calcite, aragonite, dolomite

calcite, aragonite, dolomite Calcite, aragonite, and dolomite are the most important and widespread of the carbonate minerals, of which at least 60 are known. The essential unit of carbonate composition is the CO₃²⁻ anion linked by a variety of cations, especially calcium and magnesium.

Calcite and aragonite are polymorphs of calcium carbonate (CaCO₃); they have different structures and properties and are stable under different conditions. They make up, about 50 per cent each, the skeletal material of many invertebrate groups, but with time the aragonite changes into calcite. Both can be precipitated from water, either through the agency of organisms such as algae or by inorganic processes. The resulting limestones are ultimately composed mainly of calcite and are important sedimentary rocks. Calcite is a common cement of clastic sedimentary rocks and occurs widely in mineral veins.

Both calcite and aragonite can form under metamorphic conditions. Pure calcite limestones are stable to high temperatures and pressures—the mineral simply recrystallizing, with a change in texture. Aragonite is an important constituent of so-called ‘blue schists’.

Dolomite is a double carbonate of calcium and magnesium: CaMg(CO₃)₂; there is some solid solution with calcite. Extensive bodies are believed to have formed on the sea floor by the reaction of calcite or aragonite with magnesium in the sea water; more irregular and restricted occurrences developed from magnesium-rich groundwaters reacting with already-formed limestone bodies.

Calcite is perhaps best known for its property of extreme double refraction, for its use as a polarizer in older petrological microscopes, and for the large number of crystal habits it assumes; 224 have been noted in Scotland alone.

R. Bradshaw