calcspar

Calcspar is a historical or colloquial term used to describe spar-like crystals of calcite, the mineral form of calcium carbonate (CaCO3). In contemporary terminology, calcspar generally refers to calcite crystals that are well-formed, clear, and chunky enough to resemble spar, rather than to a distinct mineral species.

Calcite, the mineral underlying calcspar, crystallizes in the trigonal (rhombohedral) system. When crystals are well formed, calcspar specimens exhibit a vitreous luster and clear to white coloration, though impurities can impart gray, yellow, or reddish hues. Key identification features include a hardness of about 3 on the Mohs scale, a specific gravity around 2.71, and perfect rhombohedral cleavage. Calcite effervesces readily with dilute acids such as hydrochloric acid, releasing carbon dioxide gas.

Calcspar commonly forms in sedimentary rocks as sparry cement, filling pore spaces and acting as a binding matrix in limestones and dolostones. It can also occur as large crystals in voids or veins within carbonate rocks. The term is often used to distinguish coarse, clear calcite crystals from more massive or fine-grained calcite in petrographic descriptions.

Distinguishing calcspar from other spar minerals is usually a matter of composition and acid reaction. Gypsum spar, for example, is a calcium sulfate and has different softness and chemical behavior. Modern geologists typically refer to the material simply as calcite, using “spar” to describe the crystal habit when appropriate.

See also: Calcite, Spar (geology).