Transmitted Light
is U(+) (very diagnostic!), but can have 2V of up to 20º. Trigonal. PPL: colorless, low relief, usually anhedral. Never shows cleavage or alteration. CPL: dark gray to white. Parallel extinction (usually undulating), ES(+). Very common, easy to confuse with cordierite, nepheline(!) and untwinned, unaltered feldspars.
is U(-), can have 2V up to 25º. Trigonal, pseudo-cubic. 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. Rare mineral, of cavities of acidic volcanic rocks.
is U(-). Very rare mineral. Trigonal. 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
is U(+), but may be anomalous biaxial. Rare mineral. Low relief, prismatic, tetragonal. Perfect (001) cleavage. Parallel extinction, ES(+/-). Interference colors ranging to anomalous, may appear gray to beige. It is usually secondary, occurring in cavities of basaltic volcanic rocks.
is U(+) or U(-). Rare mineral. Hexagonal, from nepheline syenites and alkaline granites. Medium relief, usually anhedral and/or in veins. One perfect to indistinct cleavage, parallel extinction, without twins or zoning. In XPL, blue colors, anomalous. In hand specimen it has a strong pink color, easy to identify.
may be pseudo-uniaxial, being biaxial. Heulandite is a zeolite with low to medium relief, in XPL with colors of lower 1st order,
is U(-), but can have 2V from 0-6º. Hexagonal. Usually interstitial (anhedral). Low relief, cleavage not visible. Longitudinal sections with parallel extinction and ES(-). In volcanic rocks, it may be zoned. Easily alters to clays, calcite, sodalite and others. Similar to quartz!
is U(-), forms small long prismatic crystals with rounded ends, with parallel extinction and ES(-). The basal sections tend to be hexagonal and isotropic. Hexagonal. Cleavage absent. In carbonatites it can form large crystals and be biaxial (2V angle up to 20º). Twins are rare, no zonation. It is a common mineral in igneous and metamorphic rocks.
is U(+) or U(-). Trigonal, in prismatic or tabular crystals, with square or elongated basal sections. Poor cleavages. In XPL may have anomalous bluish interference colors. Parallel extinction, rare twins and zonation. Forms small crystals in the matrix of silica-poor volcanic rocks. Similar to plagioclases.
is U(-), but can have 2V up to 6º. Rare mineral, Medium relief, no visible cleavage, prismatic, hexagonal basal sections. Hexagonal. 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.
may be pseudo-uniaxial, being biaxial. Chabazite is a zeolite with low to medium relief, in XPL shows colors of lower 1st order,
