Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The word dolomite is also used to describe the sedimentary carbonate rock, which is composed predominantly of the mineral dolomite (also known as dolostone). The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, tan, gray, or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium and magnesium ions. It does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid as calcite does. Crystal twinning is common.

Solid solution exists between dolomite, the iron-dominant ankerite and the manganese-dominant kutnohorite.[10] 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. Lead, zinc, and cobalt also substitute in the structure for magnesium.

Recent research has found modern dolomite formation under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. It is often thought that dolomite will develop only with the help of sulfate-reducing bacteria (e.g. Desulfovibrio brasiliensis).However, promising new research on low-temperature dolomite formation indicates that low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter. Dolomite. Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999. Those laboratory experiments showed how the initial precipitation of a metastable "precursor" (such as magnesium calcite) will change gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and re-precipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "breaking Ostwald's step rule". There is some evidence for a biogenic occurrence of dolomite. One example is that of the formation of dolomite in the urinary bladder of a Dalmatian dog, possibly as the result of an illness or infection.