calcium carbonate
calcium carbonate
White powder or colorless crystals; CaCO3. One of the most stable, common, and widely dispersed of materials. It occurs in nature as aragonite, calcite, chalk, limestone, lithographic stone, marble, marl, and travertine. Referred to as whiting, it has many uses in ceramics to introduce calcium oxide (CaO). Also used as a separator in glass firing.
Calcium carbonate (molecular weight, 100.09) crystallizes in two crystal systems: hexagonal rhombohedral or hexagonal as calcite, and orthorhombic as aragonite. Hexagonal calcium carbonate (calcite) is colorless, white, yellowish, or rarely pale gray, red, green, blue, or violet; sp gr, 2.710 (at 18 degrees C); Mohs hardness, 3; melting point, 1,339 degrees C (at 1,025 atm); decomposes at 898.6 degrees C; and soluble in water, in acids, and in ammonium chloride solution. Orthorhombic calcium carbonate (aragonite) is colorless, white, yellow, reddish, bluish, or black; sp gr, 2.93, ranging from 2.85 to 2.94; Mohs hardness, 3.5 to 4.0; transforms to calcite at 520 degrees C; decomposes at 825 degrees C; and soluble in water, in acids, and in ammonium chloride solution.
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