Transmitted Light
Very common cubic mineral, occurs in many types of rocks.
PPL: Sometimes garnets show a well-defined pink color, may suggest pleochroism, but does not. Usually in rounded grains. High relief, very fractured and without cleavage. It can have many inclusions.
XPL: Isotropic, completely black.
Images: Pink garnets in PPL.
Orthorhombic pyroxene common in ultramafic and basaltic rocks.
PPL: Distinct pleochroism in pink, yellow-brown or pale green.Short prismatic, granular, basal section with 4 or 8 sides and 2 cleavages at ~90º.
XPL: Parallel extinction! Colors up to 1st order orange (lower than the colors of clinopiroxenes!). ES(+), common zonation, rare twins. B(+/-), 2V from 58-86°.
Images: Enstatite in PP, rotated 90 degrees.
Common monoclinic mica, but never occurs in volcanic rocks.
PPL: Colorless, but may show weak pleochroism in pink. Lamellar habit. Medium-high relief, usually one perfect cleavage.
XPL: Intense colors up to 3rd order. ES(+), B(-), (2V=30-47º). Difficult to distinguish from talc, lepidolite, pyrophyllite and paragonite.
Images: Muscovite in PPL and in XPL.
Less common orthorhombic nesosilicate of the Al2SiO5 Group, typical of high P and T pelitic metamorphic rocks.
PPL: Colorless, but the variety "fibrolite" shows a well-defined pink pleochroism. Long prismatic crystals with fractures perpendicular to the elongation, acicular or in fibrous masses (fibrolite). Basal sections are square with diagonal cleavage.
XPL: Extinction paralell (symmetrical in basal sections), ES(+), without twins or zoning. B(+), 2V from 21-30º.
Images:
More or less common orthorhombic nesosilicate of the Al2SiO5 Group, occurs in some types of metamorphic rocks.
PPL: Usually colorless, but may show weak pale pink pleochroism. Perfect {110} cleavage, in basal sections they intersect at 89º.
XPL: Parallel extinction, ES(-) (important!), gray interference colors. No twinning/zoning. Black carbon inclusions common.
Images: Andalusite in PPL and in XPL.
Very rare orthorhombic nesosilicate, of Al-rich metamorphic rocks.
PPL: Usually pleochroic in pale blue colors, but moderate to strong pleochroism between colorless and pink may occur. High relief, cleavage not visible.
XPL: Upper 1st order colors. Parallel extinction, ES(-), twins are common. B(-), with 2V = 20 – 52º.
Images: Dumortierite in PPL, rotated 90 degrees.
Rare monoclinic sorosilicate of Mn-rich metamorphic rocks.
PPL: Strongly pleochroic in red (may be pink, yellow, orange or violet). High to extremely high relief, 1 perfect cleavage. Long prisms, sections with 6 sides.
XPL: Intense colors, masked by the red color of the mineral. 0-34º or parallel extinction. ES(+/-), B(+/-).
Images: Piemontite in PPL.
Very common orthorhombic sulphate, typical of evaporitic sequences.
PPL: Usually colorless, but may show a distinct pleochroism in pink with very light yellow and violet. Low relief, 3 perfect cleavages. Anhedral, granular, massive to fibrous.
XPL: Extinction parallel to cleavages. Intense interference colors up to 3rd order green, can show twins, ES(+/-), B(+), 2V = 36-45º. Occurs associated with gypsum (XPL=gray).
Images: Anhydrite in PPL and in XPL.
Somewhat rarer monoclinic phyllosilicate, typical mineral of rocks undersaturated in silica and Ca-rich.
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.
Rare orthorhombic sorosilicate, occurs in some specific types of rocks.
PPL: Moderate pink to yellow pleochroism in the variety thulite (of Mn). High relief, short to radiated prisms with rhombic or hexagonal basal sections. One perfect cleavage.
XPL: Anomalous or lower 1st order colors. Extinction paralell, ES (-), B(+) with 2V from 0 to 69º.
Images:
Rare mineral with pleochroism in rosy violet colors, may be blue. In XPL, anomalous interference colors.