_Extinction._—As calcite is uniaxial, basal sections remain dark during rotation. Extinction angles with respect to the cleavage cracks vary with the position of the section. _▄Convergent Light:▄_ Basal sections, even when very thin, show distinct interference figure, with cross and rings. Optical character (−). ▄Distinguished from▄: (_a_) Other CARBONATES.—By ease with which it is attacked by cold dilute acids, test can be made on slide after removing cover. (_b_) MAGNESIUM-BEARING CALCITE.—By micro-chemical tests. (_c_) TITANITE (Sphene).—See under the latter. REMARKS: Calcite is very widely distributed, in addition to the extensive sedimentary limestone deposits. Common limestone consists of dense aggregates of crystalline grains. Calcite is often a secondary product of the lime-bearing silicates in the more basic eruptive rocks. Pseudomorphs of calcite after olivine are noteworthy. Coarse aggregates of calcite occur in the crystalline schists and contact rocks. Calcite is exceedingly plastic to pressure and mechanical deformation may be recognized by curving of the cleavage cracks, crumpling of the twin lamellæ and “wavy” extinction. Calcite is easily attacked and completely dissolved with effervescence by cold dilute acids. H., 3. Sp. gr., 2.72. DOLOMITE. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: CaMg(CO_{3})_{2}, when pure CaO = 30.4, MgO = 21.7, CO_{2} = 47.8. Proportions of Mg and Ca vary, and Fe and Mn also occur. _ć_ = α. ▄Usual Appearance in Sections:▄ In rocks chiefly as crystals, even dense homogeneous aggregates showing tendency towards crystalline boundaries (saccharoidal structure). Crystals almost always unit rhombohedron (10̄11) with tendency to curved surfaces. _Index of Refraction._—_n′_ = 1.622 (α = 1.503, γ = 1.682, γ − α = 0.179), a little higher than that of calcite. For variation in appearance of surface with polarized light, see under calcite. The microscopic characters are similar to those of calcite, from which it may be _distinguished_ by not being so easily attacked by cold dilute acid (test can be made on slide with cover off), by tendency towards crystalline boundaries, by absence of twin lamellæ (or when present parallel to _−2R._ (20̄21), hence about parallel to shorter diagonals of cleavage rhombs), and by micro-chemical tests. The distinction at times may be very difficult. REMARKS: Occurs in sedimentary formations and as crystals in limestone and other rocks. In certain rocks the dolomite crystals may not have a very good “bond,” and a “drusy” structure may also be characteristic of the cavities between dolomite crystals in rocks. Only slightly attacked by cold dilute acids, but if acid is heated it dissolves easily with effervescence. APATITE.