Dolomite, named for the French geologist D.G. Dolomieu, is a mineral containing calcium and magnesium carbonate, as well as trace heavy metals. It is a double salt made up of approximately 60% calcium carbonate (equivalent to 24% calcium) and 40% magnesium carbonate (equivalent to 12% magnesium). Dolomite is also known as magnesium limestone and earlier was called compound-spar, bitter-spar, rhomb-spar and pearl-spar. The Dolomites, a mountain region in the South Tyrolese Alps, are named for the presence of dolomite in the mountains.
Dolomite was at one time a popular nutritional supplement for calcium and magnesium. It is still marketed as a nutritional supplement, but is no longer popular. The reason for this is that in the early 1980s analysis of dolomite nutritional supplements revealed them to contain substantial amounts of lead, as well as other toxic elements, such as mercury, arsenic and cadmium. In addition, much better supplementary forms of calcium and magnesium are available. Dolomite is still used in several parts of the world as a liming agent, to raise the pH of the soil, and as a fertilizer to maintain soil magnesium levels. It is also used to make magnesia, which has medical applications.
Dolomite is a mineral CaMg (CO3)2 consisting of a calcium magnesium carbonate found in crystals and in beds as dolostone. A pure form of dolostone would be rare, however; it usually intergrades with limestone and is referred to as dolomitic limestone. There is uncertainty as to the cause of its formation, as vast deposits are present in ancient rock, but it is very rarely found being produced in modern environments. This is referred to as the "Dolomite Problem". Dolomite accounts for about 10% of sedimentary rock, including much that would have been produced near the surface of the earth. However, experiments have only been able to synthesize dolomite under the high temperatures and pressures present in deeper layers.
Recent research has shown modern dolomite formation under anaerobic conditions in hypersaline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. Similar processes have been discovered in the Coorong region of South Australia. It is now thought to develop under these conditions only with the help of sulfate-reducing bacteria. The fact that conditions were better for the survival of these bacteria on the ancient Earth may explain the Dolomite Problem. This joins other research in pointing out many new interesting links between large-scale geology and small-scale microbiology.
Dolomite is after calcite the second most important and abundant of the carbonate minerals. Chemically and structurally it may be regarded as calcite with half the calcium ions replaced by magnesium. Iron or manganese may substitute for magnesium in dolomite, forming isostructural series with ankerite and Kutnahorite. The crystal structure, hexagonal-rhombohedral, is similar to that of calcite, with alternate layers of calcium ions totally replaced by magnesium. This ordered arrangement of cations slightly impairs the overall symmetry of the structure but is essential to the stability of the mineral. Hardness is 4.5-5, specific gravity 2.85, luster vitreous to pearly, color ranges from colorless to white with green, brown, or pink tints, and cleavage is perfect in three directions.
Like calcite, dolomite occurs in virtually all geologic settings: in igneous rocks as carbonatite, in metamorphic rocks as marble, and in hydrothermal deposits. Also like calcite, the most abundant occurrences are in sedimentary rocks; rock composed primarily of dolomite is sometimes referred to as dolostone. Such rocks form vast deposits; in Italy, the Alpine range known as the Dolomites is almost entirely composed of dolomite. However, unlike calcite, dolomite's sedimentary origin is enigmatic. Although the most stable carbonate mineral where magnesium is abundant in the marine environment, it is unknown as a primary mineral. The vast, ancient deposits apparently formed from primary calcite or aragonite by diagenesis, yet this process is not observed in modern marine environments.
Dolomite is quarried for building and ornamental stone, road stone, and the production of refractory brick. It is the principal ore of magnesium metal and the source of the magnesium used by the chemical industry. Crystal habits include saddle shaped rhombohedral twins and simple rhombs some with slightly curved faces, also prismatic, massive, granular and rock forming. Streak is white. Calcite is far more common and effervesces easily when acid is applied to it. But this is not the case with dolomite which only weakly bubbles with acid and only when the acid is warm or the dolomite is powdered. Dolomite is also slightly harder, denser and never forms scalenohedrons (calcite's most typical habit).
Dolomite can be several different colors, but colorless and white are very common. However it is dolomite's pink color that sets another unique characteristic for dolomite. Crystals of dolomite are well known for their typical beautiful pink color, pearly luster and unusual crystal habit and it is these clusters that make very attractive specimens.Associated Minerals: include calcite, sulfide ore minerals, fluorite, barite.