Dolomite is composed predominantly of the mineral dolomite with a stoichiometric ratio of 50% or greater content
of magnesium replacing calcium, often as a result of diagenesis. Limestone that is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as magnesian limestone.
History[]
The mineral was given its name in March 1792 by Nicolas de Saussure. Hacquet and Dolomieu met in Laibach (Ljubljana) in 1784, which may have contributed to Dolomieu's work.
Properties[]
The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, gray to pink, commonly curved (saddle shape) crystals, although it is usually massive. Unlike calcite, dolomite is a double carbonate, having a different structural arrangement, and it does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid unless it is scratched or in powdered form.
Crystal Twinning:[]
Crystal twinning is common. A solid solution series exists between dolomite and iron rich ankerite. Small amounts of iron in the structure give the crystals a yellow to brown tint. Manganese substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color.
Formation[]
Vast deposits are present in the geological record, but the mineral is relatively rare in modern environments.
Laboratory synthesis of stoichiometric dolomite has been carried out only at temperatures of greater than 100 °C (conditions typical of burial in sedimentary basins), even though much dolomite in the rock record appears to have formed in low-temperature conditions. The high temperature is likely to speed up the movement of calcium and magnesium ions so that they can find their places in the ordered structure within a reasonable amount of time. This suggests that the lack of dolomite that is being formed today is likely due to kinetic factors, i.e. due to the lack of kinetic energy or temperature.
Coral Atolls:[]
Dolomitization of calcite also occurs at certain depths of coral atolls where water is undersaturated in calcium carbonate but saturated in dolomite. Convection created by tides and sea currents enhance this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role.