of natural history, to which, after the death of Daubenton, he had been elected in January 1800. His course of lectures concluded, he revisited Switzerland. Returning thence he reached the residence of his brother-in-law at Château-Neuf, in the department of Saône-et-Loire, where he was seized with a fever, to which in a few days he succumbed, on the 26th of November 1801.
Dolomieu’s geological theories are remarkable for originality and boldness of conception. The materials constituting the primordial globe he held to have arranged themselves according to their specific gravities, so as to have constituted a fluid central sphere, a solid crust external to this, next a stratum of water, and lastly the atmosphere. Where water penetrated through the crust, solidification took place in the underlying fluid mass, which enlarging in consequence produced rifts in the superincumbent rocks. Water rushing down through the rifts became decomposed, and the resulting effervescence occasioned submarine volcanoes. The crust of the earth he believed to be continually increasing in thickness, owing to the deposition of aqueous rocks, and to the gradual solidification of the molten interior, so that the volcanic eruptions and other geological phenomena of former must have been of far greater magnitude and frequency than those of recent times.
See Lacépède, “Éloge historique de Dolomieu,” in Mémoires de la classe des sciences de l’Institut (1806); Thomson, in Annals of Philosophy, vol. xii. p. 161 (1808).
DOLOMITE, a mineral species consisting of calcium and magnesium carbonate, CaMg(CO3)2, and occurring as rhombohedral 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; MgCO3, 45·65%). Normal dolomite is thus not an isomorphous mixture of calcium and magnesium carbonates, but a double salt; and any variations in composition are to be explained by the isomorphous mixing of this double salt with carbonates of calcium, iron, magnesium, manganese, and rarely of zinc and cobalt.
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| Fig. 1. |
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| Fig. 2. |
In crystalline form dolomite is very similar to calcite, belonging to the same group of rhombohedral carbonates; the primitive rhombohedron, r (100), parallel to the faces of which there are perfect cleavages, has interfacial angles of 73° 45′, the angle of the cleavage rhombohedron of calcite being 74° 55′. 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 (fig. 1), sometimes to an extraordinary degree giving rise to saddle-shaped crystals (fig. 2). Crystals with plane faces are usually twinned, there being an interpenetration of two rhombohedra with the vertical axes parallel. The secondary twin-lamination, parallel to the obtuse rhombohedron e (110), so common in calcite, does not exist in dolomite. In the degree of symmetry possessed by the crystals there is, however, an important difference between calcite and dolomite; the former has the full number of planes and axes of symmetry of a rhombohedral crystal, whilst the latter is hemihedral with parallel faces, having only an axis of triad symmetry and a centre of symmetry. This lower degree of symmetry, which is the same as that of dioptase and phenacite, is occasionally shown by the presence of an obliquely placed rhombohedron, and also by the want of symmetry in the etching and elasticity figures on the faces of the primitive rhombohedron.
Dolomite is both harder (H. = 312–4) 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 (see below, Dolomite Rock). 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, who in 1791 noted that certain Tyrolese calcareous rocks and Italian marbles effervesce only slightly in contact with acid; this name was for many years 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 crystallized specimens are, however, usually found in metalliferous deposits; for example, in the iron mines of Traversella near Ivrea in Piedmont (as large twinned rhombohedra) and Cleator Moor in Cumberland; in the deposits of lead and zinc ores at Alston in Cumberland, Laxey in the Isle of Man, Joplin in Missouri; and in the silver veins of Schemnitz in Hungary and Guanajuato in Mexico.
Several varieties of dolomite have been distinguished, depending on differences in structure and chemical composition. Miemite is a crystallized or columnar variety, of a pale asparagus-green colour, from Miemo near Volterra in Tuscany; taraspite is a similar variety from Tarasp in Switzerland. Gurhofite, from Gurhof near Aggsbach in Lower Austria, is snow-white, compact and porcellanous. Brossite, from the Brosso valley near Ivrea in Piedmont, and tharandite, from Tharand in Saxony, are crystallized varieties containing iron. Closely related is the species ankerite (q.v.). (L. J. S.)
Dolomite Rock.—The rock dolomite, also known as dolomitic or magnesian limestone, consists principally of the mineral of the same name, but often contains admixture of other substances, such as calcite, quartz, carbonate and oxides of iron, argillaceous material, and chert or chalcedony. Dolomites when very pure and well crystallized may be snowy white (e.g. some examples from the eastern Alps), but are commonly yellow, creamy, brownish or grey from the presence of impurities. They tend to be crystalline, though on a fine scale, and appear under the microscope composed of small sharply angular rhombohedra, with a perfect cleavage and very strong double refraction. They can be often recognized by this, but are most certainly distinguished from similar limestones or marbles by tests with weak acid. Dolomite dissolves only very slowly in dilute hydrochloric acid in the cold, but readily when the acid is warmed; limestones are freely attacked by the acid in either state. Magnesian limestones, which contain both dolomite and calcite, may be etched by exposing polished surfaces for a brief time to cold weak acid; the calcite is removed, leaving small pits or depressions. The distribution of the calcite may be rendered more clear by using ferric chloride solution. This is decomposed, leaving a yellow stain of ferric hydrate where the calcite occurred. Alternatively, a solution of aluminium chloride will serve; this precipitates gelatinous alumina on contact with calcite and the film can be stained with aniline dyes (Lemberg’s solution). The dolomite is not affected by these processes.
Dolomites of compact structure have a higher specific gravity than limestones, but they very often have a cavernous or drusy character, the walls of the hollows being lined with small crystals of dolomite with a pearly lustre and rounded faces. They are also slightly harder, and for these and other reasons they last better as building stones and wear better when used for paving or road-mending. Dolomites are rarely fossiliferous, as the process of dolomitization tends to destroy any organic remains originally present. As compared with limestones they are less frequently well bedded, but there are exceptions to this rule. Many
