction of micro-twinning (also called polysynthetic twinning), consisting of numerous parallel-sided twins. Zoning is also common and is best seen by variations in extinction angles and by the distribution of inclusions. The pink stain picked up by plagioclase is more pronounced towards the anorthite (Ca-rich) end-member; however, Ca-rich plagioclase is less resistant to weathering and therefore less common in sandstones than Na-rich plagioclase (albite).

Polysynthetic (micro) twinning of plagioclase.


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Microcline is the easier of the two alkali f eldspars to recognize, owing to its characteristic tartan (plaid) twinning, which consists of two nearly perpendicular directions of micro-twins (one according to the albite law, and one according to the pericline law). The twins are characteristiclly spindle-shaped, compared with the regularly parallel twins of plagioclase. Microcline also commonly undergoes exsolution to form perthite, an intergrowth of albite and potassium feldspar.

Tartan twinning in microcline. Muscovite and biotite in lower right.

Microcline twins characteristically taper or thicken and thin.


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Large gray grain in center displays tartan twinning disrupted by perthitic texture.

Orthoclase sometimes shows simple twinning (Carlsbad law), but is more commonly untwinned. As a result, orthoclase is quite easily mistaken for quartz. Although one can get an optical sign on the grain, the simplest solution is to get stained thin sections and look for the yellow stain.

Yellow-stained orthoclase with planar, cleavage-controlled sides.


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Same grain, crossed polars; note lack of twinning.

Lithic (rock) fragments Chert is microcrystalline quartz and therefore occurs as a mass of very fine crystals with low first-order interference colors (gray and white) and low relief.

Multiple chert grains showing low first-order colors, small crystal sizes.

Shale fragments are fine-grained sedimentary rock fragments. They are often brown, with silt-sized quartz grains and disseminated opaque iron-oxide or iron-sulfide (pyrite) grains. Because they are mechanically soft, shale fragments are commonly deformed during compaction. They are thus often confused with matrix and sometimes known as pseudomatrix.


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Dark and laminated shale clast, deformed by surrounding quartz grains.

Same thin section, crossed polars.

Volcanic fragments can be highly variable in appearance, but are commonly recognized by the presence of tiny blades of feldspar (with low first-order white and gray interference colors) within an exceptionally fine groundmass. Volcanic fragments can be mistaken for chert, but the elongate feldspar grains are typically diagnostic.


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Speckled volcanic grain on left, clear plagioclase on right.

Same thin-section, crossed polars. Note white plagioclase laths in volcanic grain.

Metamorphic fragments are commonly recognized by various types of schist or phyllite that display a strong planar fabric, often defined by stretched grains of quartz and aligned crystals of muscovite. Accessory minerals Muscovite grains are easily recognizable from their clear color in plane-polarized light, second-order interference colors, and well-developed cleavage. Muscovite is commonly deformed by compaction, causing it to wrap around other grains.


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Clear muscovite grain in plane-polarized light, with strong cleavage.

Same grain, showing second-order colors in crossed polars. Note nearby biotite.

Biotite grains are elongate, have prominent cleavage, and are dark brown to brownish-green in planepolarized light. Biotite grains commonly have numerous dark black spots in plane-polarized light, owing t o radiation damage. Biotite has 3rd-order to 4th-order interference colors, but these are commonly masked by the strong brown absorption colors. Biotite is also commonly deformed by compaction.


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Brown biotite with strong cleavage. Plane-polarized light.

Same grain in cross-polarized light.

Chlorite grains are elongate, have good cleavage, and are pleochroic from nearly colorless to dark green. Chlorite typically has first-order white or yellow interference colors, but anomalous colors like brown, blue, or purple are common. Chlorite can be confused with green (iron-poor) biotite, although brown biotite is more common. Like all micas, chlorite is commonly deformed by compaction.


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Comparison of chlorite (green) and adjacent (biotite), uncrossed polars.

Same thin section in crossed polars, note lower birefringence of chlorite.

Zircon grains are small, but have strikingly high relief and similarly striking bright high-order colors, up to third and fourth-order.


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GEOL 4500 - Sedimentary Geology Small zircon grain with high relief i n plane-polarized light.

Same grain under crossed polars, showing high-order colors.

Hornblende grains are pleochroic in plane-polarized light, varying from light green to dark green or brown as the stage is rotated. Hornblende has low to moderate birefringence and therefore displays upper first to lower second-order interference colors; however, the interference color is commonly obscured by the green to brown absorption color seen in plane-polarized light.

Green hornblende grain, plane-polarized light.


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Same grain, showing pleochroism (change in color) as stage i s rotated.

Same grain, crossed polars. Note surrounding quartz grains.

Glauconite is characterized by its green color and speckled appearance in plane-polarized and crosspolarized light. It commonly occurs as oval peloids, but it can also occur as infillings of fossils and as a cement.


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Green glauconite grains among quartz and pink-stained plagioclase grains.

Same grains, crossed polars. Note speckled texture of glauconite.

Chalcedony is characterized by its radial fibrous textures and low first-order interference colors. Chalcedony can occur as a grain and as a cement.

Matrix Matrix is clay and silt, and it therefore appears as extremely fine-grained material. Note that matrix typically has a darker, dirtier appearance than chert. Matrix can be difficult to distinguish from compacted and deformed shale clasts, which are sometimes called pseudomatrix.

Cement Cement precipitates onto existing grains or matrix and therefore, the oldest cements in a rock are those that are closest to the grains. In some cases, small radiating crystal may be visible next to grains, and in others, large crystals may occupy entire pores. Because cement grows in pore space, it necessarily occupies a smaller area of a thin section than do grains.


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There are many types of cements, but we will focus on the two most common (quartz and chert), plus the relatively common iron oxide cements (hematite, goethite, limonite). Quartz cement Quartz cement is common, but can be hard to detect because it commonly grows in optical continuity with quartz grains. In some cases, a thin coating of clay or iron oxides around the grain makes it easier to distinguish the cement from the grain. Where such coatings are absent, the rock may look like a metamorphic quartzite, with highly angular grains in a tightly fitted fabric. Thick coatings of clay or iron oxide may prevent quartz cement from growing syntaxially. Quartz cement can be sourced from biogenic silica (such as diatoms, radiolaria, or sponges), quartz dust, unstable silicate grains (e.g., pyroxene, plagioclase, amphibole, etc.), and pressure solution.

Note rim of iron oxides separating overgrowths from syntaxial grains.

Overgrowths can be difficult to distinguish from grains without iron coatings.

Carbonate cement


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Carbonate cement displays fourth-order interference colors of pale pink and green, as well as high relief. Calcite is the most common carbonate cement, but dolomite and siderite can also be important locally. Carbonate cement can occur as drusy spar , with small crystals growing on grains, passing into larger crystals of calcite in the center of pores, such as in the photograph to the left. Carbonate cement can also be poikilotopic , in which single large crystals grow and envelop many grains. Poikilotopic cement can be recognized as large areas of cement that are in optical continuity and therefore go extinct at the same time under crossed polars. The growth of calcite cement commonly displaces grains, giving rise to a fabric in which quartz grains apparently float within the cement. Calcite is frequently one of the first cements to precipitate. In marine sediments, calcite cement is commonly derived from the dissolution of biogenic carbonate.

Calcite cement characterized by high-order pink interference color.

Calcite cement commonly displaces grains, causing grains to be widely spaced.

Iron oxide cements Hematite is generally opaque in plane-polarized light. It may appear brick red in plane-polarized light if the thin section is cut thinner than normal (lower photograph). Hematite cement is particularly common in http://strata.uga.edu/4500/labs/silicipetrography/

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terrestrial sediments.

Large opaque void-filling hematite cement, overlying deformed shale clast.

Goethite cement can appear similar to hematite, but is generally more translucent and is also brick red in both crossed and uncrossed polars. Limonite cement is yellow to brown, and translucent. Weathering can convert originally hematite and pyrite cements to goethite and limonite.

Dark brown, nearly opaque limonite cement.


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Inserting the substage condenser can make the brown color of limonite apparent.

Thin rims of translucent brown limonite cement, locally opaque where thicker.

Other cements include chert, chalcedony, feldspars, clays, gypsum, anhydrite, celestite, and barite. Epoxy Epoxy can be confusing at first, particularly when it occurs because of a large open pore or because grains were removed during the process of making a thin section. Epoxy is speckled in plane-polarized light, but always opaque in crossed polars. There are also bubbles under crossed polars that have what looks t o be an interference cross.


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Large area of speckled epoxy, with four quartz grains at right. Pla ne-polarized.

Same view, crossed polars. Note bubbles with interference crosses.

Large epoxy-filled area of plucked grains in lower right.


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Same view in crossed polars. Large black areas are epoxy.

Return to the Siliciclastic Thin-Sections Lab Return to Syllabus


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GEOL 4500 - Sedimentary Geology

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