Dolomite rock

(petrology) A limestone whose carbonate fraction contains more than 50% dolomite. Also known as dolomite rock; dolostone.

Sedimentary rock containing more than 50% by weight of the mineral dolomite [CaMg(CO₃)₂]. The term dolostone is used synonymously. Since dolomites usually form by replacement of preexisting limestones, it is often possible to see relict grains, fossils, and sedimentary structures preserved in dolomite rocks. More often, however, original textures are obliterated. See also Dolomite; Limestone.

Field differentiation of dolomite from calcite (CaCO₃) is most easily accomplished by application of dilute hydrochloric acid. Calcite is strongly effervescent in acid, whereas dolomite is usually very weakly effervescent unless scratched or ground into a fine powder. Dolomite also often weathers to a brown color on outcrop because of the common substitution of iron for magnesium in the dolomite structure. X-ray diffraction analysis is the most reliable method for the differentiation of dolomite from calcite, and also the best method for characterization of the degree of ordering of a particular dolomite sample. See also X-ray diffraction.

Dolomite rocks are predominantly monomineralic. The most common noncarbonate minerals are quartz (either authigenic or detrital), clay minerals, pyrite, and glauconite. Evaporite minerals or their replacements are also common.

Geologists have been unable to decipher the exact conditions of dolomite formation. This so-called dolomite problem revolves around several questions relating to the stoichiometry of the reaction in which dolomite is formed, the fact that dolomite is not common in young marine sediments, and the type of geological setting in which ancient dolomites formed.

The relative proportions of dolomite and calcite have changed through geologic time. Dolomitic rocks are dominant in the Precambrian and early Paleozoic, whereas calcitic rocks become dominant in the late Mesozoic and continue to dominate through the present. Several alternative hypotheses have been offered to explain this.

In a few geological settings, dolomite is forming within the sediments at the present time. One general setting includes a wide variety of supratidal ponds, lagoons, tidal flats, and sabkhas such as the Solar Lake in Israel, the Coorong Lagoon in Australia, the bank tops of Andros Island in the Bahamas, and the Sabkha of Abu Dhabi. Each of these environments is hot, more saline than seawater, and rich in organic matter. A second general setting of modern dolomite formation is in continental margin sediments underlying productive coastal oceanic upwelling zones. Examples include the Peru-Chile margin, the southern California borderlands, the Gulf of California, and the Walvis Ridge west of Namibia (southwestern Africa). This second set of geological environments is characterized by lower water temperature and normal marine salinity. Like the first set, these environments are rich in organic matter. The association of dolomite formation with abundant organic carbon seems to be the one common thread linking each dolomite occurrence and possibly controlling dolomite precipitation.

Dolomites are extremely important oil reservoir rocks. This is partly a result of the high porosity of many dolomite rocks and partly the result of the association between dolomite formation and the presence of sedimentary organic matter.

Dolomites are also the main host rocks for lead and zinc ore deposits. Rocks of this type are known as Mississippi Valley ore deposits. Neither the origin of the ore-forming solutions nor the association between the lead-zinc ores and the dolomite rocks has been satisfactorily explained. See also Petroleum geology; Sedimentary rocks.

Limestone that contains more than 10% but less than 80% of the mineral dolomite.