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is U(+) (very diagnostic!), but can have 2V of up to 20º. Trigonal.
Very common mineral, occurs in many types of rocks.
PPL: colorless, low relief, usually anhedral. Never shows cleavage or alteration.
XPL: dark gray to white. Parallel extinction (usually undulating), ES(+). Very common, easy to confuse with cordierite, nepheline(!) and untwinned, unaltered feldspars.
Images: Quartz in PPL and in XPL.
is U(-), can have 2V up to 25º. Trigonal, pseudo-cubic.
Very rare tectosilicate of high-Si volcanic rocks.
PPL: colorless with very low relief and without cleavage. Never alters.
XPL: almost isotropic (very low birefringence - 0.003). Extinction tends to parallel, common lamellar twins. Various habits.
is U(-). Trigonal.
Rather rare tectosilicate, often found in skarns.
PPL: Prismatic, columnar crystals, may be granular. Two cleavages that intersect at 90º in the basal sections.
XPL: colors of up to 2nd order, can be strong. Parallel extinction, ES(-), without twins, typically with many inclusions. Feldspars are similar, but biaxial.
Images: Scapolite in PPL and in XPL.
is U(+), but may be anomalous biaxial. Tetragonal.
Common phyllosilicate, typical as secondary mineral in basic igneous rocks.
PPL: Low relief, prismatic, tetragonal. Perfect (001) cleavage.
XPL: Parallel extinction, ES(+/-). Interference colors ranging to anomalous, may appear gray to beige.
Images: Apophyllite in PPL and in XPL.
is U(+) or U(-). Hexagonal.
Rare mineral, of nepheline syenites and alkaline granites.
PPL: Medium relief, usually anhedral and/or in veins. One perfect to indistinct cleavage.
XPL: Parallel extinction, without twins or zoning. Blue anomalous colors. In hand specimen it has a strong pink color, easy to identify.
Images: Eudyalite in PPL and in XPL.
Biaxial zeolite, but may be pseudo-uniaxial.
Very common tectosilicate, may be present in several geological situations.
PPL: Low to medium relief, one perfect cleavage.
XPL: Colors of lower 1st order. Twins and zoning common. Very similar to several other common zeolites.
Images: Heulandite in PPL and in XPL.
is U(-), but can have 2V from 0-6º. Hexagonal.
Rock-forming feldspatoid of Si-low magmatic rocks.
PPL: Usually interstitial (anhedral). Low relief, cleavage not visible.
XPL: Longitudinal sections with parallel extinction and ES(-). In volcanic rocks, it may be zoned. Easily alters to clays, calcite, sodalite and others.
Images: Nepheline in PPL and in XPL.
is U(-). Hexagonal.
The only rock-forming phosphate, is present in many types of rocks.
PPL: Usually forms small long-prismatic crystals with rounded ends, with parallel extinction. The basal sections tend to be six-sided and isotropic. In carbonatites it form large crystals and is biaxial (2V angle up to 20º). Cleavage absent.
XPL: Gray colors. ES(-), twins are rare, no zonation.
Images: Apatite in volcanic rock in PPL and in XPL.
is U(+) or U(-). Tetragonal.
Rare sorosilicate, typical of alkaline volcanic rocks.
PPL: prismatic or tabular crystals, with square or elongated basal sections. Poor cleavages.
XPL: may have anomalous bluish interference colors. Parallel extinction, rare twins and zonation. Forms small crystals in the matrix of silica-poor volcanic rocks. Habits similar to plagioclases.
Images: Melilite in PPL and in XPL.
is U(-), but can have 2V up to 6º. Hexagonal.
Rare cyclosilicate, found only in rhyolites and in granitic pegmatites.
PPL: Medium relief, no visible cleavage, prismatic, hexagonal basal sections.
XPL: Parallel extinction, ES(-), rare twins, no zonation. It rarely alters. Very hard (Mohs: 7.5-8), may be thick in thin section. Very similar to several common minerals.
Images: Beryl and muscovite in PPL and in XPL.
Biaxial zeolite, but may be pseudo-uniaxial. Pseudo-cubic.
Uncommon tectosilicate, occurs as secondary mineral in volcanic rocks.
PPL: Low to medium relief, may show pseudo-cubic outlines.
XPL: Colors of lower 1st order. Twins and zoning common.
Images: Chabazite in PPL and in XPL.
