Dolomite

Dolomite is a double carbonate that crystallizes in the trigonal system, distinguished from calcite by its stoichiometric 1:1 composition of Ca²⁺ and Mg²⁺. It is named after French naturalist Déodat Gratet de Dolomieu, who in 1791 described the dolomitic rocks of the Alps. It forms predominantly in marine diagenetic environments, where magnesium-rich fluids alter preexisting calcite, or directly from precipitation in lagoons and salt basins. Its hardness of 3.5–4 on the Mohs scale makes it fragile compared to feldspars. A crucial diagnostic feature is its slow, weak reaction with dilute hydrochloric acid at room temperature, whereas calcite reacts vigorously: this property is fundamental for field identification. Dolomite frequently associates with calcite, siderite, barite, and quartz in sedimentary rocks.

Crystal system: trigonal (space group R3̄). Unit cell parameters: a = 4.804 Å, c = 15.95 Å (at 25 °C). Density: 2.86 g/cm³. Hardness: 3.5–4 Mohs; perfect cleavage along 1011. Refractive index: nω = 1.503, nε = 1.679 (negative birefringent, δ ≈ 0.176). Luster: vitreous. Color: colorless, white, pink, yellow; variable UV fluorescence (often absent). Chemical composition: CaMg(CO₃)₂; Ca 21.7%, Mg 13.4%, C 5.0%, O 58.9% (wt%). Solubility: insoluble in cold dilute HCl (diagnostic test), slowly soluble in hot HCl. IR spectroscopy: characteristic absorptions at 2515, 2605 cm⁻¹ (C–O stretching), 1430 cm⁻¹ (CO₃²⁻ bending). Genesis: marine diagenesis, precipitation from supersaturated solutions, metasomatic replacement of calcite. Mineral associations: calcite, siderite, barite, ankerite, fluorite, quartz. Varieties: ferriferous dolomite (with Fe²⁺), manganiferous dolomite (with Mn²⁺).