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7 Garnet, Olivine, and Other Isolated Tetrahedral Silicates
7 Garnet, Olivine, and Other Isolated
Tetrahedral Silicates
7.1 Garnet
(Fe,Ca,Mg,Mn)3(Fe,Al,Cr)2Si3O12
Occurrence—Garnet occurs in a wide variety of metamorphic rocks and in some igneous rocks. Mg-rich garnet (pyrope) is typically found in ultramafic rocks; Fe-rich garnet (almandine) is common in mica schists and gneisses; Mn-rich garnet (spessartine) is found in granites and pegmatites; Ca-rich garnet (grossular or andradite) is found in skarns, calc-silicates, and marbles.
Distinguishing Features—Garnet is one of the few common isotropic minerals. It has high relief and no cleavage but typically fractures. Pyralspite garnets (solid solutions of pyrope-almandine-spessartine) are normally red, but the color may not show in thin section. Garnet may appear nearly colorless but usually is very pale pink or tan, sometimes red, brown, green, or gray; rarely darker. It is nearly always compositionally zoned, which can sometimes be seen optically, and commonly contains inclusions.
Garnet has several typical appearances in thin section. Irregular polygons or subhedral to anhedral crystals are common. Euhedral crystals are commonly six-sided (for example, Figures 7.1.1 and 7.1.2).
Similar Minerals—Spinel is also isotropic and may occasionally be confused with garnet. Perovskite (CaTi03), another isotropic mineral, resembles garnet but has much higher relief and is extremely rare.
garnet
Optical Properties ■■ Cubic – isotropic ■■ n = 1.71-1.87; high relief ■■ Typically colorless, very light-pink or tan ■■ No cleavage but fracturing is common ■■ Inclusions are common
Fig 7.1.1 Garnet Schist
This sample from near Poughkeepsie, NY contains a sea of fine-grained muscovite and quartz (with some minor graphite) and two large garnets. The garnets are euhedral and 6-sided, typical of low- to medium-grade metamorphic rocks. The flaky opaque mineral in the matrix is graphite. The large opaque crystals may be ilmenite. FOV = 3.5 mm.
This sample from near Poughkeepsie, NY, shows a large euhedral garnet (black in the XP view), fine-grained muscovite intermixed with some minor graphite, and a blade of chloritoid (twinned and showing good cleavage and gray interference colors in XP). A small flake of biotite is between the garnet and the chloritoid. The fractured garnet contains two large opaque flakes of an opaque (PP). FOV = 3.5 mm.
This schist comes from western Massachusetts. In the PP view, we see a conspicuous large pink fractured garnet. It is accompanied by lesser amounts of yellow pleochroic staurolite and brown pleochroic biotite. Muscovite and quartz appear as clear minerals with lower relief than the garnet and staurolite.There is probably some feldspar present, too, but it is hard to pick out. The opaque minerals include both magnetite and ilmenite, but we only know this because they have been analyzed with an electron microprobe. In XP, the garnet appears black; the staurolite shows 1st-order yellow and gray colors. The biotite show 2nd-order colors and birds eye extinction. The darkest brown biotite (lower left) contains zircon inclusions surrounded by pleochroic halos. Muscovite shows 2nd- and 3rd-order pastel colors. Quartz is 1st-order gray and white.
This schist from central Nepal contains a high-relief slightly pink garnet porphyroblast with spiral inclusion trails of colorless quartz and black ilmenite (PP). The matrix contains low-relief colorless quartz, moderate-relief colorless muscovite, and brown biotite (PP). In XP, the isotropic garnet contrasts with the 1st-order gray to white quartz to emphasize the spiral inclusion trails. Muscovite and biotite show 2nd-order colors (XP). The triangular-shaped, quartz-rich regions to the right and left of the garnet, called pressure shadows, are typical next to garnets in sheared rocks. FOV = 9 mm.
This single garnet from an aluminum-rich granite is distinctly pink (PP). Its high refractive index causes fractures and grain margin to stand out sharply as dark lines. Its isotropic character (no interference colors) distinguishes garnet from all other common minerals with high refractive indices. The bright cracks seen in cross-polarized light contain thin films of calcite. Photos from Dr. Kurt Hollocher. FOV = 3 mm.
In this schist from Southern Vermont, a large, high-relief, pinkish garnet with inclusions of colorless quartz and black ilmenite, is surrounded by fine-grained, colorless, low-relief quartz, brown biotite, and colorless, moderate-relief muscovite (PP). In XP, the garnet is isotropic, and the inclusions form a spiral trail in the garnet core, with a clockwise (dextral) rotational sense. Muscovite shows its typical high-order interference colors, quartz is 1st-order white, and biotite’s brown color masks is interference colors. FOV = 3.5 mm.
In this eclogite from Southern Switzerland, coarse crystals of pinkish, high-relief garnet are surrounded by fine- grained, greenish, moderate-relief omphacite (Na-rich, high-pressure pyroxene), and a few crystals of relatively low-relief, colorless phengite (high-pressure muscovite) (PP). Minor blue-green amphibole alteration and a few grains of quartz are present, too. The tiny opaque minerals are rutile. In XP, the garnet is isotropic, and has birefringent inclusions clustered in its core. Omphacite shows low interference colors. Phengite displays birds-eye extinction, high colors for some grains, but low colors (grays) for flakes oriented parallel to the plane of the section. FOV=8 mm.
This famous garnet peridotite from the western Alps, Italy, contains large crystals of high-relief colorless to slightly pink garnet. The garnet is surrounded by a brown rim of fibrous kelyphite (an alteration material containing multiple minerals, especially amphibole; PP). The matrix consists almost entirely of high-relief colorless olivine that is partially altered to low-relief colorless to tan serpentine (PP). A vein of serpentine also cuts across the garnet. In XP, the garnet is isotropic, olivine shows up to 2nd-order colors, and serpentine is 1st-order gray to white. FOV = 9 mm.
mostly a solid solution of Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite)
Occurrence—Olivine is a major mineral in many ultramafic and mafic igneous rocks. It is also found in metamorphic rocks including mafic gneisses and some marbles and calc-silicates.
Distinguishing Features—Olivine has very-high relief and is generally colorless, but Fe-rich varieties may be very faintly pleochroic yellow. Grains are typically equant and anhedral; euhedral grains in basalts may appear six-sided. Twinning is uncommon and poorly developed when present.
Olivine has one good and one poor cleavage that are rarely seen in thin sections. Irregular curved fractures are, however, common (for example, Figure 7.2.1). Olivine displays 2nd-order interference colors; typically pastel violet, blue, or green.
Olvine altering to iddingsite. Photo from www.alexstrekeisen.it.
Olivine may alter to iddingsite (distinctive red to yellow-brown mixture of fine-grained clay, oxides, and oxyhydroxides), serpentine, or chlorite.
Similar Minerals—Diopside is occasionally confused with olivine, but diopside has better cleavage, lower relief, and lower birefringence. Epidote has a different color, usually faintly pleochroic green or yellow-green, and often displays anomalous interference colors. Chondrodite can have similar relief and interference colors, but is usually pleochroic in brown or red-brown.
olivine
Optical Properties ■■ Orthorhombic – biaxial (+) or (-) ■■ 2V = 47-54° (Fe-rich) to 85-90° (Mg-rich) ■■ α = 1.641-1.835, β = 1.651-1.877, γ = 1.670-1.886 ■■ δ = 0.035-0.051, maximum colors are upper second order ■■ Extinction is parallel to cleavage and crystal outlines
Fig 7.2.1 Websterite
This olivine websterite xenolith from Cima Volcanic Field, California (Mojave National Preserve) shows one large grain of olivine (center with a major fracture) surrounded by clinopyroxene and orthopyroxene. The two pyroxenes cannot be reliably distinguished in these views, but they have cleavage whereas the olivine is cleavage free. FOV = 4.5 mm.
Large olivine crystal in a xenolith from the Cima Volcanic Field, California, with distinct 2nd-order blue interference colors. The olivine is surrounded by fine-grained magnetite, clinopyroxene and minor plagioclase, which is hard to identify at this magnification. FOV = 4.5 mm.
Phenocrysts (large crystals) of olivine, augite (Cpx), and plagioclase in a porphyry from near Fish Lake, Oregon, are surrounded by a fine-grained groundmass of volcanic glass and small mineral grains that cannot be identified but are most likely plagioclase. The phenocrysts are elongate laths of plagioclase, several more equant grains of olivine, and two grains of augite. In PP light all three are nearly colorless, but the augite (hard to pick out in PP) has a slightly greenish tint. In XP light, the olivine has 2nd-order pastel interference colors that vary within individual grains. The largest olivine grain, a quarter of the way down from the top (center of the photo) shows patchy red, violet, blue and green interference colors. The two largest augite grains are next to it – one up and to the right and one below and slightly left. Both show somewhat anomalous yellow-brown interference colors. The field of view is about 2.5 mm.
This basalt from Mauna Loa Volcano, Hawaii, contains large colorless phenocrysts of olivine (Ol), smaller grayish phenocrysts of augite (Cpx), and even smaller laths of plagioclase (Pl). These minerals are surrounded by a very dark groundmass of plagioclase, augite, ilmenite, and almost no olivine, but the grains are hard to see at this magnification (PP). The olivine is fractured at random and also along some weak cleavages. In contrast, an augite grain in the lower left displays two cleavages intersecting at 90° (PP). In the XP view, augite shows mostly 1st-order interference colors, with one grain showing low 2nd-order blue. Plagioclase has its usual 1st-order gray and white colors and shows twinning. FOV = 5 mm
This olivine porphyry comes from Boulder County, Colorado. It contains phenocrysts of olivine and smaller phenocrysts of augite. In the PP view, the olivine is clear and the augite is clear but covered with mottled green patches of chlorite. Some olivine rims appear slightly altered. The matrix, hard to discern, contain primarily elongate plagioclase lathes with small grains of augite, olivine, and magnetite. In XP, the olivine shows 2nd-order interference colors that are patchy within individual grains. Augite interference colors are mostly masked by the green color of the grains, but are also 2nd-order. FOV = 2.5 mm
This calcite marble is dotted by rounded forsterite crystals and equant diopside crystals. The forsterite is clear and the diopside almost clear – just slightly grayish – in the PP view. Two of the diopside grains are oriented so they display cleavages intersecting at 90°. The rock also has many small to tiny opaque graphite grains. In XP, the olivine crystals offer striking, 2nd-order colors, whereas the diopside grains display only 1st-order colors. FOV = 2.5 mm
Occurrence—Kyanite is primarily a metamorphic mineral found in medium- or high-pressure mica schists and gneisses, also occasionally in eclogites.
Distinguishing Features—Kyanite has very high relief. It displays one perfect cleavage and one good cleavage parallel to the long crystal dimension. Parting at nearly 85° to the length of a crystal may appear as another cleavage. It occurs as broad elongate tabs, laths, or blades, less commonly as narrow prisms. Crystals may appear bent. Simple twins are common; polysynthetic twins occur rarely.
Most kyanite is colorless, but it can be weakly pleochroic with colors ranging from pale yellow to pale grayish-blue or medium-blue. Rare samples may show stronger blue colors.
Kyanite displays upper-1st order interference colors, typically yellow but sometimes red.
Similar Minerals—Kyanite is similar to its polymorphs andalusite and sillimanite but can be distinguished by its cleavage and parting, higher relief, and inclined extinction (although the angle can be small). Orthopyroxene has a smaller 2V, parallel extinction, and much less distinct cleavage, clinozoisite has different cleavage, and lawsonite is biaxial (+) and has different cleavage.
kyanite
Optical Properties ■■ Triclinic; biaxial (-) ■■ 2V = 82° ■■ α = 1.711, β = 1.721, γ = 1.728, ■■ δ = 0.017, maximum interference colors are upper-2nd order. ■■ Extinction angle in long sections is 0° to 30°; in cross sections, extinction is nearly parallel.
Fig 7.3.1 Kyanite Schist
In this rock, two high-relief kyanite grains/aggregates consist of a large blade with distinct cleavage and a patchy mass showing no cleavage. The kyanite is surrounded by colorless, low-relief quartz. Quartz has low-order interference colors (black-gray-white), while the kyanite interference colors range up to 1st-order yellow. The upper left end of the kyanite blade is slightly bent, so is close to extinction, unlike the rest of the grain. Several flakes of muscovite (colorless with low relief and upper 2nd-order interference colors) are just to the right of the large kyanite blade. Field of view is about 1 mm.
This kyanite schist from near Flin Flon, Manitoba, has one large blade of kyanite at the center, surrounded mostly by (twinned) plagioclase, minor quartz, and flakes of pleochroic brown biotite. The interference color of the kyanite is typical 1st-order yellow. A single large garnet (isotropic) is on the right hand edge of the image. The plagioclase and quartz are difficult to tell apart at this magnification. The field of view is about 2.5 mm across.
This view includes mostly kyanite (high relief, cracked/cleaved/fractured) that has shattered during thin-section making. The kyanite shows good cleavage, and some grains are twinned. Its interference colors range up to 2nd-order yellow. Minor quartz occurs in the upper left and lower right corner of the image. The dark mineral, not quite isotropic in these views, is rutile. FOV ~ 1 mm.
In this specimen, gray high-relief kyanite occurs in three different optical orientations. It is surrounded by typical red-brown biotite and colorless quartz and plagioclase. In the XP view, the kyanite shows up to 1st-order orange interference colors; biotite shows typical mottled 2nd-order colors, and quartz and plagioclase show 1st-order grays and white. A few small opaque grains are also present. Photos from the Oxford Earth Sciences Image Store. FOV = 2.5 mm.
In this rock, several large kyanite crystals are light-tan to colorless (PP). They have high relief, and two show a strong cleavage parallel to their length. Relief is much higher than muscovite, which surrounds the kyanite. The kyanite crystals have interference colors up to upper 1st-order orange, much lower than the muscovite. Most grains yield slightly inclined extinction, as expected from kyanite’s triclinic symmetry. The bottom of the image is occupied by the edge of a large, high-relief, isotropic garnet crystal. Photos from Dr. Kurt Hollocher. FOV = 3 mm.
Here we see high-relief colorless, folded gray laths of kyanite in a schist from the Great Smoky Mountains, North Carolina. The kyanite is surrounded by moderate-relief brown biotite, low -relief colorless quartz, plagioclase (circled), and high-relief garnet. In XP, the kyanite shows undulose extinction (here evident as black bands) and interference colors that range from 2nd-order red and orange to 1st-order orange. Quartz and plagioclase show 1st-order gray to white interference colors, garnet is isotropic, and biotite shows intense 2nd-order colors. FOV = 8 mm.
Occurrence—Andalusite mainly occurs in low-pressure metamorphosed mudstones and, rarely, in granites that derive from metasedimentary rocks.
Distinguishing Features—Andalusite is mostly colorless, but often contains pleochroic pink or rose-red cores. It displays high-relief, low-order interference colors, and parallel extinction. Viridine is an uncommon green variety.
Andalusite crystals (PP). Photo from alexstrekeisen.it
High relief euhedral crystals with square or square-ish cross sections or coarse columnar aggregates are typical (the photo here and, for example, Fig 7.4.4). Two cleavages at about 90° can be seen in cross sections; one cleavage is visible in long sections. Cross sections commonly show twinning.
Inclusions of dark organic matter and quartz are common and may form symmetrical cross-like patterns termed chiastolite (Fig 7.4.4). Chiastolite often has color zoning, and end section cores appear darker than rims.
Similar Minerals—Andalusite is distinguished from sillimanite by being length fast (sillimanite is length slow), and having weaker birefringence, lower relief, and larger 2V. End-sections of sillimanite also show distinctive cleavage (lacking in andalusite). Kyanite has inclined extinction, higher relief, and distinct cleavage. Colored varieties of andalusite may resemble orthopyroxene, but orthopyroxene is length slow.
andalusite
Optical Properties ■■ Orthorhombic; biaxial (-) ■■ 2V = 83° to 85° ■■ α = 1.629-1.640, β = 1.633-1.644, γ = 1.638-1.651 ■■ δ = 0.009-0.011, maximum interference colors are generally 1st-order yellow ■■ Extinction is parallel in most sections; symmetrical to cleavage in cross sections ■■ Elongate crystals are length fast
Fig 7.4.1 Chiastolite Schist
This schist from Idaho contains two andalusite porphyroblasts in near perfect cross-section. Within them, abundant minute black graphite inclusions form cross patterns, and the andalusite cores are light pink. Andalusite that displays a cross is termed chiastolite. The matrix is fine-grained colorless quartz, feldspar, and muscovite, plus brown biotite and graphite (PP). Large blobs near the andalusite were once cordierite but are now highly altered. A XP image shows typical fine-grained muscovite alteration around the andalusite and in a cross-cutting fracture. The former cordierite is now pseudomorphed mostly by muscovite and chlorite. FOV = 3.5 mm.
This is an andalusite-garnet-biotite schist from Maine. In PP, colorless andalusite (And) crystals fill much of the image. They could be mistaken for quartz but show good cleavage. Garnet (Grt), biotite (Bt), quartz, and one end-on tourmaline (Tur) crystal are also visible. In XP, the andalusite grains display 1st-order gray colors. Biotite has birdseye texture. The tourmaline is black because of the end-on (c-axis vertical) view, and gives a centered uniaxial(-) interference figure in a conoscopic view. FOV = 5 mm.
Here we see a large, diamond-shaped andalusite crystal that has been mostly replaced by sericite (fine-grained muscovite), leaving only a “cross” of andalusite behind. In chiastolite, the cross may be highlighted by dark included organic matter, but not in this highly altered specimen. FOV ~ 3.5 mm.
In PP, portions of three andalusite (chiastolite variety) porphyroblasts are visible surrounded by a dark, graphite rich matrix of quartz, muscovite, and biotite. Graphite inclusions in the andalusite crystals are arranged in a typical chiastolite pattern, forming crosses in square end-view cross section. Two andalusite cleavages are faintly visible. In XP, a muscovite rim on the andalusite grains is easily seen because of muscovite’s bright interferences colors that contrast with the 1st-order gray colors of andalusite. FOV = 5 mm
This view shows a large andalusite with square inclusion cloud in its core, in a muscovite–biotite schist. Inclusion trails radiate from the corners of the square to form a chiastolite cross. A small garnet (isotropic) inclusion is below the square. The andalusite shows two {110} cleavages that intersect at about 90o. Andalusite’s birefringence is 1st-order white in this view, much like quartz and feldspars. But andalusite has higher relief and very different habit. Photos from Dr. Kurt Hollocher. FOV = 6 mm.
This andalusite schist from the Adamello contact aureole, Italy, contains porphyroblasts of moderate-relief, mostly colorless andalusite with distinctive reddish pleochroic cores (arrows; PP). The red color represents slight uptake of Mn3+ and is fairly common in andalusite. The coarse material is colorless muscovite and quartz, and brown biotite. Some colorless to gray/brown fibrous patches are fibrolite (fibrous sillimanite) (PP). Clots of high-relief fine-grained material are probably mostly sillimanite but are too fine-grained to identify with certainty. In XP, Andalusite and quartz show low 1st-order colors, muscovite and biotite have 2nd-order colors, and the sillimanite ranges up to 1st-order orange. FOV = 1.6 mm.
Occurrence—Sillimanite is a metamorphic mineral found in high-grade aluminous schists and gneisses.
Longitudinal- and end-views of sillimanite crystals.
Distinguishing Features—Sillimanite is usually colorless, and most typically appears as high-relief blades or needles. Cross sections appear square or diamond-shaped with a single distinctive diagonal cleavage. An examples can be seen in Fig 7.5.3. Coarse crystals show excellent cleavage parallel to the long dimension and have parallel extinction. Sillimanite is also found in fine-grained fibrous masses called fibrolite (Fig 7.5.2).
Sillimanite may show up to 2nd-order blue interference colors.
Similar Minerals—Sillimanite resembles andalusite (both have high relief and are colorless), but sillimanite is length slow (andalusite is length fast) and has higher birefringence and smaller 2V than andalusite. Kyanite has higher relief and 2V, but lower interference than sillimanite. Zoisite has lower birefringence but higher relief.
sillimanite
Optical Properties ■■ Orthorhombic; biaxial (+) ■■ 2V = 21° to 30° ■■ α = 1.659, β = 1.660, γ = 1.679 ■■ δ = 0.021 ■■ Extinction is parallel to faces in longitudinal sections; symmetrical to faces in cross sections ■■ Elongate crystals are length slow
Fig 7.5.1 Sillimanite Gneiss
This rock from near Kazabazua, Quebec, contains colorless needles of sillimanite (high relief and somewhat fractured), colorless quartz, reddish-brown biotite, and opaque and equant magnetite (PP). In the XP view, the quartz shows low-order white-gray interference colors, sillimanite ranges up to 2nd order purple-red, and biotite shows 2nd-order colors. FOV = 2.5 mm.
This sample, from near Poughkeepsie, New York, is dominated by a large yellow mass of staurolite on the left side (PP). The material to its right is mostly a mix of granular quartz and fine fibrous sillimanite (fibrolite). Some of the sillimanite shows upper 2nd-order interference colors but most is so fine grained that interference colors are hard to see. Minor muscovite is also present. FOV ~ 3.5 mm.
This granulite facies schist contains coarse sillimanite grains, both as elongate laths and diamond-shaped or square cross sections. Some of the cross-sections display a diagonal cleavage. The rock also contains brown biotite, high-relief isotropic garnets, and low-relief, low interference cordierite. Lower 2nd-order blue interference colors can be seen in the long sections of sillimanite. Photos from Dr. Kurt Hollocher. FOV = 3 mm.
This is a biotite–sillimanite–muscovite schist that contains wispy needles of fibrous sillimanite (fibrolite). The fibers are so thin that individual fibers show little birefringence, but thick stacks of them do, with interference colors up to 2nd-order blue, somewhat higher than kyanite and much higher than andalusite. Fiber extinction is parallel. The sillimanite has much higher relief than the muscovite around it. Sillimanite fibers may be included in many minerals, especially garnet and quartz, and can thus escape retrograde metamorphism. Photos from Dr. Kurt Hollocher. FOV = 0.6 mm.
Coarse moderate-relief, colorless sillimanite dominates this restite (the rest of a rock after a melt has been extracted), along with high-relief, pinkish garnet, orange tourmaline, and reddish-brown biotite (PP). Cross-sections of sillimanite are characteristically rectangular with an inclined cleavage. In XP, sillimanite’s interference colors range from gray for end-sections (perpendicular to the c-axis) to 1st-order red. The intense colors of tourmaline and biotite mask their interference colors, and garnet is isotropic. FOV = 3 mm.
This unusual rock from New Mexico is mostly dark-green tourmaline with some sillimanite. The sillimanite shows a range of habit – from fibrous tan mats to needles with diamond-shaped cross-sections. Low-relief, colorless plagioclase completes the assemblage (PP). In XP, sillimanite interference colors vary from 1st-order gray or yellow for end sections to 2nd-order blue for elongate needles. Plagioclase shows twins and 1st-order gray to white interference colors. FOV = 8 mm.
Occurrence—Staurolite is a metamorphic mineral common in medium-grade pelitic schists.
Poikiloblastic staurolite
Distinguishing Features—Staurolite has high relief and typically displays diagnostic pale- to strong-yellow pleochroism. It often is poikiloblastic, containing many quartz inclusions that give it a “Swiss cheese texture” (photo here and see Fig 7.6.5).
Euhedral to subhedral crystals with geometric shapes are common; short prismatic crystals may have well-formed, hexagonal cross sections. Figure 7.6.1 show an imperfect example. Penetration twins are common and lead to differences in extinction angle between different sides or ends of a crystal. Staurolite has one poor cleavage that is generally not observed.
Maximum interference colors are 1st-order yellow to red.
Similar Minerals—Staurolite is sometimes confused with tourmaline, but tourmaline is uniaxial. Epidote is lighter colored and more greenish than staurolite. Kyanite has similar relief, but usually has well-developed cleavage.
staurolite
Optical Properties ■■ Monoclinic, biaxial (+) ■■ 2V = 19° to 90° ■■ α = 1.739-1.747, β = 1.745-1.753, γ = 1.752-1.762 ■■ δ = 0.013-0.015 ■■ Extinction is virtually parallel in long sections; symmetrical in cross sections ■■ Staurolite is length slow
Fig 7.6.1 Staurolite Schist
This view shows a large somewhat moth-eaten euhedral yellow staurolite crystal from western Vermont, surrounded by a lower-relief mat of colorless muscovite and scattered opaque graphite (PP). Staurolite displays low-order interference colors (the view is nearly down an optic axis) while the muscovite shows upper order pastels of various hues. FOV = 3.5 mm.
In this specimen, radiating high-relief yellow, staurolite crystals are surrounded by low-relief, colorless quartz. Interference colors are barely 1st-order yellow. FOV = 3.5 mm.
This sample, from near Poughkeepsie, New York, is dominated by a large yellow mass of staurolite on the left side (PP). The material to its right is mostly a mix of granular quartz and fine fibrous sillimanite (fibrolite). Some of the sillimanite shows upper 2nd-order interference colors but most is so fine grained that interference colors are hard to see. Minor muscovite is also present. FOV ~ 3.5 mm.
This staurolite schist contains both brown biotite and clear muscovite. The euhedral staurolite crystals show a typical yellow color (PP). A large, high-relief garnet crystal is present; it has many tiny inclusions and a rust-colored alteration rim. Colorless quartz and plagioclase are also present, although most of the plagioclase does not show lamellar twinning in XP. This specimen comes from Williamsburg, Massachusetts. FOV = 2.5 mm.
Here we see yellow pleochroic poikilitic staurolite; inclusions of (mostly) quartz and magnetite give it a sort of “Swiss cheese” texture (common for staurolite). Relatively small rounded pinkish high-relief garnets and smaller light-brown biotite flakes are also visible in the PP view. Clear grains on the left are quartz and untwinned feldspar, and several clear muscovite flakes surround the staurolite. In the XP view, muscovite’s 2nd-order pastel interference colors stand out. Staurolite’s 1st-order interference colors are masked by the color of the mineral. The quartz and feldspar display characteristic gray and white colors, but the feldspar shows brownish discoloring due to alteration; the quartz does not. Biotite has interference colors up to 2nd-order green and yellow and display a birds eye texture. This rock comes from south-central Vermont. FOV = 3.5 mm.
This beautiful schist (meta-paleosol?) from the Austrian Alps contains high-relief pleochroic yellow staurolite. It also contains high-relief blue to green pleochroic chloritoid, moderate-relief pale-green chlorite, moderate-relief colorless muscovite, low-relief colorless quartz, and opaque magnetite (PP). In XP, chloritoid shows 1st-order gray to anomalous blue interference colors, staurolite shows up to 1st-order yellow, chlorite is an anomalous khaki green, muscovite has 2nd-order colors, quartz appears 1st-order gray to white, and magnetite remains black. FOV = 1.4 mm.
Occurrence—Chloritoid is a metamorphic mineral common in low- and medium-grade Fe- and Al-rich schists and in high-pressure schists.
Distinguishing Features—Chloritoid appears typically in various shades of very distinctive blue, “military” green, and gray. Rare Mg-rich varieties are nearly colorless.
Chloritoid forms distinctive high-relief platy elongate or tabular crystals with pseudohexagonal cross sections. Inclusions can reveal an “hour glass” pattern. One perfect and one poor cleavage sometimes give two cleavages at near to 60°. Polysynthetic twins are common.
Chloritoid displays middle- to upper-1st order interference colors, sometimes anomalous pale blue because of the strong color of the mineral.
Similar Minerals—In PP, chloritoid’s greenish color may resemble the color of chlorite. Chloritoid’s relief, however, is much higher than chlorite’s, and usually some grains in a thin section are oriented so they appear blue. Green biotite looks superficially like chloritoid but has a smaller 2V, lower relief, higher birefringence, and a single good cleavage. Na-rich amphiboles may have blue colors like chloritoid, but have lower relief, have amphibole cleavage, and the blue color commonly grades into purple. In XP views, chlortoid’s low 1st-order birefringence is higher than most chlorite, but lower than micas. The strong green/gray color of the mineral, however, usually masks interference colors.
chloritoid
Properties and Interference Figure ■■ Monoclinic; biaxial (+) ■■ 2V = 36° to 63° ■■ α = 1.713-1.728, β = 1.719-1.734, γ = 1.723-1.740 ■■ δ = 0.010-0.012 ■■ Extinction is 0° to 20° to the long dimension ■■ Elongate crystals are length fast
Fig 7.7.1 Chloritoid Schist
This view shows a cluster of high-relief, pale-blue chloritoid crystals with low-relief muscovite. Interference colors of chloritoid are characteristically pale bluish gray and contrast with muscovite’s pastel yellow-green interference colors. FOV = 3.5 mm.
This sample is a schist from Monte Altissimo, Italy. All the high-relief grains with a greenish or bluish hue are chloritoid. The largest grain shows an obvious “butterfly” twin. A sea of fine-grained muscovite surrounds the chloritoid. Photos from alexstrekeisen.it. FOV = 2 mm.
This view shows a large garnet (isotropic) and a large blade of chloritoid (right side) with relatively high relief, good cleavage (seen in PP), twinning, and 1st-order interference colors (in XP). The groundmass includes fine-grained muscovite. A small flake of biotite is between the garnet and the chloritoid. The fractured garnet contains two flakes of opaque graphite. The field of view is 3.5 mm across. This sample comes from near Poughkeepsie, New York.
This schist from central Nepal contains large, high-relief blue-green plates of chloritoid that are more or less parallel to a fine-grained matrix of moderate-relief colorless muscovite, low-relief colorless quartz, and sparse, moderate-relief pale-green chlorite (PP). Porphyroblasts of high-relief, slightly pinkish garnet show spiral inclusion trails of quartz and opaque ilmenite (PP). In XP, chloritoid displays characteristic polysynthetic twinning and anomalous blue-gray interference colors. Garnet is isotropic, and quartz and muscovite are typical – low 1st-order gray to white interference colors for quartz, and high 2nd- to 3rd-order interference colors for muscovite. FOV = 3.5 mm
This phyllite contains relatively coarse chloritoid with blue to pale yellow pleochroism (PP) and low 1st-order interference colors (XP). A fine-grained muscovite–chlorite-quartz matrix surrounds the chloritoid. Prominent polysynthetic twinning in the largest chloritoid crystal shows as parallel stripes (XP view). Photos from Dr. Kurt Hollocher. FOV = 6 mm.
This specimen contains high-relief deformed chloritoid porphyroblasts. The chloritoid displays gray-green to blue pleochroism (PP). In XP, chloritoid displays its typical 1st-order colors, largely masked by its blue color, as well as polysynthetic twins and some ductile folding (wavy extinction). Other minerals include high-relief pinkish garnet (isotropic), low-relief, pale-green chlorite, and low-relief, colorless fine-grained white mica and quartz (PP). The mica is either muscovite and/or paragonite (an Na-muscovite), but these cannot be distinguished optically. This rock is from the central Alps. FOV = 8 mm.
Occurrence—Titanite, sometimes called sphene (an old name referring to the diamond/sphenoid shape of euhedral crystals) is an often overlooked accessory mineral in many igneous and metamorphic rocks. It has also been identified in a few sedimentary rocks.
Euhedral titanite crystal displaying chocolate brown interference colors (XP)
Distinguishing Features—Titanite tends to occur as small accessory grains. Perhaps its most distinguishing characteristics are very high relief and high-order (off the chart) interference colors. Even when grains are small, the mineral stands out. There are few common minerals that display such high relief and high-order interference colors.
Euhedral crystals with diamond, wedge, or lenticular shapes are common and diagnostic (a perfect one is seen in the photo here but see alsoFig 7.8.1). Distorted hexagonal shapes or elongate parallel-sided tabs also occur. Less commonly titanite manifests as larger high-relief anhedral grains.
Titanite has one good cleavage, rarely seen. Several directions of parting may be mistaken for cleavage. Twins, sometimes polysynthetic, may be present. Rarely it can cause pleochroic halos.
Titanite may be nearly colorless but commonly displays a light-brown or greenish color. When colored, it may be pleochroic in brown, yellow, or green.
Although it has very high birefringence, interference colors may be anomalous due to the color of the mineral. Titanite’s interference colors typically appear “chocolate brown” and are hard to discern, but usually some pastels show (Fig 7.8.1).
Similar Minerals—Monazite (a rare-earth element phosphate) is another common accessory mineral, and also has high relief. Monazite, however usually occurs in much smaller grains and has lower birefringence. Epidote has lower relief and birefringence and greater 2V than titanite. Rutile is normally much more strongly colored. In high-pressure rocks rutile commonly forms cores inside titanite rims. Calcite has similarly high interference colors, but has much lower relief.
titanite
Optical Properties ■■ Monoclinic; biaxial (+) ■■ 2V = 23° to 50° ■■ α = 1.885-1.921, β = 1.896-1.927, γ = 1.993-2.081 ■■ δ = 0.108-0.160, interference colors are so high-order that they may be hard to discern ■■ Cross sections display symmetrical extinction but, due to strong dispersion, titanite may show flashes of orange and blue and not go completely extinct
Fig 7.8.1 Granite
This granite from South Carolina contains large crystals of brown high-relief titanite (PP). The characteristic shape in this and the next figure gives rise to titanite’s commercial name, sphene, which comes from the Greek root spheno, meaning wedge. The rest of the rock consists of brown biotite and low-relief colorless quartz, plagioclase and K-feldspar. In XP, titanite has super-high pastel interference colors, biotite’s brown color largely masks its 2nd-order colors, and quartz and feldspar are 1st-order gray to white. K-feldspar shows characteristic tartan (cross-hatch) twinning, and plagioclase has characteristic polysynthetic twinning. FOV = 3.5 mm.
This typical blueschist from Syros, Greece, contains wedge-shaped, very high-relief colorless to tan titanite (arrows). Other minerals include moderate-relief, light-blue to lavender glaucophane, moderately low-relief colorless muscovite, and high-relief colorless to light-yellow epidote (PP). Small, amber-colored rutile crystals occur as inclusions in some titanite. In XP, titanite’s high pearly interference colors resemble calcite interference colors, glaucophane displays up to 1st-order orange, muscovite has intense 2nd-order colors, and epidote shows mostly upper 1st- or low 2nd-order colors. FOV = 1.5 mm.
This eclogite, from Sonoma County, California, contains small scattered, high-relief titanite crystals surrounded mostly by a sea of pale green omphacite (clinopyroxene). The titanite crystals are irregular elongate grains, and some have acute terminations. One large euhedral garnet and several flakes of colorless white mica (phengite) are also present. Titanite’s interference colors are of such high order that they are hard to discern. The isotropic garnet is slightly altered along its edges to chlorite with anomalous interference colors. The omphacite displays up to 1st-order red interference colors. In a few places, where higher-order colors can be seen, the omphacite is being replaced by hornblende. The field of view is about 1.5 mm across.
A number of high-relief titanite grains are visible in PP, some inside the yellow circle. The titanite is more difficult to see in XP because of the very-high, pale interference colors. Hornblende (blue-green) and biotite (brown) stand out in PP surrounded by colorless plagioclase and colorless quartz. The plagioclase is not twinned. FOV = 1.25 mm.
Occurrence—Zircon is a widespread accessory mineral in all rock types. It is easily overlooked because it generally occurs as tiny grains.
Distinguishing Features—Zircon crystals are typically very small and uncolored, both in a rock matrix and when included in other minerals. Zircon is radioactive, containing uranium and thorium. It is often only noticed when included in biotite or chlorite, where its radioactivity produces dark circles or patches, called pleochroic halos, in the surrounding grain.
When grains can be seen, zircon appears uncolored or, less-commonly, pale brownish or greenish. It may form small prismatic crystals with pyramidal terminations or small slender rounded grains. Zircon grains show very-high relief and extreme birefringence (up to 4th-order interference colors). Zircon has one poor and one good cleavage that are rarely seen in thin section.
Similar Minerals—Zircon is sometimes confused with apatite when fine grained, but zircon has much higher birefringence and relief. It is similar to monazite, but zircon has parallel extinction; monazite does not. Additionally, monazite is commonly a more yellow color. Zircon may be mistaken for epidote or titanite, which also have high/extreme birefringence, and can be distinguished only by crystal shape (square cross-sections) or parallel extinction.
zircon
Optical Properties ■■ Tetragonal; uniaxial (+) ■■ ω = 1.925-1.931, ε = 1.985-1.993 ■■ δ = 0.060-0.062, interference figures show many isochromes ■■ Extinction is parallel to the long dimension ■■ Elongate crystals are length slow
Fig 7.9.1 Cordierite Gneiss
This view shows large brown biotite grains with cordierite and orthopyroxene from near Sioux Lookout, Ontario. The largest grain of biotite contains one conspicuous pleochroic halo (dark brown ellipse) around a small inclusion that is likely zircon (PP). In the XP view, the biotite shows up to 2nd-order green interference colors. The included zircon grain displays concentric high-order colors but they are hard to see because of small grain size. FOV = 2 mm.
This granite from from Bhutan contains high-relief prismatic zircon and moderate-relief colorless apatite, together with brown biotite and colorless quartz and plagioclase (PP). In XP, zircon shows 2nd-order colors, while apatite is 1st-order dark gray (XP). The dark color of biotite masks its interference colors, but bird’s-eye texture is evident. Quartz and feldspar show 1st-order gray to white interference colors; polysynthetic twinning in plagioclase is also apparent. FOV = 1.6 mm.
This highly-magnified view shows high-relief, blocky zircon enclosed in brown biotite in a metaluminous granite. Small colorless apatite crystals are scattered around the zircon. The zircon is surrounded by a dark zone created by radiation damage (seen in PP and in XP). Zircon birefringence typically produces 3rd- or, sometimes 4th-order interference colors. In this grain interference colors are concentric, reflecting differences in grain composition and/or radiation damage. Photos from Kurt Hollocher. FOV=0.6 mm.
Here we see high-relief, slightly pinkish zircon (circled) surrounded by dark greenish-brown biotite in a coarse granite from the northwest Himalaya. Colorless, low-relief quartz and altered K-feldspar are also present on the right side of the view. In XP, zircon shows 3rd-order interference colors. Quartz and K-feldspar show 1st-order gray to white colors, and the biotite interference colors are masked by biotite’s intense color. FOV=1.6 mm.
This ultrahigh-temperature gneiss from northern China has definitely seen better days, but the small high-relief zircons (circled) survived well (PP). Low-relief colorless feldspar and altered brown biotite didn’t fare so well, and high-relief garnet is highly fractured (PP). In XP, zircon shows upper 2nd-order interference colors, garnet is isotropic, quartz and feldspar are 1st-order gray to white (except for orange clay alteration), apatite is low 1st-order gray, and biotite’s intense color masks is interference colors. FOV = 2.5 mm.
Like many granites, this one from northwest India contains abundant small accessory mineral grains: high-relief pinkish zircon, moderate-relief colorless apatite, and high-relief titanite and epidote. Larger grains are low-relief colorless quartz and plagioclase (PP). In addition to its prismatic habit and color, zircon’s high-order interference colors distinguish it from apatite (1st-order gray colors), titanite (super high interference colors), and epidote (2nd-order colors) (XP). Quartz and plagioclase show their typical 1st-order gray to white interference colors. Twinning in plagioclase and development of myrmekite (circled near top of photo) are also apparent in XP. FOV = 1.6 mm.
Occurrence—Garnet occurs in a wide variety of metamorphic rocks and in some igneous rocks. Mg-rich garnet (pyrope) is typically found in ultramafic rocks; Fe-rich garnet (almandine) is common in mica schists and gneisses; Mn-rich garnet (spessartine) is found in granites and pegmatites; Ca-rich garnet (grossular or andradite) is found in skarns, calc-silicates, and marbles.
Occurrence—Olivine is a major mineral in many ultramafic and mafic igneous rocks. It is also found in metamorphic rocks including mafic gneisses and some marbles and calc-silicates.
Occurrence—Kyanite is primarily a metamorphic mineral found in medium- or high-pressure mica schists and gneisses, also occasionally in eclogites.
Occurrence—Andalusite mainly occurs in low-pressure metamorphosed mudstones and, rarely, in granites that derive from metasedimentary rocks.
Occurrence—Sillimanite is a metamorphic mineral found in high-grade aluminous schists and gneisses.
Occurrence—Staurolite is a metamorphic mineral common in medium-grade pelitic schists.
Occurrence—Chloritoid is a metamorphic mineral common in low- and medium-grade Fe- and Al-rich schists and in high-pressure schists.
Occurrence—Titanite, sometimes called sphene (an old name referring to the diamond/sphenoid shape of euhedral crystals) is an often overlooked accessory mineral in many igneous and metamorphic rocks. It has also been identified in a few sedimentary rocks.
Occurrence—Zircon is a widespread accessory mineral in all rock types. It is easily overlooked because it generally occurs as tiny grains.
