Atomic Positional Parameters
Ca 3a  0.0000  0.0000  0.0000
Mg 3b  0.0000  0.0000  0.5000
C1 6c  0.0000  0.0000  0.2429
O1 18f 0.2480 -0.0347  0.2440

References
H Effenberger, K Mereiter and J Zemann (1981) Zeitschrift fuer Kristallographie, 156, 233.

C Vasconcelos, JA McKenzie, S Bernasconi, D Grujic, and AJ Tien (1995) Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures. Nature, 377, 220-222.

Mineral Chemistry
Dolomite has a crystal structure similar to calcite CaCO₃, but with every other plane of calcium ions replaced by a layer of magnesiums. Because calciums and magnesiums occupy distinct structural sites in dolomite, complete solid solution between CaCO₃ and MgCO₃ is achieved only at high temperatures. At ambient temperatures, the composition of dolomite is close to CaMg(CO₃)₂.

Dolomite is found principally as a sedimentary mineral, and the origins of extensive sedimentary strata of dolomite, and their mechanisms of formation, have long been debated. A wide range of investigations into dolomite formation have been undertaken, including geochemical and hydrologic experimental studies. Recent research has suggested a link between dolomite formation and sulphate reducing bacteria (Vasconcelos et al., 1995) although the exact chemical mechanisms for formation of dolomite in this manner have yet to be fully explored. Dolomite may also be formed at higher temperatures; it is found as a mineral in hydrothermal veins, and in metamorphosed dolomite sediments, forming dolomitic marble.

Phases isostructural with dolomite are ankerite, CaFe(CO₃)₂, and the rare mineral kutnahorite, CaMn(CO₃)₂.

The Chime figure shows calcium atoms coloured grey and magnesiums green; both metal ions are coordinated by six oxygens. Note the triangular carbonate ions in the structure.