REMARKS: These minerals are found in the basic, soda-rich rocks. (S) in elæolite-syenite also in trachyte and phonolite, (H) and (N) common in phonolite and leucite-porphyry. H., 5.5 to 6. Sp. gr., 2.3. [Illustration: FIG. 37.—Haüynite, showing dark centre and border, in nepelinite. (From Cohen.) ] PEROVSKITE, Perofskite. ISOTROPIC. ISOMETRIC. COMPOSITION: CaTiO_{3}. ▄Usual Appearance in Sections▄: Microscopic, octahedral crystals or larger grains, pale brownish in color and not very transparent, darker colored in the larger grains. In reflected light grains appear yellowish with adamantine lustre. Refractive index very high (_n_ = 2.38), hence _relief_ very strong. Between _crossed nicols_, the little crystals generally appear optically normal and remain dark; but the larger crystals may show a complicated penetration twinning. ▄Distinguished from▄: GARNET (melanite) and SPINEL (picotite) by much higher refractive index and reaction for titanium and by zonal coloration of melanite. When opaque it might be mistaken for the iron ores, but has no metallic lustre. REMARKS: Found in the younger basic eruptive rocks, especially melilite-basalt. Commonly associated with the iron ores nephelite, augite and chrysolite. Insoluble in hydrochloric acid. H., 5.5. Sp. gr., 4.1. RUTILE. ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: TiO_{2}. _ć_ = c ELONGATION ∥ _ć_. ▄Usual Appearance in Sections▄: Sharp, elongated, prismatic crystals when microscopic, but granular when the individuals are large. Grains may be almost opaque, with adamantine lustre by reflected light. Knee- or heart-shaped twins, Figs. 38 and 39, common in the smaller crystals, the larger individuals may also show geniculated twinning. Small crystals sometimes form net-shaped groups (sagenite), by crossing one another at angles of 60°. Pleochroic halos may surround crystals. Color, yellowish to reddish-brown. Index of refraction very high (_n′_ = 2.712, α = 2.616, γ = 2.903), hence _relief_ marked and surface very rough. Prismatic cleavage present in larger individuals, not observed in microscopic crystals. Pleochroism and strong absorption may be noticed, especially in the larger grains, but may fail entirely. ▄Crossed Nicols▄: Double refraction _very_ strong (γ − α = 0.287). Interference colors[82] very high order, only seen in the microlitic crystals which do not appear dark due to total reflection; in other cases may not show at all. _Extinction_ parallel to prisms. In _convergent light_ optical character (+). ▄Alteration▄: May take place to a white or yellowish, fibrous or granular substance, strongly refracting, and similar to the alteration product of ilmenite. May be surrounded by grains of titanite.