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14 Other: Graphite, Apatite, Monazite, Fluorite
14.1 Graphite
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Occurrence—Graphite is a widespread accessory mineral in schists, marbles, gneisses, and other metamorphic rocks. It is rare in igneous rocks, but is known from some S-type granites (derived by partial melting of sedimentary rocks).
Distinguishing Features—Graphite is an opaque mineral, appearing black in thin section. It has a metallic luster, and is black under reflected light.
In lower-grade metamorphic rocks graphite is commonly interlayered in sheets between micas or chlorite. When abundant, an entire mass of graphite may appear black. In higher-grade rocks, it typically occurs as small flakes or scattered specks. The crystals may appear as thin tabular prisms (like tiny ilmenite) when viewed edge-on. They may appear hexagonal or pseudohexagonal when viewed down the c-axis. Graphite has one good cleavage but it is rarely visible in thin section.
Similar Minerals—Graphite can be confused with magnetite, ilmenite, or other silver, gray, or black opaque materials, but is commonly extremely fine-grained and has a flaky habit. A wavy or irregular grain shape, or layering with micas, may distinguish this mineral from opaque oxides.
graphite
Optical Properties ■■ Hexagonal, opaque
Fig 14.1.1 Pelitic Schist
This biotite-sillimanite schist from the Beartooth Mountains, Montana, contains unusually coarse and abundant accessory graphite. The graphite appears as elongate opaque flakes intimately associated with biotite (PP). Some of the flakes are more than half a millimeter long. The biotite is brown (PP) and shows 2nd-order interference colors in XP. Sillimanite, in the center of the view, is relatively fine grained with high relief. The colorless, low-relief minerals are quartz and K-feldspar (PP), but it is difficult to pick out the K-feldspar, and some of the quartz appears isotropic in XP because the view is down its c-axis. FOV = 2 mm.
In most graphitic schists, like this one, graphite forms tiny black specks (actually flakes) only a few microns across. Imaging clearly requires high magnification (40x, here) and conoscopic illumination. This is why the image appears washed out. Other minerals include brown biotite, black ilmenite (as clumps with graphite) and quartz. FOV = 0.4 mm.
In this graphitic phyllite from Dutchess County, New York, tiny black flakes of graphite darken the fine-grained matrix of quartz and muscovite (PP). Quartz shows its normal gray-white interference colors, and the muscovite only gives a hint of 2nd-order colors in some places (XP). Several small flakes of chlorite are also present (anomalous blue interference colors in XP). FOV = 3.5 mm.
This graphitic schist from the Santa Catalina subduction complex, California, shows swirls of black graphite across what appears at first to be a fine-grained matrix of low-relief colorless quartz and albite, moderate-relief colorless muscovite, and high-relief colorless epidote. A few large clusters of titanite and some tiny grains of pale green chlorite are also seen. Crossing the polars, however, reveals 5 porphyroblasts of albite, with its 1st-order white interference colors, a single large porphyroblast of epidote with anomalous blue interference colors. Quartz shows its normal gray to white interference colors, muscovite has 3rd-order interference colors, and titanite displays pastel colors (XP). Albite porphyroblasts commonly lack twinning seen in compositionally intermediate plagioclase, and can easily be mistaken for quartz, but quartz generally does not form porphyroblasts or host inclusions of graphite. FOV = 1.6 mm.
Occurrence—Apatite is a common accessory mineral in many different igneous and metamorphic rocks, but it is often overlooked – in large part because grains are usually small and colorless. Collophane, a fine-grained variety of apatite is found in some sedimentary rocks .
Distinguishing Features—Apatite is most often colorless, less commonly brownish or reddish. Inclusions, especially of graphite, may give it a gray or black hue. Apatite has moderate relief.
Apatite may form coarse anhedral crystals but more typically occurs as small stubby subhedral to euhedral prismatic crystals with a hexagonal cross section. See, for example, Fig 14.2.1. Apatite displays white and light- to dark-gray interference colors.
Apatite has several poor cleavages that are rarely seen in thin section.
Similar Minerals—When coarse, apatite is similar to quartz (another uniaxial mineral) in many ways, but apatite is optically (-), and quartz is optically (+). Quartz has much lower relief than apatite, and apatite often forms tabular or prismatic crystals. Zoisite has higher relief than apatite. Nepheline and beryl have lower relief.
apatite
Optical Properties ■■ Hexagonal; uniaxial (-) ■■ ω = 1.634-1.651, ε = 1.631-1.646 ■■ δ = 0.019-0.05 ■■ Basal (tabular) sections are usually too small to observe interference figures ■■ Parallel extinction in long sections ■■ Longitudinal (prismatic) sections are length-fast, but basal (semi-hexagonal) sections are length slow
Fig 14.2.1 Norite
Plagioclase has many apatite inclusions in this norite (plagioclase-orthopyroxene gabbro). Apatite crystals typically forms small stubby prisms, as seen here, but their relatively high relief is distinctive. The high-relief darker colored grains are orthopyroxene. FOV = 1.s mm. Photos from Dr. Kurt Hollocher.
In this carbonatite from Cape Verde, greenish calcite surrounds abundant colorless, moderate-relief apatite (PP). In XP, apatite shows its maximum interference color – dark gray. Calcite displays its typical pastel colors. A low-relief colorless hole, near the center of this image (PP), is black in crossed polars. FOV = 7 mm.
This schist from NW India has particularly large, colorless, moderate-relief apatite (circled). Accompanying black ilmenite has thin, tan, high-relief titanite rims (PP). In XP, apatite has maximum interference colors of dark gray and parallel extinction (XP). FOV = 1.6 mm.
This ultrahigh- temperature gneiss from the North China Craton has abundant moderate-relief colorless apatite (circled) with high-relief pleochroic orthopyroxene, low-relief colorless quartz and K-feldspar, and opaque magnetite (PP). In XP, apatite shows a maximum interference color of moderate gray, orthopyroxene ranges up to 1st-order red, and quartz and K-feldspar are 1st-order gray to white. FOV = 2.5 mm.
This is a thin section of the Phosphoria Formation in Idaho. The sedimentary rock contains rounded ovoids of collophane (an amorphous phosphate material with composition equivalent to apatite) with minor quartz. The collophane has a dark color due to organic inclusions; the quartz is clear (PP). In the XP view, the collophane shows no interference colors; the quartz has its usual 1st-order gray and white colors. FOV = 3.5 mm.
The large, colorless, moderately high-relief apatite (circled) with hexagonal cross-section in this calc-silicate from NW India could easily be mistaken for garnet, but garnet nearby has higher relief and a little more color (PP). Because it is an end section, this apatite in XP is nearly (but not quite) black, in contrast with garnet that is completely isotropic (jet black). Quartz is a little yellow in XP because the section is a bit too thick. FOV = 3 mm.
Occurrence—Monazite is a common accessory mineral in many different igneous and metamorphic rocks, but is often overlooked or mistaken for zircon or titanite.
Distinguishing Features—Monazite contains radioactive elements that may produce brown pleochroic halos in biotite (Figs 14.3.1, 14.3.2), chlorite, hornblende, and rarely andalusite, and yellow or orange pleochroic halos in cordierite.
Crystals are small, colorless or faintly yellowish, and have very high relief. They are commonly tiny, roughly equant blobs (Figs 14.3.1, 14.3.2) with poorly-formed crystal faces, but larger crystals may be stubby prisms with parallelogram cross-sections. Monazite has one good cleavage that is rarely seen in thin sections.
Monazite displays very high-order interference colors unless grains are thin.
Similar Minerals—Monazite has similar optical properties to zircon, which also forms pleochroic halos. But monazite does not normally form squares or rectangles in cross-section like zircon, and it has inclined extinction. Additionally, zircons are commonly colorless and monazite usually has a faint yellow color. Staurolite is more distinctly pleochroic and has much lower birefringence. High-order interference colors sometimes cause monazite to be confused with epidote or titanite.
monazite
Optical Properties ■■ Monoclinic; biaxial (+) ■■ 2V = 6° to 19° ■■ α = 1.774-1.800, β = 1.777-1.801, γ = 1.825-1.849 ■■ δ = 0.051-0.049 ■■ Maximum extinction angle is around 10° in longitudinal sections but may be hard to measure due to irregular grain shape
Fig 14.3.1 Biotite Schist
A single small grain of monazite near the center of the image forms a pleochroic halo in surrounding biotite. Other minerals are brown biotite and colorless quartz. Photos from www.science.smith.edu. FOV = 1.7 mm.
Two grains of monazite (circled) from Great Smoky Mountains, North Carolina, form pleochroic halos in brown biotite (PP) and have high order interference colors (XP). Other minerals are high relief garnet (isotropic) clouded with minute inclusions, and low-relief, colorless plagioclase with 1st-order gray interference colors and slight chemical zoning (zoned from light gray in the core towards darker gray, then lighter gray on the rim). FOV = 1.6 mm.
This thin section from New Mexico is mostly andalusite that contains many quartz and (opaque) magnetite inclusions. Near its center there is a single colorless high-relief monazite crystal. The andalusite and quartz are both nearly colorless but the andalusite is grayish. In XP, the monazite shows 2nd-order colors, the andalusite is dark gray, and quartz is 1st-order gray to white. FOV = 1.6 mm
This is similar to the previous figure — another example from New Mexico of a colorless high-relief monazite crystal surrounded by a moderate-relief colorless andalusite grain with abundant inclusions of quartz and opaque magnetite (PP). In PP, slight reddish “staining” in the andalusite attests to radiation damage. In XP, the monazite shows 2nd-order colors, the andalusite is dark gray, and quartz is 1st-order gray to white. FOV = 1.6 mm.
Occurrence—Fluorite is found in veins and carbonate-hosted ore deposits. It is also an accessory mineral in limestone or dolomite, and in igneous and metamorphic rocks.
Distinguishing Features—Fluorite is one of the few relatively common isotropic minerals. It is clear to light-purple and has high relief. Crystals may be zoned and may show concentric rings of different colors. Pleochroic halos around inclusions are common.
Fluorite has four perfect (octahedral) cleavages. Two or three are visible depending on grain orientation.
Penetration twins are common but rarely visible in thin section.
Similar Minerals—Opal has no cleavage. Sodalite has less negative relief (n=1.48-1.495) and different cleavage.
fluorite
Optical Properties ■■ Cubic; isotropic ■■ n = 1.434
Fig 14.4.1 Granite
This view shows a metaluminous granite. The large grain in the center is fluorite; quartz surrounds it. A single grain of plagioclase is on the lower left. Altered grains on the right edge are probably K-feldspar but cannot be identified unambiguously in this view. The fluorite show a very light, but typical, purplish color in the PP view. The other minerals are colorless but turbid due to alteration. In XP, the fluorite is isotropic because it is cubic. Quartz and plagioclase show 1st-order white and gray colors, and the plagioclase shows twinning. Photos from Dr. Kurt Hollocher. FOV = 1.2 mm.
In 2010, China produced over 50% of the world’s fluorite, mostly from Hunan province, in southern China. This greisen (hydrothermally altered granite) from Hunan contains only three major minerals: high negative-relief colorless to purple fluorite, low-relief colorless quartz, and moderate-relief colorless muscovite (PP). In XP, fluorite is characteristically isotropic, quartz shows 1st-order gray to white colors, and muscovite has 3rd-order colors. Photos courtesy of Dr. Hao Yang, Jilin University.
Occurrence—Graphite is a widespread accessory mineral in schists, marbles, gneisses, and other metamorphic rocks. It is rare in igneous rocks, but is known from some S-type granites (derived by partial melting of sedimentary rocks).
Occurrence—Apatite is a common accessory mineral in many different igneous and metamorphic rocks, but it is often overlooked – in large part because grains are usually small and colorless. Collophane, a fine-grained variety of apatite is found in some sedimentary rocks .
Occurrence—Monazite is a common accessory mineral in many different igneous and metamorphic rocks, but is often overlooked or mistaken for zircon or titanite.
