REMARKS: Almost entirely confined to younger eruptive rocks, phonolite, tephrite and other leucite rocks and their tuffs. Often found with plagioclase, nephelite, augite, etc. It is more or less attacked by hot hydrochloric acid. H., 5.5 to 6. Sp. gr., 2.4 to 2.5. The isolation of leucite from rock powder can be better accomplished by specific gravity than by chemical methods. ANALCITE. ISOTROPIC. ISOMETRIC. COMPOSITION: NaAlSi_{2}O_{6} + H_{2}O. ▄Usual Appearance in Sections▄: Secondary colorless grains, with no very characteristic microstructure or properties. Cleavage parallel to cube (100) usually seen. Index of refraction low (n = 1.488), hence rather rough surface. Between _crossed nicols_ may show optical anomalies, but not so marked as in garnet. _Distinguished from_: LEUCITE, SODALITE and NEPHELITE. These minerals are most easily confused with analcite and recourse must be had to chemical tests, detection of optical anomalies, gelatinization test or turbidity by heating. REMARKS: Occurs as a secondary product (commonly from nephelite or leucite) in alkali-rich eruptive rocks. Considered also as a primary mineral in igneous rocks.[81] Gelatinizes with hydrochloric acid, and becomes turbid by heating. H., 5.5. Sp. gr., 2.25. SODALITE GROUP. Sodalite, Haüynite (Haüyne) and Noselite (Nosean). ISOTROPIC. ISOMETRIC. COMPOSITION: Sodalite, 3NaAlSiO_{4} + NaCl. Haüynite, 2(Na_{2}Ca)Al_{2}(SiO_{4})_{2} + (Na_{2}Ca)SO_{4}. Noselite, 2Na_{2}Al_{2}Si_{2}O_{8} + Na_{2}SO_{4}. ▄Usual Appearance in Sections▄: Dodecahedral rounded crystals or (S) irregular grains. Colorless, yellowish, brownish, greenish to deep blue. Refractive index low [_n_ = 1.483(S) to 1.503(H)], hence the surface appears rather rough in sodalite and slightly rough in haüynite. Inclusions, abundant and characteristic, often rod-like and arranged regularly, making section translucent and especially dark at border, Fig. 37. Dodecahedral cleavage sometimes seen. Optical anomalies may occur. ▄Alteration▄: Takes place easily to aggregates of natrolite, other zeolites, mica, etc. ▄Distinguished from▄: (_a_) ONE ANOTHER only by chemical tests. Gelatinization test with hydrochloric acid will show in addition to jelly, salt crystals for (S), abundant gypsum crystals (CaSO_{2} + 2H_{2}O) for (H) and few if any gypsum crystals (absence of Ca) for (N). (H) and (N) turn blue when heated, but test will not work if minerals are decomposed. When treated with hydrochloric acid and nitrate of silver the black sulphide of silver will show on (H) and (N), but the white chloride on (S). (_b_) NEPHELITE (Elæolite) by being isotropic. (_c_) ANALCITE by no turbidity when heated.