Transmitted Light
Most common mica, alters to chlorite, which is green in PPL.
PPL: Strongly pleochroic in brown (may be in green, yellow, reddish, yellow or red). Moderate relief. One perfect cleavage if not altered.
XPL: 3rd-4th order colors, masked by the strong mineral color. Paralell and mottled extinction. ES(+).
Images: Biotite in PPL, rotated 90 degrees.
Very common monoclinic nesosilicate found in many types of rock.
PPL: Brown color, may have gray or yellow tones, may have weak pleochroism. Granular, only sometimes with rhombic basal sections as in the images below. High relief. Fractured, 1 good cleavage, often absent.
XPL: Its color is almost the same as in PPL. Zoning and twins occur. B(+), (2V=17-56º). The strong color make it difficult to see the interference figure.
Images: Titanite in PPL, rotated 90 degrees.
Cubic garnet exclusive to alkaline volcanic rocks.
PPL: Strong brown color, may suggest pleochroism, but does not. Mineral generally in polygonal grains of high relief, without cleavage. May have inclusions.
XPL: Isotropic, completely black.
Images: Melanite garnet in PPL.
A somewhat rarer monoclinic amphibole, of intermediate and acidic igneous rocks.
PPL: Strong pleochroism between deep brown and reddish brown. Basal sections with 2 cleavages at 56º and 124º. Moderate to high relief.
XPL: Extinction 0-19º. Intense 1st to 2nd order colors, frequent zoning, B(-). Similar to biotite, but without mottled extinction.
Images: Brown hornblende in PPL, rotated 90 degrees.
Relatively rare alteration of biotite or phlogopite.
PPL: Pleochroic in pale shades of brown; may be colorless. Low to medium relief, perfect cleavage, typical micaceous habit.
XP: Colors until the end of the 1st order. Extinction tends to parallel, but is not mottled! SE(+), B(-), 2V from 0 to 15º.
Images: Vermiculite in PPL, rotated 90 degrees.
Rarer triclinic phyllosilicate of some specific metamorphic rocks.
PPL: Strong pleochroism in various shades of yellow and brown to greenish. Medium-high relief, 1 perfect cleavage. Lamellae, fibers and needles.
XPL: Intense 2nd to 4th order colors. Paralell extinction. B(-) with 2V from 0º (pseudo-uniaxial) to 30º. Looks like biotite, but has no mottled extinction!
Images: Stilpnomelane in PPL, rotated 90 degrees.
Somewhat rarer monoclinic phyllosilicate, typical mineral of rocks undersaturated in silica and rich in Ca.
PPL: colorless, but may have moderate brown pleochroism in sections perpendicular to (001); sections parallel to (001) almost without pleochroism. Moderate relief, perfect cleavage, lamellar habit.
XPL: Paralell and mottled extinction. ES(+) and B(-) with small 2V, can simulate to be uniaxial. Very similar to biotite.
Images: Phlogopite with calcite in PPL, rotated 90 degrees.
Rather common monoclinic sorosilicate, of the Allanite Group.
PPL: Usually pleochroic in red-brown (in green is possible). High relief, no visible cleavage. Anhedral or tabular grains. Zoning common.
XPL: Colors up to 3rd order, masked by the mineral color. Dark halos around the grains may occur. Twins are common. B(-/+). Similar to brown hornblende. May be metamict (almost isotropic).
Images: Allanite in PPL, rotated 90 degrees.
Relatively rare tetragonal Sn-oxide, occurs associated with granites.
PPL: Weak to strong pleochroism in brownish and reddish (may be orange or green). Very high relief, cleavage not visible.
XPL: Its own color masks the very high interference colors. Extinction tends to parallel. ES(+), twins are very common. U(+), can have anomalous 2V up to 38º.
Images: Cassiterite in PPL, rotated 90 degrees.
Very common tetragonal Ti-oxide, accessory in many types of rocks.
PPL: Weakly pleochroic in red-brown to yellow-brown. Very high relief, granular or acicular crystals. The color is almost the same in PPL and XPL (like titanite).
XPL: Parallel extinction, ES(+), twins possible. No zoning, no alteration, dark halos (due to radioactivity) possible. U(+).
Images: Rutile in PPL.
Less common trigonal cyclosilicate of the Tourmaline Group (>40 different minerals).
PPL: Strong pleochroism in brown (may be in many different colors like yellow, blue, green, black and red-orange). High relief, cleavage not visible, fractures perpendicular to elongation. Basal sections (without pleochroism!) triangular or pseudo-hexagonal, if idiomorphic.
XPL: Paralell extinction, ES(-), frequent zonation. U(-), can have 2V of 5º.
Images: Tourmaline in PPL.
Very common monoclinic amphibole.
PPL: Strong pleochroism in green, yellow-green to bluish-green. Two cleavages at 56º-124º in the basal sections (pseudo-hexagonal, rhombic, if idiomorphic). Long to short prismatic, medium-high relief.
XPL: Shows 1st to end 2nd order colors, oblique extinction (14-22º), ES(+), twins and zoning are common, B(-).
Images: Green hornblende in PPL.
Very common tetragonal nesosilicate, occurs in many types of rock.
PPL: Weak pleochroism in brown if not colorless. High relief, cleavage not visible, roundish grains, may have a dark halo around it.
XPL: 3rd to 4th order colors. Frequent zoning. Paralell extinction , ES(+), rare twins. U(+), 2V of 10º is possible. Similar to monazite!
Images: Zircon in PPL.
Rare orthorhombic amphibole exclusive of metamorphic rocks.
PPL: When rich in Fe, moderate pleochroism in brown colors in various shades (including blue). Typically asbestiform (fibrous) habit. Moderate relief.
XPL: Parallel extinction, ES(+). No twins nor zoning. B(+/-), 2V = 57 – 80 degrees
Images: Anthophyllite in PPL and in XPL.
Very rare monoclinic amphibole of Ca-poor metamorphic rocks.
PPL: Usually colorless, but can be pleochroic between pale brown or pale green, stronger when Fe-rich. Forms clusters of prismatic crystals. In the basal section (rhombic!), 2 cleavages at 56/124º.
XPL: Extinction 15-21º. Colors up to 2nd and 3rd order. ES(+), B(+/-).
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