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DOLOMITE
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DOLOMITE , a See also:mineral See also:species consisting of See also:calcium and See also:magnesium carbonate, CaMg (CO3)2, and occurring as See also:rhombohedral crystals or large See also:rock-masses. Analyses of most well-crystallized specimens correspond closely with the above See also:formula, the two See also:carbonates being See also: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 See also:double See also:salt; and any See also:variations in See also:composition are to be explained by the isomorphous mixing of this double salt with carbonates of calcium, See also:iron, magnesium, See also:manganese, and rarely of See also:zinc and See also:cobalt. In crystalline See also:form dolomite is very similar to See also:calcite, belonging to the same See also:group of rhombohedral carbonates; the See also:primitive rhombohedron, r (See also:loo), parallel to the faces of which there are perfect cleavages, has interfacial angles of 730 45', the See also:angle of the cleavage rhombohedron of calcite being 74° 55'• A specially characteristic feature is that this rhombohedron is fre- quently the only form present on the crystals (in calcite it is rare except in See also:combination with other forms); the faces are also usually curved (fig. I), sometimes to an extraordinary degree giving rise to See also:saddle-shaped crystals (fig. 2). Crystals with See also:plane faces are usually twinned, there being an interpenetration of two rhombohedra with the See also:vertical axes parallel. The secondary twin-lamination, parallel to the obtuse rhombohedron e (Ho), so See also: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 See also:axis of triad symmetry and a centre of symmetry. This See also:lower degree of symmetry, which is the same as that of See also:dioptase and See also:phenacite, is occasionally shown by the presence of an obliquely placed rhombohedron, and also by the want of symmetry in the See also:etching and See also:elasticity figures on the faces of the primitive rhombohedron. Dolomite is both harder (H.= 3–4 and denser (sp. gr.2.85) than calcite. The two minerals may also be readily distinguishedby the fact that dolomite is not acted upon by See also: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 See also:colour is usually See also: Gurhofite, from Gurhof near Aggsbach in Lower See also:Austria, is See also:snow-white, compact and porcellanous. Brossite, from the Brosso valley near Ivrea in Piedmont, and tharandite, from Tharand in See also:Saxony, are crystallized varieties containing iron. Closely related is the species See also:ankerite (q.v.). (L. J. S.) Dolomite Rock.—The rock dolomite, also known as dolomitic or magnesian See also:limestone, consists principally of the mineral of the same name, but often contains admixture of other substances, such as calcite, See also:quartz, carbonate and oxides of iron, argillaceous material, and chert or See also:chalcedony. See also:Dolomites when very pure and well crystallized may be snowy white (e.g. some examples from the eastern See also:Alps), but are commonly yellow, creamy, brownish or See also:grey from the presence of impurities. They tend to be crystalline, though on a See also:fine See also:scale, and appear under the See also:microscope composed of small sharply angular rhombohedra, with a perfect cleavage and very strong double See also: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 See also:state. Magnesian See also:lime-stones, which contain both dolomite and calcite, may be etched by exposing polished surfaces for a brief See also:time to cold weak acid; the calcite is removed, leaving small pits or depressions. The See also:distribution of the calcite may be rendered more clear by using ferric chloride See also:solution. This is decomposed, leaving a yellow stain of ferric See also:hydrate where the calcite occurred. Alternatively, a solution of See also:aluminium chloride will serve; this precipitates gelantinous alumina on contact with calcite and the film can be stained with See also:aniline dyes (See also: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 See also: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 See also:building stones and See also:wear better when used for paving or road-mending. Dolomites are rarely fossiliferous, as the See also: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 See also:rule. Many dolomites, particularly those of the See also:north of See also:England, show a very remarkable concretionary structure. The beds look as if made up of rounded balls of all sizes from a See also:foot or two in See also:diameter down-wards. Often they are See also:stuck together like piles of shot or bunches of grapes. They are composed of fibrous radiate calcite crystals, which by some See also:kind of concretionary See also:action have segregated from the dolomitic material and grouped themselves together in this way. Other concretions from these beds resemble bunches of See also:corals, tufts of See also:plants, or present various See also:strange imitative forms. Dolomite, unlike calcite, is not secreted by marine animals to build up the hard parts of their skeletons, and it is generally agreed also that dolomite is only very rarely and under exceptional conditions deposited directly from solution in water. On the other See also:hand, there is much See also:evidence to show that limestones may absorb or be partly replaced by magnesium carbonate, and the double salt dolomite substituted for calcite by one of those processes which are described as " metasomatic. Thus the Carboniferous limestones of various parts of See also:Britain pass into dolomites along lines of See also:joint, fissure or See also:fault, or occasionally along certain bedding planes. At the same time the rock becomes crystalline; its See also:minute structure is altered, its fossils are effaced, and as dolomite has a higher specific gravity than limestone, contraction results and cavities are formed. The prevalence of crystalline, concretionary and drusy structures in dolomite can thus be simply explained. The process may actually be studied in many " magnesian limestones," in which by means of the microscope we may trace the See also:gradual growth of dolomite crystals taking See also:place simultaneously with the destruction of the See also:original features of the limestone. See also:Recent investigations in See also:coral reefs show that these changes are going on at the present See also:day at no considerable depths and in rocks which have not See also:long consolidated. All this goes to prove that the double carbonate of calcium and magnesium is under certain conditions a more See also:stable salt than either of the See also:simple carbonates, and that these conditions recur in nature with considerable frequency. Experiments have proved that at moderately high temperatures (roo° to zoo° C.) solutions of magnesium salts will convert calcite into dolomite in the laboratory, and that See also:aragonite is even more readily affected than calcite. The See also:analogy with dolomitization of limestones is strong but not See also:complete, as the latter process must take place at See also:ordinary temperatures 'and approximately under atmospheric pressures. No completely satisfactory explanation of the See also:change, from the standpoint of the geologist, has as yet been advanced; though much See also:light has been thrown upon the problem. Many limestones are See also:rich in aragonite, but this in course of time tends to re-crystallize as calcite. Magnesium salts are abundant in See also:sea-water, and in the See also:waters of evaporating enclosed coral lagoons and of many bitter lakes. Calcite is more soluble than dolomite in water saturated with carbonic acid and would tend to be slowly removed from a limestone, while the dolomite increased in relative proportion. Dolomite also being denser than calcite may be supposed to replace it more readily when pressure is increased. These and many other factors probably co-operate to effect the transmutation of limestones into dolomites. Examples of dolomitization may be obtained in practically every See also:geological formation in which limestones occur. The See also:oldest rocks are most generally affected, e.g. the See also:Cambrian lime-stones of See also:Scotland, but the change occurs, as has already been stated, even in the upraised coral reefs of the See also:Indian and Pacific oceans which are very recent formations. It is very interesting to See also:note that dolomites are very frequent among rocks which indicate that See also:desert or salt-See also:lake conditions prevailed at the time of their See also:deposit. The dolomite or magnesian limestone of the See also:English See also:Permian is an instance of this. The explanation may be found in the fact that the waters of bitter lakes are usually rich in magnesium salts which, percolating through beds of limestone, would convert them into dolomite. Among the most famous dolomites are those of the Dolomite Alps of See also:Tirol. They are of Triassic See also:age and yield remarkably picturesque See also:mountain scenery; it is believed that some were originally coral reefs; they are now highly crystalline and often contain interesting minerals and ores. The See also:galena limestone of the North See also:American Trenton rocks is mostly a dolomite. Dolomites furnish' excellent-building stones, and those of the north-See also:east of England (See also:Mansfield See also: Additional information and CommentsThere are no comments yet for this article.
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