ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: Silicates of Ca, Al _ć_ = a. ELONGATION ∥ a′. and Na. ▄Usual Appearance in Sections▄: Colorless grains, lath-like individuals or prisms (dipyre, in contact metamorphic limestone). Index of refraction (_n′_ = 1.551 to 1.584;, α = 1.542 to 1.558, γ = 1.555 to 1.597) about the same as quartz, hence usually no _relief_ and surface smooth. Cleavage distinct parallel to square prism. Inclusions (carbonaceous) may be abundant in contact rocks. ▄Crossed Nicols▄: Double refraction usually strong, but varies (γ − α = 0.013 to 0.039), increases with the Ca percentage. Interference colors upper 1st or 2d order, more brilliant than those of most of the colorless minerals. Basal sections (showing cleavages intersecting at 90°) isotropic. _Extinction_ parallel in longitudinal sections. In _convergent light_ basal sections show distinct uniaxial interference figure; optical character (−). ▄Alteration▄: Takes place easily to a fibrous substance or to kaolin, muscovite, etc. ▄Distinguished from▄: (_a_) FELDSPARS (not showing twinning) and IOLITE (Cordierite) by uniaxial character, cleavage and higher order interference colors. (_b_) QUARTZ by cleavage, higher order interference colors and optical character; quartz is (+) (_c_) APATITE (in grains) by lower index of refraction, cleavage and higher order interference colors. REMARKS: Found especially in metamorphosed diabases and gabbros (Norwegian); also in gneisses, crystalline schists, metamorphosed limestones, etc. Dipyre occurs in contact zones of limestones and schists, where it might be confused with andalusite, but cross-sections show uniaxial character. The minerals of this group are more or less soluble in hydrochloric acid. When the scapolite contains Cl, the following test can be made on fresh material. Treat with a solution of silver nitrate in hydrofluoric acid and the jelly will be impregnated with chloride of silver which will turn brown. H., 5.5. Sp. gr., 2.68. VESUVIANITE, Idocrase. ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Ca_{6}Al_{3}(OH.F)(SiO_{4})_{6}. ▄Usual Appearance in Sections▄: Grains or prismatic crystals. Almost colorless to reddish (when containing Mn). Index of refraction high (_n′_ = 1.715, α = 1.701 to 1.726, γ = 1.705 to 1.732), hence _relief_ marked and surface rough. Cleavage imperfect, parallel to prism. Pleochroism generally very faint.