Transmitted Light
it may be a mica (biotite, muscovite, phlogopite, lepidolite, vermiculite), talc, pirophyllite or brucite.
It is a trigonal carbonate with typical “relief pleochroism” or “chagrin change". May be calcite, dolomite, siderite, magnesite or rhodocrosite. It is NOT possible to differentiate these carbonates from each other under the petrographic microscope.
Very common pyroxene. Monoclinic. Anhedral, equidimensional or prismatic grains. Perfect basal sections have 4 or 8 sides. 2 cleavages at ~90º in basal sections. Oblique extinction of 35-48º. ES(+). Twins are common. Hourglass texture occurs in augite-Ti. B(+), 2V = 25-61º. Pigeonite and diopside similar.
Common pyroxene. Monoclinic. Weak pale green pleochroism possible. 2 cleavages at ~90º in basal sections. Granular to short prismatic. Medium-high relief. In XPL, colors up to 2nd order orange. Oblique extinction of 38-48º, B(+), 2V of 58-62º. Very similar to augite, pigeonite and hedenbergite!
Very common. In volcanic rocks, it is hexagonal, octagonal or skeletal. In igneous/metamorphic rocks, anhedral. Orthorhombic, no cleavage. Many fractures with alteration products (gray = serpentine; red = iddingsite). In XPL, strong and vivid, colors, many different, up to 3rd order. Extinction //, ES(+/-). B(+/-), 2V = 46-98º
High relief, cleavage not visible, grains rounded, with a dark halo around them if in biotite or hornblende. Tetragonal. In XPL, intense 3rd to 4th order colors. Extinction paralell, ES(+), rare twins, frequent zonation. U(+), but a 2V of 10º is possible. Similar to monazite. Can become metamict (= isotropic).
High relief, cleavage normally not visible, may have pale colors (yellow, green). Monoclinic. Rounded, prismatic or wedge-shaped grains, may show dark halo around them. In CPL, intense 2nd and 3rd order colors. Parallel extinction, rare twins. B(+), 2V from 6-19°. Similar to zircon, epidote, etc.
Amphibole. Monoclinic, long prisms or acicular aggregates. Always colorless, if with greenish pleochroism it is actinolite. Basal sections with 2 cleavages at 56-124º. In XPL, 2nd order colors. Extinction 10-15º, ES(+),twins possible. B(-) with 2V from 85º. Occurs in marbles, mafic and ultramafic metamorphic rocks and blue schists. Wollastonite is similar, but its interference colors are lower.
Long prismatic crystals with fractures perpendicular to the elongation, acicular or in fibrous masses (fibrolite). Orthorhombic. Basal sections are square with diagonal cleavage. Extinction paralell (symmetrical in basal sections), ES(+), without twins or zoning. B(+), 2V from 21-30º. Typical of high P and T pelitic metamorphic rocks.
Prismatic, acicular, radiated or granular with pseudohexagonal sections. Monoclinic, one perfect cleavage. In XPL, intense and/or anomalous blue colors. Zoning common. Extinction paralell to cleavage or oblique of 30º. ES(+/-), B(+), 2V of 40-90º. Epidote and pumpellyite are similar.
Rare amphibole. Forms parallel to subradiate aggregates of prismatic crystals. Basal section form rhombs with 2 cleavages at 56 and 124º. May be pleochroic in pale green/brown. Monoclinic. Extinction 15-21º, ES(+), common thin polysynthetic twins, B(+/-), 2V of 68-97º.
Rare pyroxene. Monoclinic. In PPL, colorless, can be pleochroic in very pale green. High relief. In CPL, colors up to 3rd order. Oblique extinction, Twins common, B(+). Identical to augite and diopside, but restricted to metamorphic rocks of eclogite facies: eclogites, kimberlites, ophiolites, blueschists.
Very rare pyroxene. Monoclinic. Anhedral, subhedral, fibrous. Basal sections with 2 cleavages intersecting at 56-124º. In XPL, colors up to middle 2nd order. Oblique extinction (33-40º), simple twins possible. B(+) 2V=67-80º. Typical of high pressure, medium temperature metamorphic rocks
Crystals tabular, prismatic or radiated, can be in bow-tie patterns. Orthorhombic. Medium relief. Good {001} cleavage. In XPL, 2nd order or anomalous colors (blue or brown). Extinction parallel, undulating or in chess pattern. ES(+/-). Twins rare, zoning possible. B(+), 2V from 64-71°.
Small tabular euhedral crystals. Orthorhombic. Medium to high relief. Cleavages intersect at 67°. In XPL, colors from 1st order orange to 2nd order blue. Longitudinal sections show parallel extinction and ES(-). Basal sections with symmetrical extinction. Twinning possible. B(+) with 2V=76-87º.
