3 Micas and Other Sheet Silicates

Bird’s-Eye Extinction
Some sheet silicates (notably biotite and muscovite, but also others) commonly show bird’s-eye extinction under crossed polars. It’s named after bird’s eye maple, and appears as a pebbly texture, especially when a mineral grain nears extinction. This video discusses bird’s-eye extinction and how it can be used to help identify minerals in thin section.

3.1 Serpentine

Mg3Si2O5(OH)4

Occurrence—Serpentine is most commonly a secondary mineral in mafic and ultramafic igneous rocks, generally forming by alteration of olivine or pyroxene. It is typically associated with fine-grained magnetite or chromite (see for example Figure 3.1.2). Serpentine is also found in marbles, associated with carbonates, forsterite, dolomite, or magnesite.  It may occur as replacements for individual grains, in massive aggregates, or in veins. A rock that consists mostly of serpentine is a serpentinite (which is the state rock of California).

Serpentine vein in soapstone (XP)

Distinguishing Features—Serpentine has three polymorphs: antigorite, lizardite, and chrysotile. They cannot be definitively distinguished without crystallographic or spectroscopic analysis, but different forms can sometimes have slightly different properties. Antigorite is micaceous or flaky, and is commonly in foliated or scaly masses. Lizardite is finer grained and may display a net-like pattern and undulatory extinction. Chrysotile is especially distinctive as fibrous masses in veins or mats. Antigorite has one perfect cleavage. Cleavage is not visible for other serpentine minerals.

Serpentine is generally colorless to green. It may be slightly pleochroic from clear to pale green to yellow green. It has low birefringence (maximum interference colors are 1st-order yellow).

Serpentine is normally an alteration product, often as pseudomorphs after mafic minerals. It typically appears as an aggregate of small anhedral crystals and may exhibit a coarse “alligator skin” texture (Fig 3.1.4).

Similar Minerals—Chlorite is typically biaxial (+), more pleochroic, and displays lower or anomalous interference colors. Micas have much higher birefringence, and muscovite is generally not pleochroic, whereas biotite is much more strongly colored. Brucite is uniaxial, rare, and displays anomalous interference colors. Fibrous amphiboles have higher refractive indices and birefringence than chrysotile.

serpentine

Optical Properties
■■ Monoclinic; biaxial (-) or (+)
■■ 2V variable
■■ α = 1.50-1.55, β = 1.50-1.60, γ= 1.50-1.60
■■ δ = 0.006-0.014, maximum interference colors are normally 1st-order gray or white, more rarely 1st-order yellow.
■■ Interference colors may be somewhat anomalous if crystals have a strong green color
■■ Interference figures are often hard to obtain due to fine grain size
■■ Extinction is parallel to cleavage
■■ Length slow

 

Fig 3.1.1 Serpentinite

This serpentinite from Eden Mills, Vermont, consists mostly of fine-grained serpentine with some opaque magnetite. Most of the serpentine shows 1st-order gray and white colors but some shows 1st-order yellows. A few serpentine clusters are radiating splays. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.1.2 Serpentinite

This is a serpentinite from Windsor County, Vermont. Most of the material on the right side of the view is a mix of serpentine and magnetite. The left side contains talc. The serpentine displays only 1st-order interference colors; the talc displays extremely high-order interference colors. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.1.3 Serpentinite

This serpentinite from Windsor County, Vermont, is mostly clear to light-green antigorite (PP), perhaps the most common variety of serpentine. The high-relief material forming a sickle-shaped patch near the center of the view is talc. In XP, serpentine has 1st-roder gray to white interference colors. The talc has such high-order interference colors (pearl white), they are hard to see. Some patchy magnetite (opaque) can be seen near the right edge of the image.

■■ Larger photos: PP XP

Fig 3.1.4 Serpentine

Yellow-green serpentine veins have replaced colorless olivine in this rock (PP). Serpentine displays its usual 1st-order interference colors, whereas olivine displays 2nd-order colors, mostly red-orange (XP). FOV = 2 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP

Fig 3.1.5 Altered Peridotite

In this highly altered peridotite, serpentine is fine-grained greenish-gray in veins and patches (PP). The colorless grains are highly cracked pyroxene and possibly olivine. Several grains of dark-brown (almost black) spinel are present; the largest is labeled Sp. Olivine shows only 1st-order gray interference colors and the pyroxene grains show up to 1st-order orange colors (XP). The spinel is isotropic. FOV = 15 mm. Photos modified from virtualmicroscope.org.

■■ Larger photos: PP XP

 Fig 3.1.6 Altered Harzburgite

Antigorite in this metamorphosed harzburgite (opx- and ol-rich peridotite), is platy, unlike fibrous chrysotile, colorless to pale green, and in some ways resembles chlorite. Like chlorite, antigorite can have anomalous Berlin blue or brown interference colors, but, unlike chlorite, both colors can appear in the same rock at different orientations. Both can be seen in this thin section. The interference colors for chlorite, in contrast, depend on composition –  almost entirely anomalous blue (Fe-rich), anomalous brown (Mg-rich), or anomalous violet (intermediate composition), irrespective of orientation. Photos from Dr. Kurt Hollocher. FOV = 1.2 mm.

■■ Larger photos: PP XP

Fig 3.1.7 Kimberlite

This altered kimberlite from Colorado contains abundant low-relief brownish serpentine and variable-relief brownish calcite, both as fine grains in the matrix and as coarse clots (PP). Large masses of serpentine pseudomorph olivine. Fine-grained brown biotite, too small to discern in this view, lends color to the matrix (PP). In XP, serpentine has characteristic 1st- order gray interference colors and a veiny texture, and calcite is pearly. FOV = 8.5 mm.

■■ Larger photos: PP XP

 Fig 3.1.8 Serpentinite

In PP light, a casual look at this thin section of a serpentinite from coastal California might appear to show a single black oxide grain – is there anything else present? But XP reveals the beauty of this rock, with its folded bands of light- to dark-gray serpentine. Reexamine it in PP, and closer inspection reveals serpentine’s slight variations in shading, from colorless to extremely pale-green or tan. FOV = 3.5 mm.

■■ Larger photos: PP XP

 

 

3.2 Clay Minerals

Clays have highly variable compositions; most are hydrated aluminosilicates

Occurrence—Clay minerals form as products of weathering and are abundant in a variety of sedimentary rocks (Figs 3.2.1, 3.2.2, 3.2.3, 3.2.4) and in soils. They also form in secondary veins, and as fine grained alteration products on or in coarser grained minerals (Fig 3.2.6).

In thin sections clay minerals may appear as replacement patches in feldspars, micas, and in many other silicates. Incipient alteration to clays can turn normally clear minerals cloudy (Fig 3.2.5). Generally, clay minerals are too fine-grained to see individual grains.

Distinguishing Features—In sedimentary rocks, clays appear as fine clasts, with up to 2nd-order interference colors. In altered rocks, clays normally appear as aggregates of fine scales or shards, or just discoloration in or on other minerals. Clays are normally colorless in thin section but fine grain size may give them a darker gray or brown hue.

Similar Minerals—All the many different clay minerals are similar, and many other secondary minerals, such as zeolites, may be mistaken for clays. Glauconite, sometimes considered a clay mineral, but more appropriately grouped with micas, is a conspicuous green mineral found in some sandstones.

Optical Properties—Most clay minerals are monoclinic; biaxial (- or +), although a few are triclinic, orthorhombic, or even hexagonal (uniaxial). Indices of refraction are normally 1.5-1.6, so relief is low. Some coarse clays may display up to middle 2nd-order interference colors, but kaolinite has very low birefringence. Fine-grained clay masses may appear nearly isotropic. In most cases, grain size is to small to permit interference figures.

 

Fig 3.2.1 Shale

This graphite-rich Maplewood Shale from near Rochester, New York, consists mostly of clay minerals (dark) and quartz (colorless in PP). The clay mineral grains are, however, too fine-grained to see clearly.  FOV = 1.5 mm.

■■ Larger photos: PP XP

Fig 3.2.2 Shale

This shale from the Llewellyn Fm. in St. Clair, Pennsylvania contains clay (gray to brown) and quartz (colorless). FOV = 1.5 mm.

■■ Larger photos: PP XP

Fig 3.2.3 Illite in Mudstone

This mudstone contains large stacks of colorless, moderate-relief illite in a fine-grained matrix whose dark color is likely also due to clays (PP). The white area at the bottom is a hole in the thin section. In XP, illite shows upper 1st- to lower 2nd-order colors. Note the small scale: FOV = 0.350 mm. Photos courtesy of Dr. Hao Yang, Jilin University.

■■ Larger photos: PP XP

Fig 3.2.4 Kaolinite in Mudstone

This mudstone contains large stacks of colorless, moderate-relief kaolinite in a fine-grained brownish matrix whose color is likely due to clays (PP). A high-relief grain of bluish polishing compound is also present. In XP, kaolinite shows 1st-order colors. FOV = 1.4 mm. Photos courtesy of Dr. Hao Yang, Jilin University.

■■ Larger photos: PP XP

Fig 3.2.5 Altered Granite

In this highly altered granite from St. Austell, England, K-feldspar and plagioclase crystals have altered to fine-grained brown clay, probably kaolinite. Quartz grains resisted alteration and remain colorless. In XP, clay masses appear nearly isotropic, quartz has typical gray to white interference colors, and small flakes of remnant biotite show up to 2nd-order colors. FOV = 35 mm.

■■ Larger photos: PP XP

Fig 3.2.6 Saponite in Altered Basalt

This altered basalt from Jilin province, northeast China, contains bright green saponite (a type of smectite clay), formed from olivine, which appears as relict high-relief slightly colored grains in a sea of low-relief colorless plagioclase (PP). In XP, the dark-green color of saponite persists, relict olivine shows 1st- to-2nd order colors, and plagioclase is 1st-order gray to white with pronounced twins. Note the small scale: FOV = 0.350 mm. Photos courtesy of Dr. Hao Yang, Jilin University.

■■ Larger photos: PP XP

3.3 Pyrophyllite

Al2Si4O10(OH)2

Occurrence—Pyrophyllite is found in low- and medium-grade metamorphosed shales; more rarely as a secondary mineral in felsic volcanics and as a vein or ore mineral in metabauxites. It is typically fine grained and difficult to identify in thin sections of common rocks.

Distinguishing Features—Pyrophyllite looks much like muscovite. It is colorless in thin section, has one perfect cleavage, and has up to 3rd-order interference colors.

Patches or aggregates with curved, radial, or distorted crystals are common. Fine-grained aggregates are also possible. Twinning may be present but is difficult to see. In some cases, pyrophyllite displays bird’s-eye extinction (link to video).

Similar Minerals—Muscovite and talc may be confused with pyrophyllite, but both muscovite and talc have a smaller 2V, and muscovite crystals are normally not curved or distorted. Pyrophyllite is always associated with quartz. Talc is never found in aluminous rocks. Kaolinite has lower birefringence.

pyrophyllite

Optical Properties
■■ Triclinic; biaxial (-)
■■ 2V = 53° to 62°
■■ α = l.554, β = 1.588, γ= 1.601
■■ δ = 0.045-0.048. High birefringence yields up to 3rd-order interference colors; sections parallel to cleavage show white or gray colors.
■■ Cleavage flakes show a centered biaxial figure. Relief is low to moderate. Extinction is parallel (or nearly so) to cleavage and long dimension
■■ Slow direction is parallel to good cleavage.

 

Fig 3.3.1 Pyrophyllite Schist

This rock form Graves Mountain, Georgia, contains mostly pyrophyllite. Pyrophyllite is similar to muscovite and talc (colorless in PP, bright 2nd-order interference in XP), but may form needles or splays as can be seen here. FOV = 30 mm. The images are from rockPTX.com.

■■ Larger photos: PP XP

3.4 Talc

Mg3Si4O10(OH)2

Occurrence—Talc is a major mineral in some low-grade metamorphic rocks, including marbles and ultramafic rocks. It is a secondary mineral in altered mafic and ultramafic igneous rocks where it may be found with serpentine.

Distinguishing Features—Talc is a colorless sheet silicate that displays 3rd-order pastel and patchy interference colors. The pastel colors and rock associations are keys to identification.

Talc typically forms platy masses or fine to coarse fibrous aggregates with subparallel alignment. Bent fibers and shreds are common.

Talc has one good cleavage.

Similar Minerals—Talc resembles muscovite and pyrophyllite, but has higher-order interference color, lower 2V, and does not occur in aluminous rocks that are likely to have muscovite or pyrophyllite. Pyrophyllite is associated with quartz, whereas talc-quartz assemblages are rare. Brucite is uniaxial (+). Gibbsite is biaxial (+), has lower birefringence, and has oblique extinction.

talc

Optical Properties
■■ Monoclinic; biaxial (-)
■■ 2V = 0° to 30°
■■ α = 1.539-1.550, β = 1.589-1.594, γ = 1.589-1.596
■■ δ = 0.046-0.050. Maximum interference colors are upper 3rd-order
■■ Cleavage flakes typically show low-order interference colors. Relief is low.
■■ Extinction angle is commonly just a few degrees, so extinction is nearly parallel to cleavage
■■ Length slow

 

Fig 3.4.1 Verde Antique

This is a variety of a facing stone called verde antique. The right side of the view contains a mix of colorless to light-green antigorite (serpentine) and magnetite. The left side is all talc. The serpentine shows low 1st-order colors that are slightly anomalous in some places. The talc shows 3rd-order colors but they are somewhat masked by the coarse texture of the mineral grains. This rock comes from Rochester, Vermont. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.4.2 Talc-Actinolite Schist

This is a classic talc-actinolite schist from the Santa Catalina subduction complex, California. It contains three colorless minerals – moderately low-relief talc, moderately low-relief chlorite, and moderate-relief actinolite (PP). The large end section of actinolite show good amphibole cleavage. We know this is actinolite rather than tremolite because it is green in hand sample. In XP, talc has distinctive 2nd-order interference colors, chlorite is 1st-order gray, and actinolite ranges up to low 2nd-order colors. FOV = 3 mm.

■■ Larger photos: PP XP

 

 

Fig 3.4.3 Talc Schist

Talc and tremolite appear somewhat similar (PP) in this schist from the Adirondack Mountains, New York. In XP view, however, the  finer-grained talc displays high-order pastel interference colors, while coarse blades of tremolite have high 2nd-order colors. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.4.4 Soapstone

The talc in this soapstone (metamorphosed harzburgite) is easy to identify in hand sample because of its soapy feel. In thin section, talc is colorless and resembles muscovite or colorless phlogopite (PP), but displays higher-order interference colors (XP). Photos from Dr. Kurt Hollocher. FOV = 3 mm.

■■ Larger photos: PP XP

Fig 3.4.5 Talc Marble

This talc ore from the Mojave Desert is mostly moderate-relief, strongly twinned calcite with swirls of fine-grained, brownish talc (PP). A relatively large grain of talc is circled. In XP, talc shows high-order colors, more pastel than muscovite (not present in this thin section), while calcite is pearly, with distinct twins. FOV=3 mm.

■■ Larger photos: PP XP

Fig 3.4.6 Talc-Garnet-Chloritoid Schist.

This rock contains obvious large garnet porphyroblasts in a matrix of fine clear talc, blue glaucophane, and green chloritoid. The garnets contain many small rutile inclusions (PP). In the XP view, the garnets are isotropic, talc shows bright 3rd-order colors, the glaucophane displays 2nd-order colors, and the chloritoid mostly 1st-order grays. These photos come from jm-derochette.be. The rock is from Austria. FOV = 7 mm.

■■ Larger photos: PP XP

3.5 Biotite

K(Mg,Fe)3AlSi3O10(F,OH)2

Occurrence—Biotite occurs in a wide variety of igneous and metamorphic rocks; it is quite rare in sedimentary rocks, but appears occasionally in immature sedimentary rocks.

Distinguishing Features—Biotite is typically brown to yellow in thin section, but red or green hues are not uncommon. It is pleochroic and shows one excellent cleavage.

Biotite with bird’s eye texture and a pleochroic halo (XP)

Biotite grains have mottled, or bird’s-eye, extinction in many/most thin sections (link to video). They may contain pleochroic halos around small included crystals of zircon or monazite.

In thin section, biotite commonly occurs as elongate rectangular flakes with a single cleavage. More equant plates or tabs are common too, depending on grain orientation.  Rarely, flakes show hexagonal symmetry. Flakes commonly combine to form masses or aggregates, and can be bent. Twins may be present but can be difficult to see.

Similar Minerals—Biotite is distinguished from hornblende by cleavage (one cleavage, not two), color (usually more brown or red brown than green), relief (lower), and a smaller extinction angle. Amphiboles have two cleavages at about 60° and 120°, biotite has one cleavage. Biotite is difficult to distinguish from stilpnomelane, a brown brittle mica. Stilpnomelane occurs in low-temperature, high-pressure rocks where biotite is unstable, and gives a pseudouniaxial figure and has higher relief.

Color—Most biotites are pleochroic (X = colorless, light tan, pale greenish brown, or pale green, Y=Z= brown, olive brown, dark green, or dark red-brown). Phlogopite (Mg-biotite) is colorless to pale brown and occurs in Fe-poor rocks such as calc-silicate rocks.

biotite

Optical Properties
■■ Monoclinic (pseudohexagonal); biaxial (-)
■■ 2V = 0° to 33° for most biotite
■■ α = 1.571-1.616, β = 1.609-1.696, γ = 1.610-1.697; moderate relief
■■ δ = 0.028-0.081, strong interference colors range up to 2nd-order red, but they may be hard to see due to the color of the mineral
■■ Flakes lying on cleavage show very low-order colors, largely masked by the mineral’s color
■■ Interference figures may appear uniaxial depending on composition
■■ Normally parallel extinction, but extinction angle may be up to 3°
■■ Distorted plates show wavy extinction
■■ Cleavage direction is slow

 

 

Fig 3.5.1 Biotite-Cordierite Gneiss

This view shows biotite in a high-grade cordierite-orthopyroxene rock from near Sioux Lookout, Ontario The biotite is pleochroic in browns and tans and contains one conspicuous pleochroic halo (burn mark) around a small inclusion that is probably zircon (PP). The cordierite and orthopyroxene are colorless. In XP, biotite shows up to 2nd-order green interference colors. The biotite grain in the lower left has one good cleavage, but the large grain in the center does not because the view is more nearly perpendicular to the cleavage. It does, however, show a partially developed “bird’s eye” pattern (XP). FOV = 2 mm.

■■ Larger photos: PP XP

Fig 3.5.2 Biotite-Muscovite Schist

In this mica schist from Manhattan, NY, biotite is pleochroic (various shades of green to brown; PP). The near-vertical elongate flake near the center, and a few other grains, colorless in PP light and showing higher-order interference colors, are muscovite. Quartz occurs in the lower right. The long flaky habit is typical of biotite (and muscovite) as are the mottled extinction and interference colors (XP view). FOV = 4.5 mm.

■■ Larger photos: PP XP

Fig 3.5.3 Biotite Gneiss

This biotite gneiss from Worcester County, Massachusetts, contains strung-out brown biotite flakes and colorless quartz and microcline; the microcline shows incipient alteration to clays (PP). In the XP view, the biotite shows up to mid 2nd-order interference colors; the quartz and feldspar show their typical low 1st-order colors. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.5.4 Biotite Hornfels

This biotite hornfels from Riverside County, California, contains very small flakes of brown biotite mingled with colorless quartz. Some of the quartz grains are considerably larger than the biotite grains, but overall the rock is too fine-grained to see the minerals clearly at this magnification. FOV = 1.5 mm.

■■ Larger photos: PP XP

Fig 3.5.5 High-T Gneiss

Biotite in this ultrahigh-temperature gneiss from Brazil is distinctly reddish because of high Ti content (PP). High-relief pleochroic orthopyroxene and low-relief colorless feldspar and quartz are also present (PP). In XP, biotite shows 2nd-order colors, orthopyroxene shows 1st-order yellow color and parallel extinction, and feldspar and quartz are characteristically 1st-order gray to white. Both biotite and feldspar show myrmekitic (wormy) intergrowths with quartz, which is typical of high-temperature growth from a partial melt. Sample courtesy of Dr Shujuan Jiao, Chinese Academy of Sciences. FOV = 5 mm.

■■ Larger photos: PP XP

Fig 3.5.6 Phlogopite Marble

This marble from Pickens County, Georgia , contains abundant calcite (with obvious twins) and one large flake of light-tan phlogopite (Mg-rich biotite) on the right edge. Phlogopite is generally less strongly colored than Fe- rich varieties of biotite. A few small  high-relief apatite crystals are scattered around the phlogopite. The phlogopite shows high 2nd-order interference colors; the calcite shows extremely high pastel colors (pearl white; XP). FOV = 3.5 mm.

■■ Larger photos: PP XP

3.6 Muscovite

KAl2(AlSi3)O10(F,OH)2

Occurrence—Muscovite is found in felsic to intermediate igneous rocks, in a wide variety of metamorphic rocks and, less commonly, in immature sedimentary rocks. A fine grained variety of muscovite (sericite) forms as an alteration product of feldspars and a few other minerals. High pressure muscovite is called phengite.

Distinguishing Features—Muscovite is almost always colorless mica with moderate relief. A rare Cr-bearing variety of muscovite, called fuchsite, has a diagnostic green color.

Muscovite forms tabular crystals, rectangular flakes, or laths when coarse; scaly aggregates or shreds when fine. It generally shows one excellent cleavage. Twinning is common but hard to detect.

Muscovite displaying bird’s eye (sometimes called “pebbly”) texture (XP)

Muscovite has 2nd-order interference colors, up to yellow or red.  Under crossed polars it commonly displays bird’s-eye extinction (link to video).

Similar Minerals—Some other rarer white micas (paragonite and margarite) and lepidolite have similar properties and cannot be distinguished in thin section. Chemical analyses, XRD, or other tests are needed to distinguish them. Some clay minerals, too, appear superficially like muscovite in thin section.

muscovite

Optical Properties
■■ Monoclinic; biaxial (-)
■■ 2V = 35° to 50°
■■ α = 1.552-1.770, β = 1.582-1.619, γ = 1.588-1.624; moderate relief may vary with stage rotation
■■ δ = 0.036-0.054; up to 2nd-order yellow or red interference colors; sections parallel to cleavage show 1st-order white
■■ Cleavage fragments yield a nearly centered Bxa figures.
■■ Generally parallel extinction or a very small extinction angle
■■ Cleavage direction is length slow

 

Fig 3.6.1 Muscovite-Biotite Granite

This is a two-mica granite from near Concord, New Hampshire. In the PP view, clear minerals are muscovite, quartz, and microcline. Two small flakes of brown biotite can also be seen. In the XP view, muscovite shows upper 2nd-order pink-purple colors. The other minerals show 1st-order grays and white. The large grain right-of-center show unusual twinning in one direction. It is topped by patches of sericite (fine muscovite). Quartz has wavy extinction, and several small grains of plagioclase show albite twinning. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.6.2 Biotite-Muscovite Schist

This is a view of the Manhattan Schist from New York City. In the PP view, colorless grains are mostly quartz and muscovite, with lesser amounts of plagioclase and kyanite. The kyanite has high relief and stands above the other minerals. Brown biotite is also present. In the XP view, quartz, plagioclase and kyanite show typical 1st-order interference colors; the plagioclase is twinned. Muscovite displays up to high 2nd-order colors. Biotite’s interference colors are also 2nd-order but are hard to see because of the color of the mineral. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.6.3 Greenschist

This graphitic schist from central Nepal contains sheaves of clear wavy muscovite, high-relief colorless to yellow staurolite filled with tiny black specks of graphite, large crystals of high-relief, slightly pink garnet, and brown biotite. The more equant clear minerals are fine-grained low-relief colorless quartz and plagioclase (PP). In XP, the muscovite and biotite both show variable 2nd-order colors. Staurolite has low 1st-order colors, garnet is isotropic, quartz and plagioclase are 1st-order gray to white. Plagioclase in this rock does not show twinning, and so looks like quartz. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.6.4 Biotite Schist

In this biotite schist from Gilpin County, Colorado, the mica is mostly muscovite (colorless) with subordinate biotite (brown), easily distinguished in PP. In XP, the two micas have similar interference colors, but biotite’s are somewhat masked by the overall brown color of the mineral. Equant quartz grains are evident in XP. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.6.5 Kyanite Schist

In this muscovite-biotite-kyanite schist, the two micas (colorless muscovite and brown biotite) have flaky shapes and show good cleavage (PP). Both also display bird’s eye texture in XP. The kyanite (almost all on the left side of the image) has high relief  and displays 1st-order interference colors. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.6.6 Muscovite-Chlorite Schist

Moderate-relief colorless muscovite is intensely folded in this schist from New Mexico. In contrast, the moderate-relief, pale-green pleochroic chlorite shows no deformation (PP). Low-relief, colorless quartz is abundant. In XP, the folding in muscovite creates variations in extinction angle that delineate fold patterns; chlorite shows a beautiful anomalous purple interference color, while quartz is characteristically 1st-order gray to white. FOV = 8 mm.

■■ Larger photos: PP XP

Fig 3.6.7 California Eclogite

This famous eclogite from Jenner, California contains moderate-relief colorless muscovite, high-relief slightly pink garnet, high-relief pale green omphacite (a Na-rich clinopyroxene), moderately high-relief pale-blue glaucophane, and high-relief composites of amber-colored rutile surrounded by thin rims of colorless titanite (PP). Muscovite normally doesn’t coexist with pyroxenes, except in high-pressure rocks like blueschists and eclogites. Titanite rimming rutile is a common texture in blueschists and eclogites. In XP, muscovite shows high 2nd-order colors and birds eye texture, garnet is isotropic, omphacite shows 1st-order, and glaucophane has intense low 2nd-order colors. The high-reliefs and strong colors of rutile and titanite mask their pastel interference colors. FOV = 1.4 mm.

■■ Larger photos: PP XP

Fig 3.6.8 Fuchsite Schist

This schist from Santa Catalina Island, California, contains moderate-relief, blue-green pleochroic fuchsite, which is a rare chromium- bearing muscovite. Fuchsite is sometimes described as having an “emerald” hue in thin section, but its greenish color is often faint (PP). It typically forms scaly masses instead of sheets showing good cleavage like most normal muscovite. In XP, the fine-grained fuchsite shows typical muscovite high-order interference colors. The rest of the rock is mostly quartz, which is slightly yellow in XP, indicating the section is a little too thick. FOV=3mm.

■■ Larger photos: PP XP

 

3.7 Stilpnomelane

K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O)

Occurrence—Stilpnomelane is common in low-grade high pressure metamorphic rocks. It is generally associated with chlorite, muscovite, garnet, actinolite, calcite, or epidote.

Distinguishing Features—Stilpnomelane is a high-relief dark brown, pale yellow, or green mica-like mineral with strong pleochroism. Its strong brown color and radiating habit are distinctive.

Stilpnomelane may form long elongate grains that in some cases seem to pile up on top of each other. It also occurs in sheaves or radiating aggregates/splays.

Stilpnomelane commonly shows two cleavages. It has one good and one fair cleavage that intersect at 90° in properly oriented crystals.

Similar Minerals—Stilpnomelane is hard to distinguish from biotite, but stilpnomelane generally has a deeper brown color. Stilpnomelane’s basal cleavage is less perfect than biotite’s and is intersected by an additional cleavage. Bird’s-eye extinction is absent or weakly present, and stilpnomelane’s birefringence may be greater than that of biotite.

stilpnomelane

Optical Properties
■■ Monoclinic; biaxial (-)
■■ 2V≈0; biaxial interference figure appears uniaxial
■■ α = 1.543-1.634, β = 1.576-1.745, γ = 1.576-1.745, but varies with composition
■■ δ = 0.030-0.110; birefringence varies with composition
■■ Maximum interference colors may be up to 6th-order, but the strong mineral color tends to mask them
■■ Relief is high, and may vary with stage rotation
■■ Extinction angle with the principle cleavage is close to 0°
■■ Principal cleavage direction is slow

Fig 3.7.1 Stilpnomelane Schist

This greenschist contains brown stilpnomelane, green chlorite, and colorless quartz (PP). In the XP view, the stilpnomelane has 2nd- to 3rd-order colors that barely show because of the color of the mineral. The quartz has gray to white interference colors, and the chlorite displays anomalous interference colors – dark inky blue to black. FOV = 1 mm. The photos come from the Oxford Earth Sciences Image Store.

■■ Larger photos: PP XP

Fig 3.7.2 Stilpnomelane Schist

All the brown and dark-colored needles are stilpnomelane, surrounded by colorless quartz (PP). Some of the stilpnomelane clusters have a bow-tie texture. Stilpnomelane’s color is so strong that it masks any interference colors in the XP view. FOV = 7 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP

 

 

Fig 3.7.3 Stilpnomelane Schist

This stilpnomelane schist, from the same quarry as Figure 3.7.2, contains mostly brown stilpnomelane and colorless quartz, but also a few small blue needles of glaucophane in the lower left part of the image  (PP). The glaucophane is difficult to find in the XP view. FOV = 3.5 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP

3.8 Chlorite

(Mg,Fe,Al)6(Si,Al)4O10(OH)8

Occurrence—Chlorite is common in low- and medium-grade metamorphic rocks, both as a primary mineral and as a secondary mineral forming after biotite and other mafic silicates in both igneous and metamorphic rocks.

Distinguishing Features—There are many varieties of chlorite and properties vary somewhat.

In thin section, chlorite occurs as massive patches, flaky or scaly crystals, or thin to thick tabs. It is a common replacement for other minerals. In some cases it is pseudohexagonal. Polysynthetic twinning is common but generally hard to see.

Chlorite showing anomalous purple interference colors (XP). Photo from https://open.agh.edu.pl/.

Color is variable but normally light greenish or army green, more rarely shades of brown or yellow. Chlorite is normally pleochroic (although it may be faint) and displays low 1st-order or anomalous interference colors (blue, green, purple, or brown).

Chlorite has one excellent cleavage, but the cleavage may not be apparent in small grains.

Similar Minerals—Epidote, zoisite, pumpellyite and a few other minerals commonly exhibit anomalous interference colors, but are much higher relief, and generally don’t show green pleochroism. Chorite’s generally light-green color, form, and associations usually serve to distinguish it.

chlorite

Optical Properties
■■ Monoclinic; biaxial (+) or (-)
■■ 2V = 0° to 50°
■■ α = 1.571-1.588, β = 1.571-1.588, γ = 1.576-1.597
■■ δ = 0.006-0.020; interference colors are very weak
■■ Anomalous interference colors (blue, purple, or brown, depending on Fe-Mg composition) are typical
■■ Rarely chlorite shows 1st-order light-yellow colors
■■ Relief is moderate to high
■■ Extinction angle varies from 0° to about 10°
■■ Crystals showing cleavage are length fast

 

Fig 3.8.5 Blueschist

This blueschist from Panoche Pass, CA, contains pale green chlorite, blue glaucophane, colorless white mica flakes (bottom center and right) and many small high-relief epidote grains (PP). In XP light the chlorite shows anomalous interference colors, the glaucophane shows 2nd-order interference colors that are not generally typical of this mineral. The epidote grains are hard to pick out in XP but the mica flakes show typical mottled 2nd-order interference colors.  FOV = 2.5 mm.

■■ Larger photos: PP XP

 

Fig 3.8.1 Chlorite Schist

This greenschist contains abundant green chlorite, lesser amounts of colorless muscovite, and high-relief epidote and titanite (PP). In XP, chlorite shows anomalous army-green colors, muscovite has its typical bright 2nd-order interference colors, and epidote displays very intense colors. Titanite’s colors are so pastel and washed out that they can barely be seen. FOV = 2 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP

Fig 3.8.2 Paragonite Schist

This is the Gassetts schist from southeastern Vermont. The colorless minerals on the left are paragonite and quartz. Paragonite is a white mica nearly identical to muscovite; the distinction between the two cannot be made in thin section. Light-green chlorite and pinkish garnet are also prominent. The paragonite contains inclusions of opaque magnetite, and the garnet contains small inclusions of reddish rutile. In the XP view, the chlorite shows anomalous interference colors, the paragonite typical 2nd-order colors, and the garnet is opaque. FOV = 3.5 mm.

■■ Larger photos: PP XP

Fig 3.8.3 Altered Amphibolite

Chlorite commonly replaces primary minerals in rocks. Here, a diamond-shaped hornblende crystal has been replaced by chlorite (light green PP, greenish gray interference colors XP), epidote (clear PP, high-order interference colors XP) and magnetite (opaque). The other replacement materials are colorless feldspar and quartz. FOV = 1.5 mm.

■■ Larger photos: PP XP

Fig 3.8.4 Metamorphosed Shale

This low-grade  metashale from ner Poughkeepsie, NY, contains chlorite (light green in PP; anomalous blue in XP) surrounding a large flake of graphite. Most of the other colorless material is muscovite and quartz; tiny grains of graphite are scattered throughout in the wavy layers. FOV = 1.5 mm.

■■ Larger photos: PP XP

Fig 3.8.5 Blueschist

This blueschist from Panoche Pass, CA, contains pale green chlorite, blue glaucophane, colorless white mica flakes (bottom center and right) and many small high-relief epidote grains (PP). In XP light the chlorite shows anomalous interference colors, the glaucophane shows 2nd-order interference colors that are not generally typical of this mineral. The epidote grains are hard to pick out in XP but the mica flakes show typical mottled 2nd-order interference colors.  FOV = 2.5 mm.

■■ Larger photos: PP XP

 Fig 3.8.6 Amphibolite
Chlorite (Chl) is pale green in PP and aligned in the foliation plane with blue-green hornblende (Hbl) laths.  Colorless plagioclase and quartz are also visible along with a few grains of brown biotite.  Chlorite displays anomalous 1st-order gray-blue colors in a distinctive texture in XP in contrast to the higher 1st-order red-orange to blue hornblende colors.  The plagioclase is untwinned.  This rock comes from near Hanover, New Hampshire. FOV = 2.5 mm.■■ Larger photos: PP XP

3.9 Prehnite

Ca2Al2Si3O10(OH)2

Occurrence—Prehnite is a low-grade metamorphic mineral in mafic rocks (prehnite-pumpellyite facies) and also commonly occurs as an alteration product in basalts and other igneous rocks. It generally occurs in veins or vugs.

Distinguishing Features—Prehnite is colorless and has 2nd-order interference colors. It has one good cleavage. It commonly forms round globs or sheaf-like aggregates, but may show a “bow tie” or radiating structure. Fine polysynthetic twins in two directions might be seen in XP light.

Similar Minerals—Prehnite may be confused with lawsonite, but lawsonite has lower birefringence, higher relief, and different mineral associations (e.g., with glaucophane).

prehnite

Optical Properties
■■ Orthorhombic; biaxial (+)
■■ 2V = 65° to 69°
■■ α = 1.611-1.630, β = 1.617-1.641, γ = 1.632-1.669
■■ δ = 0.021-0.039; maximum interference colors are low to upper 2nd-order Relief is moderate to high
■■ Extinction is parallel; may be wavy

Fig 3.9.1 Altered Basalt

This colorless (PP) prehnite from an altered basalt from the Loanhead Quarry, Scotland, has high-order interference colors. Although prehnite in other rocks can show anomalous interference colors, these are typical 2nd-order colors. FOV = 2 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP

Fig 3.9.2 Altered Basalt

Colorless (PP) prehnite with opaque magnetite from an altered basalt at Oibicella, Italy. The interference colors (XP) are lower 2nd-order only, so are lower than the colors seen in Figure 3.9.1. FOV = 9 mm. Photos from www.alexstrekeisen.it.

■■ Larger photos: PP XP