Dolomite, or BITTER SPAR, a mineral consisting of double calcium and magnesium carbonate. The proportions of the two carbonates are very variable, and occasionally the mineral contains a considerable percentage of ferrous carbonate. It crystallises in rhombohedral forms, the faces of the crystals being often curved; its lustre is somewhat pearly or vitreous, and its colour usually white, but variously coloured kinds are not uncommon, such as reddish, brown, green, gray, and black. It effervesces feebly with cold acid. The pure crystallised varieties are known as Pearl Spar. Those which contain a notable proportion of ferrous carbonate are called Brown Spar (q.v.) or Ankerite. Besides these, columnar fibrous and granular or saccharoid varieties are known. Magnesian limestone—a rock occurring abundantly in the Permian System (q.v.)—is composed essentially of bitter spar, and hence is often called dolomite. It is usually white, gray, or yellow in colour, and finely crystalline. In some varieties cellular spaces occur which are often lined with crystals of dolomite.
Other varieties are composed of botryoidal or irregularly-shaped concretionary masses. Most limestones contain some magnesian carbonate, but it is only when the percentage of this salt is considerable that they are called magnesian limestones or dolomites. Magnesian limestone is often used as a building-stone; and it is also burned and made into mortar, but the lime obtained from it remains much longer caustic than lime from common limestone, and is considered of less value for agricultural purposes. In some districts, however, this lime is preferred to purer limes for application to hill-pastures.—Dolomite is named after the geologist Deodat Guy de Dolomieu (1750-1801), who was born at Dolomieu in Dauphiné.