▄Crossed Nicols▄: Double refraction weak (γ − α =0.009), like quartz. Interference colors middle first order, white to yellow. _Extinction_ in general parallel to cleavage cracks. In _convergent light_, Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_; axial angle large (hyperbolas only seen without ellipses) (2_E_ = 64° to 150°); optical character (−). ▄Alteration▄: Takes place readily, forming greenish mica-like decomposition products, the decomposition commencing along the crevices or about the inclusions. ▄Distinguished from▄: QUARTZ by observation in convergent light (quartz is uniaxial), decomposition and pleochroism or pleochroic halos. The section can also be treated with hydrofluosilicic acid, when the evaporated solution yields characteristic prismatic crystals of magnesium fluosilicate. REMARKS: Found in gneiss, hornstone, granite, granulite, etc., and in some volcanic rocks. It is often associated with garnet, biotite, sillimanite, etc. In a thick section heating to redness makes the pleochroism more distinct. Iolite is only slightly acted on by acids. H., 7 to 7.5. Sp. gr., 2.6. It is hard to make a mechanical separation from quartz, on account of similarity in sp. gr. NATROLITE. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = c. ELONGATION ∥ c′. Na_{2}Al_{2}Si_{2}O_{10} + 2H_{2}O. ▄Usual Appearance in Sections▄: Aggregates of colorless, fibrous crystals, which may have sphærulitic structure, showing a dark cross between crossed nicols. Index of refraction lower than balsam (_n′_ = 1.483, α = 1.478, γ = 1.490), hence (in large crystals) the surface would appear rather rough. ▄Crossed Nicols▄: Double refraction weak (γ − α = 0.012). Interference colors the middle first order (yellow, etc.), a little higher than those of quartz. _Extinction_ parallel to fibres. Optical character (+). REMARKS: Never a primary mineral in rocks, but found in igneous rocks filling amygdaloidal cavities, and also as a very common alteration product of sodalite, noselite, nephelite and acid plagioclases. It gelatinizes easily with hydrochloric acid. H., 5 to 5.5. Sp. gr., 2.2. OTHER ZEOLITES. COMPOSITION: Hydrous silicates; Al, Ca and Na being the chief bases. ▄Usual Appearance in Sections▄: The form depends on the individual mineral species, but the majority appear in elongated crystals or fibers. They are all colorless and most of them have a small index of refraction, hence no _relief_ (prehnite has distinct relief). ▄Crossed Nicols▄: The double refraction is generally very weak (between that of nephelite and quartz), giving very low order interference colors (prehnite and thomsonite have strong double refraction). REMARKS: The zeolites are always secondary minerals in rocks. They gelatinize with hydrochloric acid.