Carbonate minerals rhombohedral

Aragonite is the only one of the four carbonate minerals examined that does not have a calcite-type rhombohedral crystal structure. For all the minerals examined, with the exception of aragonite, the two solution saturation states studied represent supersaturated conditions, because at a saturation state of 1.2 with respect to calcite, the seawater solution is undersaturated (0.8) with respect to aragonite. 

Fisler DK, Gale JD, Cygan RT (2000) A shell model for the simulation of rhombohedral carbonate minerals and their point defects. Amer Mineral 85 217-224 Florke RWG (1967) Die Modifikationenvon Si02. Fortschr Mineral 44 181-230... 

The other carbonate mineral in common use as a filler is dolomite calcium magnesium carbonate (CaC03. MgC03). It is in the trigonal crystal system and is usually found as rhombohedral crystals with curved composite faces. Dolomite is harder (3.5), denser (2.85), and slightly more acid resistant than calcite, bnt in general, the mineral properties and properties in a polymer are similar. 

The commonly found carbonate minerals are calcite (CaCOg), dolomite (CaMg(C03)2), and magnesite (MgCOs). The former two minerals make up large volumes of carbonate rock, the ultimate repository for atmospheric carbon dioxide. These minerals have essentially the same crystal structure with rhombohedral symmetry which is characterized by excellent rhombohe-dral cleavages. 

Barber, D. J., Wenk, H.-R. (1979). Deformation twinning in calcite, dolomite, and other rhombohedral carbonates. Phys. Chem. Minerals, 5,141-65. 

Brown, N.E., Navrotsky, A., Nord, G.L., Baneijee, S.K. (1993) Hematite (Fe203)-ilmenite (FeTiOs) solid solutions Determinations of FeTi order from magnetic properties. Am Mineral 78 941-951 Burton, B.P. (1987) Theoretical analysis of cation ordering in binary rhombohedral carbonate systems. Am Mineral 72 329-336... 

The structure of calcium carbonate, CaC03, in the form of the mineral calcite has already been briefly described. The structure (fig. 10 03) is rhombohedral, and in terms of the cell indicated (which is not the smallest possible cell) it may be described as an arrangement in which Ca2+ ions occupy the corners and the centres of the faces of the cell while the C032 ions lie at the centre of the cell and at the mid-... 

Dolomite is strictly speaking the double carbonate containing 54 to 58% of CaCOa and 40 to 44 % of MgCOs. The mineral has a hexagonal-rhombohedral crystal structure. The term is, however, frequently used to describe dolomitic Umestone. 

Description and general properties. Dolomite is a massive calcareous sedimentary rock made of the mineral dolomite [CaMg(C03)j, rhombohedral], first identified by the French geologist D. Dolomieu in 1791 and named by H. Saussure after its discoverer. Dolomite occurs as huge geological formations such as in the northeastern Italian Alps called the Dolo-miti. Usually dolomite as a rock contains, apart from dolomite, other carbonates (e.g., calcite. 

Calcium carbonate exists in three crystal modifications, aragonite, calcite and vaterite, but only the calcite form is of real importance. Because of calcite s perfect rhombohedral cleavage, it is a soft mineral with a Mohs hardness of 3.0. It has a specific gravity of 2.7 and is birefringent having refractive indices of 1.65 and 1.48. 

Among the three polymorphs of crystalline calciiun carbonate, calcite is the equilibrium phase, most commonly adopting a rhombohedral morphology. In biologically produced minerals, however, the rhombohedral morphology is rarely adopted. Single-crystalline calcite fibers are observed most prominently in sea-urchin teeth and bacterial deposits [67-69]. They do not follow the crystallographic symmetry of calcite. Such calcite fibers were prepared in vitro via a solution-precursor-solid mechanism [70]. 

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