Dolomite

[Chemical composition] Mg in the component can be replaced by Fe, Mn, Co, and Zn. Among them, CaMg[CO ₃ ] ₂ —CaFe[CO ₃ ] ₂ can be in the complete isomorphous series; when Fe>Mg, it is called iron dolomite. The substitution of Fe and Mn is limited, and the end of the Mn, CaMn[CO ₃ ] _{2, is} called manganese dolomite. It can also form variants such as lead dolomite, cobalt dolomite, and zinc dolomite.

[Crystal Structure] Trigonal System; Rhombic unit cell: a _rh = 0.601 nm; α = 47 ° 36'; Z = 1; hexagonal unit cell: a _h = 0.481 nm, c _h = 1.601 nm; Z = 3. The crystal structure is similar to calcite . The difference lies in the structural position occupied by Ca ^{2+ in} the crystal structure of calcite, of which 1/2 is occupied by Mg in dolomite; Ca ²⁺ and Mg ²⁺ are layered regularly in the direction of the vertical cubic axis. The alternating arrangement, thus causing the crystal structure of dolomite to be less symmetrical than calcite. When Fe and Mn in dolomite replace Mg, it can cause cell unit to increase.

[Form] The crystal is often rhombohedral. It is not as diverse as calcite. The crystal face is often bent into a saddle shape (Fig. H-7(a), H-7(b)). The saddle can be seen under the microscope by cutting the slice. The morphology has a crystal-deformed phase-embedded structure and a wavy extinction phenomenon (Fig. H-7(c)). Often The twin crystal is formed, the latter double parallel to the dolomite cleavage plane length, the short diagonal line is different from the calcite. Some dolomites appear {02 1} split, caused by twin crystals. The aggregates are often granular, dense, and sometimes porous and kidney-like.

(c)

(a) crystal morphology, (b) saddle-like morphology, (c) photographs of many saddle-like dolomite crystal aggregates

Figure H-7 saddle-like dolomite crystal

((a), (b) quoted from Pan Zhaoyu, 1993, (b) according to Sadebeck, 1876)

[Physical properties] Pure is mostly white, iron-containing gray-dark brown, iron-bearing dolomite weathering, the surface becomes brown;

Glass luster. Cleavage {1011} is complete, and the cleavage plane is often curved. Hardness 3.5 to 4. The relative density of 2.85 increases with the increase of the content of Fe, Mn, Pb and Zn in the composition. Some dolomites produce bright orange red light under the action of cathode rays.

[genesis and occurrence] Dolomite is a kind of mineral widely distributed in nature, mainly composed of sedimentation and hydrothermal. It is the main mineral that makes up dolomite and dolomitic limestone. Dolomite is also one of the main constituent minerals of carbonate rock genesis. Magnesium-containing substance or dolomitic limestone deterioration or contact metamorphism dolomite marble can be formed in the area. At higher stages of metamorphism, dolomite can be broken down into periclase and brucite.

[Identification characteristics] The crystal face is often curved saddle shape. The difference with calcite is that the cold hydrochloric acid does not violently foam, and the foaming is intense after the heating, and the direction of the double crystal grain is also different from that of calcite. In addition, the dyeing method can be used to distinguish the two: 0.2 mol/L HCl is added to the alizarin red sulfur solution, the dolomite is not dyed, and the calcite is dyed reddish purple.

[Main use] Used as a refractory material and a flux in the production of blast furnace ironmaking; some dolomite can be used as a raw material for extracting magnesium. Dolomite marble can be used as a good building stone after processing.

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