Dolomitization, process by which limestone is altered into dolomite; when limestone comes into contact with magnesium-rich water, the mineral dolomite, calcium and magnesium carbonate, CaMg(CO3)2, replaces the calcite (calcium carbonate, CaCO3) in the rock, volume for volume. Dolomitization involves recrystallization on a large scale. The dolomite mineral grains often show distinct faces, are of more or less uniform size throughout, and are larger than the calcite crystals in the limestone. When the recrystallization is not complete, the dolomite crystals are scattered throughout a calcite matrix. Sometimes rocks are formed that show patches of dolomite mottling the limestone where the magnesium-rich waters are thought to have filtered through the rock; such mottling does not appear to be controlled by bedding structures, however, and thus may be a product of the unmixing (separation) from a crystalline mixture containing calcium and magnesium carbonates of magnesium carbonate, which then forms dolomite.
In the process of dolomitization the dolomite crystals cut across original calcite grains, fossils, and oölites (spherical modules of calcite) and sometimes include quartz grains within their boundaries. Dolomites do not preserve the textures of the original limestone nor the fossils therein. Sometimes the only evidence that the limestone was formed from grains of older rocks is scattered quartz grains. Fossils are uncommon in dolomites but sometimes remain as faint shadows outlining the original shape without showing internal detail or as molds with poor detail and filled with tiny dolomite crystals.