Dolomite (mineral)

Did You Know?

• The mineral was first named by Nicolas-Théodore de Saussure in March 1792, after Déodat Gratet de Dolomieu.
• Dolomieu first described it as a rock in 1791, from buildings in Rome and samples from the Tyrolean Alps.
• Carl Linnaeus probably first described the mineral in 1768.

Properties

The mineral crystallizes in the trigonal-rhombohedral system and forms white, tan, gray, or pink crystals. It is a double carbonate with alternating calcium and magnesium ions. Unlike calcite, it does not rapidly effervesce in cold dilute hydrochloric acid unless in fine powder form. Crystal twinning is common. Solid solution exists with ankerite and kutnohorite; iron tints crystals yellow to brown, while manganese gives a rosy pink color. Lead, zinc, and cobalt can substitute for magnesium.

Formation

Modern dolomite forms under anaerobic conditions in supersaturated saline lagoons, such as Lagoa Vermelha and Brejo do Espinho in Brazil, and along sabkhas in the Persian Gulf. It also occurs in sedimentary basins bearing gas hydrates and hypersaline lakes. Nucleation is often aided by sulfate-reducing bacteria like Desulfovibrio brasiliensis, though other microbial metabolisms also mediate formation. Low-temperature dolomite appears in supersaturated environments rich in extracellular polymeric substances and microbial cell surfaces, likely due to complexation of magnesium and calcium by carboxylic acids.

Vast deposits exist in the geological record, but the mineral is rare in the Cenozoic and modern settings. Inorganic low-temperature syntheses remain unreproducible; initial precipitation yields a metastable precursor like magnesium calcite, which may transform into partially ordered dolomite through dissolution and re-precipitation, a process termed breaking Ostwald's step rule. High diagenetic temperatures from groundwater along fault systems or deeply buried limestone also drive dolomitization.

Dolomite occurs in continental saline lakes in Australia under conditions of high salinity, high Mg/Ca ratios, and high alkalinity. In some Neogene platforms not subjected to elevated temperatures, long-term subsurface biosphere activity and mixing of diagenetic fluids influenced by Milankovitch cycles may play a role.

Uses

Dolomite serves as an ornamental stone, concrete aggregate, and source of magnesium oxide, and is used in the Pidgeon process for magnesium production. It is an important petroleum reservoir rock and host for Mississippi Valley-Type ore deposits of lead, zinc, and copper. Where calcite limestone is costly, dolomite substitutes as a flux in iron and steel smelting. Large quantities are used in float glass production. In horticulture, it buffers pH and supplies magnesium in soils and potting mixes. Dolomitic lime raises pH in pastures and in soils affected by mining. It buffers pH in marine aquariums. Calcined dolomite catalyzes tar destruction in biomass gasification. Particle physics detectors are built under dolomite layers to insulate against cosmic rays without adding background radiation. Large transparent crystals from Eugui, Spain, are valued by collectors.