Calcite sulphate

Calcium Calcite, sulphates, organic association, silicates, phosphates... 

Fig. 35. Hexagonal and trigonal systems. (See also Figs. 24, 27 and 29.) a. Hexagonal- type unit cell. 6. Apatite, 3Ca3(P04)I.CaFB. Class 6/rm c. riydrocinchonine sulphate hydrate, (C19H940N9)9.H2S04. llHaO. Class 6m. d. Rhombohedral-type uriit cell, e. A habit of calcite, CaC()s. Class 3m. /. KBr03. Class 3m.

The optical properties of crystals are usually quite reliable criteria for identification but occasionally crystals have sifbmicroscopic cracks and cavities, and although appearing quite normal, give refractive indices lower than those of an entirely solid crystal. This phenomenon, which is obviously very misleading, is fortunately very rare, but has been observed in anhydrite (calcium sulphate) and calcite (calcium carbonate) prepared in the laboratory. In cases erf doubt, X-ray powder photographs should be taken—see Chapter V. 

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

The mineralogical composition of Sahara dust particles shows the predominance of aluminosilicates (clays). Illite is also present in many cases while quartz particles are rare. Scanning Electron Microscopy (SEM) results on dust composition transported over different regions in the Mediterranean Basin have shown that Al-rich clay minerals such as illite and kaolinite are very common in PM10 for Cypms and dominant for Crete. Dust particles are also very rich in calcium which is distributed between calcite, dolomite and sulphates and Ca-Si particles (e.g. smectites) whereas iron oxides are often detected [43]. 

The dissociation pressure of calcite reaches 0.101 kPa (1 atm) at 894°C (S20) and the decarbonation reaction is highly endothermic (Section 3.1.4). The rate of decarbonation becomes significant at 500-600°C if a sufficiently low partial pressure of COj is maintained or if the calcite is intimately mixed with materials, such as quartz or clay mineral decomposition products, that react with the calcium oxide. Even in a precalciner, such mixing occurs, aided by agglomeration caused by the presence of low-temperature sulphate melts. 

Calcium (3.4% of lithosphere) occurs as carbonate in the minerals aragonite and calcite and the rocks limestone, chalk and marble, and as sulphate in anhydrite, CaS04, and gypsum, CaS04.2H20. All are plentiful and widely distributed. The metal, which has both h.c.p. and c.c.p. forms, is made (Fig. 138) by electrolysing the fused chloride, CaClg, a by-product of the Solvay process. 

Spiro, B., 1977. Bacterial sulphate reduction and calcite precipitation in hypersaline deposition of bituminous shales. Nature, 269 235—237. 

These two reactions contribute C to the pore waters in the sulphate reduction and, particularly, the suboxic zones. Methane seepages on the seafloor are accompanied by the formation of authigenic calcite and aragonite that are highly enriched in (Hovland el al 1987). 

Eogenetic calcite cement dominates in shallow marine siliciclastic sediments, and accompanies sulphate reduction and methane oxidation (Kantor-... 

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