COMPOSITION: R″_{3}R‴_{2}(SiO_{4})_{3}. R″ is Ca, Mg, Fe or Mn; R‴ is Al, Fe‴, or Cr, rarely Ti. ▄Usual Appearance in Sections▄: Irregular grains, Fig. 13, or simple crystals, showing forms (110) and (211), alone or in combination, Fig. 14 _b_. Zonal structure not infrequent, especially in the titanium varieties, Fig. 34. _Color._—Colorless, or nearly so, to yellowish, reddish or brownish. _Index of Refraction._—_n_ = 1.750–1.856, hence _relief_ high and surface very rough. _Fracture._—Irregular cracks occur, but no cleavage noticed. ▄Crossed Nicols▄: As garnets are isotropic, sections remain dark during complete rotation. Optical anomalies may however occur, but are generally confined to, titanium free, lime garnets and manganese garnets. The effect being to divide the crystal symmetrically into different areas, “dodecahedral structure.” [Illustration: FIG. 34.—Garnet, with zonal structure, in gneiss. (From Cohen.) ] ▄Alteration▄: Garnets are usually fresh, but may be found altered to chlorite or hornblende. ▄Distinguished from▄: SPINEL and PEROVSKITE.—See under the latter. REMARKS: Found principally in granulites, metamorphic rocks, contact rocks, crystalline schists, etc. Certain varieties may be found in eruptive rocks or olivine rocks. May form pegmatitic borders with pyroxene, spinel, etc. Garnets are practically insoluble in hydrochloric acid. H., 6.5 to 7.5. Sp. gr., 3.4 to 4.3. The insolubility in acids and the high sp. gr. help in separating garnet from a powdered rock. LEUCITE. ISOTROPIC. ISOMETRIC.[80] COMPOSITION: KAl(SiO_{3})_{2}. ▄Usual Appearance in Sections▄: Crystals or grains, which vary greatly in size. Cross-sections often nearly round. When very small and free from inclusions may be easily overlooked. Sometimes the grains are surrounded by tangentially arranged needles of different minerals. [Illustration: FIG. 35.—Leucite, with radial and tangential inclusions, Vesuvius Lava. (From Cohen.) ] _Color._—Colorless. _Index of Refraction._—_n_ = 1.509, hence no _relief_ and generally smooth surface. _Fracture._—May be noticed, but no cleavage observed. _Inclusions._—Common, radially or zonally arranged, consisting of minerals or glass, Fig. 35. ▄Crossed Nicols▄: The smaller crystals appear isotropic; the larger crystals show characteristic intersecting systems of twin lamellæ, Fig. 36. [Illustration: FIG. 36.—Leucite, showing complicated, interpenetration twinning between crossed nicols. ] _Double Refraction._—Very weak (γ − α = 0.001). In thin sections it may be necessary to use a sensitive color plate to prove double refraction. _Interference Colors._—Very low 1st order, dark gray, etc. ▄Alteration▄: Quite frequent to fibrous or granular zeolites. ▄Distinguished from▄: ANALCITE—see under analcite.