▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.001 to 0.006), may vary in different portions of the same crystal (optical anomalies due to the mineral being at times a mixture of isomorphous individuals). Interference colors very low 1st order, dark gray, etc., may often appear zonal. Basal sections isotropic when normal, but may show division into biaxial portions. _Extinction_ parallel in sections elongated ∥ _ć_ axis. In convergent light basal sections show a faint cross when normal; optical character generally (−). ▄Alteration▄: Not known in rock-making vesuvianite. ▄Distinguished from▄: EPIDOTE (Pistacite) by the very low order interference colors. CORUNDUM by weaker double refraction. GARNET (Grossularite), ZOISITE, and APATITE may be easily confused with this mineral and hard to distinguish from it. REMARKS: Found in limestones, that have undergone alteration by contact with igneous rocks, and in metamorphic schists. Also may occur in dense (nephrite-like) aggregates in serpentine. It is insoluble in hydrochloric acid. H., 6.5. Sp. gr., 3.3 to 3.8. MELILITE. ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: Na_{2}(Ca,Mg)_{11}(Al,Fe)_{4}Si_{9}O_{36}?. _ć_ = a. ELONGATION ∥ c′. ▄Usual Appearance in Sections▄: Almost colorless, tabular (∥ base) crystals or irregular grains or shreds. Sections very commonly lath-shaped, and often characterized by the peculiar “peg-structure,”[84] the lines or markings being ∥ _ć_ (⟂ elongation of the section). Index of refraction (_n′_ = 1.630, α = 1.629, γ = 1.631) higher than that of the other associated colorless materials, hence _relief_ rather marked. Cleavage, parallel to base, very imperfect. ▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.003), and diminishes with a decrease of Al. Interference colors the lower 1st order, grays, etc.; anomalous interference colors may show. _Extinction_ parallel to cleavage or the peculiar markings or lines. Optical character usually (−), but when poor in Al (+). ▄Alteration▄: Takes place frequently to a fibrous aggregate. ▄Distinguished from▄: NEPHELITE and FELDSPAR by higher relief, shape, “peg-structure” and usual dull appearance with reflected light. REMARKS: Abundant in the leucite and nephelite rocks (associated with these minerals and with augite, perovskite and chrysolite), and takes the place of a feldspar in the melilite-basalt. It gelatinizes easily with hydrochloric acid. H., 5. Sp. gr., 2.9. GRAPHITE. HEXAGONAL. COMPOSITION: C. ▄Usual Appearance in Sections▄: Minute particles, or flakes and grains of irregular shape, seldom crystallized. _Opaque_, and by reflected light, black with metallic lustre.