DOLOMITE, a mineral species consisting of calcium and magnesium carbonate, and occurring as rhombo hedral crystals or large rock-masses. Analyses of most well crystallized specimens correspond closely with the above formula, the two carbonates being present in equal molecular proportions (CaCO3,54.35; Normal dolomite is thus not an isomorphous mixture of calcium and magnesium carbonates, but a double salt. In crystalline form it is very similar to calcite, belonging to the same group of rhombohedral carbonates; the primitive rhombohedron, parallel to the faces of which there are perfect cleavages, has interfacial angles of 73° 45'. A specially characteristic feature is that this rhombohedron is frequently the only form present on the crystals (in calcite it is rare except in combination with other forms) ; the faces are also usually curved, sometimes to an extraordinary degree, giving rise to saddle-shaped crystals. In the degree of symmetry possessed by the crystals there is, however, an important difference between calcite and dolomite; the latter is hemihedral with parallel faces, having only an axis of triad symmetry and a centre of symmetry.
Dolomite is both harder (H = and denser (sp. gr. 2.85) than calcite. The two minerals may also be readily distinguished by the fact that dolomite is not acted upon by cold, dilute acids. Crystals of dolomite vary from transparent to translucent, and often exhibit a pearly lustre, especially when the faces are curved;, the colour is usually white or yellowish.
The crystallized mineral was first examined chemically by P. Woulfe in 1779, and was named compound-spar by R. Kirwan in 1784 ; other early names are bitter-spar, rhomb-spar and pearl spar (but these included other rhombohedral carbonates). The name dolomite (dolomie of N. T. de Saussure, 1792) is in honour of the French geologist, D. G. Dolomieu; this name was originally applied to the rock only, but was later extended to the crystallized mineral, first in the form dolomite-spar.
In the white crystalline dolomite-rock of the Binnenthal near Brieg in Switzerland beautiful water-clear crystals of dolomite are found ; and crystallized masses occur embedded in serpentine, talc-schist and other magnesian silicate rocks. The best crystal lized specimens are, however, usually found in metalliferous deposits ; for example, in the iron mines of Traversella near Ivrea, Piedmont (as large twinned rhombohedra) and Cleator Moor, Cumberland ; in the deposits of lead and zinc ores at Alston, Cumberland, Laxey in the Isle of Man and Joplin in Missouri. (L. J. S.)