▄Usual Appearance in Sections▄: Nephelite in short hexagonal prisms and grains in the younger volcanic rocks, hence sections rectangular or hexagonal, Fig. 43; elæolite allotriomorphic in the older plutonic rocks. _Color._—Colorless. _Index of Refraction._—_n′_ = 1.539 (α = 1.538, γ = 1.542), hence no relief and surface smooth. _Cleavage._—Imperfect, parallel to prism (10̄10) and base (0001). More marked in elæolite than in nephelite, especially when decomposition has commenced. [Illustration: FIG. 43.—Nephelite sections, showing zonal inclusions. (From Reinisch.) ] _Inclusions._—Microscopic needles of augite, etc., also fluid and gas. Mostly in zones. Elæolite may be much clouded by inclusions and alteration products. ▄Polarized Light▄: _Pleochroism._—None. _▄Crossed Nicols▄_: _Double Refraction._—Very weak (γ − α = 0.004), may only be detected by using a test-plate. _Interference Colors._—The lower first order, grayish-white, etc., a little lower than the feldspar colors. _Extinction._—As the mineral is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction takes place and is parallel to cleavage lines when these appear. _▄Convergent Light▄_: Basal sections show a broad cross, without rings. Optical character (−). ▄Alteration▄: Takes place easily to fibrous zeolites (natrolite), or in certain rocks to mica. ▄Distinguished from▄: OTHER MINERALS by gelatinization test and staining with fuchsine. When present in small interstitial individuals (as is often the case in basalts) it is very difficult to distinguish without this test; but it must be remembered that other minerals, zeolites, etc., will also gelatinize. QUARTZ has stronger double refraction, rarely shows hexagonal outline, is always fresh and optically (+). FELDSPAR is biaxial and often shows twinning. ANALCITE.—See under the latter. REMARKS: Nephelite bears the same relation to elæolite as sanidine does to orthoclase. It occurs only in the younger volcanic rocks; with sanidine in phonolite, with plagioclase in tephrite, without feldspar in nepheline-basalt, and with leucite in leucite-basalt. It is not found with primary quartz. Elæolite occurs with orthoclase in elæolite-syenite, etc. nephelite and elæolite frequently occur with the sodalite group. Nephelite gelatinizes with acids. H., 5.5 to 6. Sp. gr., 2.5 to 2.6. TOURMALINE, Schorl. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: Uncertain, _ć_ = a. ELONGATION ∥ a′. R_{18}B_{2}(SiO_{5})_{4}. R chiefly Al, K, Mn, Ca, Mg, Li. ▄Usual Appearance in Sections▄: Staff-like individuals, bunched or in radiating aggregates, Fig. 44 B, or prismatic crystals, Fig. 44 A. Basal sections may be nine-sided. _Color._—Varies greatly, grayish-blue, brown and green most common. Li-tourmaline (rare in rocks) is colorless. Zonal structure may be indicated by differences in color.