Calcium carbonate occurrence

The often smooth top surface to the calcrete suggests that the calcrete formed by the precipitation of Ca (as carbonate) from sub-surface laterally and vertically flowing vadose water sourced from rainfall, which penetrates easily through the overlying sand. The sub-surface has been plugged with calcrete preventing vertical downwards penetration of vadose water. Instead, vadose water would move laterally until it evaporates or is removed by transpiration. Precipitation of calcium carbonate at the surface of the calcrete would occur, thus contributing to the laminated occurrence of the calcrete. 

Occurrence. Carbon is distributed very widely in nature as calcium carbonate (limestone). Coal, petroleum and natural gas are chiefly hydrocarbons. Carbon is found as carbon dioxide in the atmosphere of the earth and dissolved in all natural waters. The atmosphere of Mars contains 96% C02. It is plentiful in the sun, stars, comets and the atmospheres of most planets. 

In 1839, H- Rose said that the ordinary commercial carbonate liquefied when slowly heated in a retort whereas, in 1870, E. Divers found scarcely any liquefaction. The older carbonate when distilled with anhydrous calcium chloride gave ammonium chloride, calcium carbonate, and carbon dioxide, whereas the newer carbonate gave in addition ammonium carbamate. The solubility of the newer carbonate is about twice as great as the old and the aq. soln. is not charged with carbon dioxide. R. Phillips and E. Divers have also reported as rare the occurrence of the hydrocarbonate in commercial carbonate. In consequence of these differences it is necessary to know whether the old or the new carbonate is in question when discussing the properties of the commercial carbonate. Sometimes the sesquicarbonate is to be understood. 

Finally, this tripartite cyclicity is also seen in the frequency of occurrence of Phanerozoic ironstones and oolites (Figure 10.18). As sea level withdrew from the continents and continental freeboard increased, shallow-water areas with the requisite environmental conditions necessary to form oolite and ironstone deposits decreased in extent. Thus, as calcium carbonate deposidon increased on slopes and in the deep sea, carbonate oolite and ironstone deposition on shelves and banks nearly ceased. 

In Chapter 15 it was mentioned that hard water sometimes contains a large amount of dissolved calcium carbonate. Ordinarily we think of calcium carbonate as an insoluble substance, and the question about its occurrence in hard water is an interesting one. 

Beachrock occurs predominantly on tropical to subtropical ocean coasts and islands (e.g. Russell, 1962 Krumbein, 1979 Scoffin and Stoddart, 1983), but it is also found in the Mediterranean (Alexandersson, 1972 El-Sayed, 1988 Strasser et al., 1989), the Black and Caspian Seas (Zenkovitch, 1967, pp. 183-186), South Africa (Siesser, 1974 Cooper, 1991) and as far north as 57° latitude (Knox, 1973 Kneale and Viles, 2000). Beachrock is also reported to form on the coasts of freshwater lakes (Binkley et al., 1980 Jones et al., 1997). Beachrock occurrences from polar regions, i.e. north and south of 60° latitude have not been reported. Collectively, this distributional evidence identifies warm climates with pore waters rich in calcium carbonate as essential cement-precipitation prerequisites for beachrock formation. 

Occurrence Calcium carbonate is one of the most stable, common, and widely dispersed materials. It occurs in nature as aragonite, oyster shells, calcite, chalk, limestone, marble, marl, and travertine, especially in Indiana (structural limestone), Vermont (marble), Italy (travertine), and England (chalk). 

Where a skeletal source cannot be identified, calcium carbonate (CaC03) grains and finegrained muds may be of abiological origin. The most famous occurrences occur in shallow, warm, saline waters of the Bahamas and the Arabian Gulf. In these areas two distinctive morphologies are present, ooids and needle muds (Fig. 1). 

Atlantic Ocean Nodule abundance in the Atlantic Ocean appears to be more limited than in the Pacific or Indian Oceans, probably as a result of its relatively high sedimentation rates. Another feature which inhibits nodule abundance in the Atlantic is that much of the seafloor is above the calcium carbonate compensation depth (CCD). The areas of the Atlantic where nodules do occur in appreciable amounts are those where sedimentation is inhibited. The deep water basins on either side of the Mid-Atlantic Ridge which are below the CCD and which accumulate only limited sediment contain nodules in reasonable abundance, particularly in the western Atlantic. Similarly, there is a widespread occurrence of nodules and encrustations in the Drake Passage-Scotia Sea area probably due to the strong bottom currents under the Circum-Antarctic current inhibiting sediment deposition in this region. Abundant nodule deposits on the Blake Plateau can also be related to high bottom currents. 

Many minerals containing oxy anions, other than silicates, form primarily in sedimentary conditions that is, in aqneons solntion. Minerals containing the carbonate or sulfate ions are not likely to be formed at the high temperatures of igneous processes because both anions decompose at high temperatnres. For example, calcium carbonate decomposes to CaO and CO2. The solnbility rnles can provide a rationalization for observations such as the fact that halite (NaCl) is formed mainly in evaporated lakes with high concentrations of sodium and chloride ions, or the relatively infrequent occurrence of chal-canthite in copper mines, except where aqneons solntions may be concentrated and/or evaporated. 

Shortite [Named after the American mineralogist, Maxwell Naylor Short (1889-1952)1 (ICSD 16495 and PDF 21-1348) Na3Ca,(C03)3 M= 306.16 15.02 wt.%Na 26.18 wt.% Ca 11.77 wt.% C 47.03 wt.% 0 (Nitrates, carbonates, and borates) Orthorhombic a = 496.1 pm b = 1103 pm c= 712pm (Z = 2) P.G.mm2 S.G.Amm2 Biaxial a= 1.531 1.555 r= 1.570 27=75" 3 2600 (2610) Habit wedge-shaped, equant, or short prismatic crystals. Color colorless to pale yellow. Diaphaneity transparent. Luster vitreous. Streak n.a. Geavage (010). Fracture conchoidaL Decon oses in water releasing an insoluble residue of calcium carbonate. Fluorescent under UV radiation. Occurrence with calcite and pyrite. 

Donnay G. and Donnay J. D. H. Ewaldite, a new barium calcium carbonate I. Occurrence of ewaldite in syntactic intergrowths with mackelveyite. Tschermaks miner, petrogr. Mitt., 15, 1971, 185-200. 

The predominance of calcium carbonate in filled plastics is primarily related to its widespread occurrence as white and pure mineral deposits, combined with the low cost of processing. However, in many large volume applications, it gives important functional benefits and would still be the filler of choice, even if prices were doubled or trebled. 

The occurrence or exclusion of plant species in karst enviromnents has been studied in detail in temperate latitudes, and this has led to the ecological classification of plants as calcicoles, species frequently growing on calcium carbonate-rich soils, and calcifuges, species that avoid, or are not found, on those soils (Kinzel, 1983). This differentiation, however, is not necessarily related to physiological properties or modification of the competitive ability of those plant species under natural conditions. 

Prepared calcium carbonates are purified forms of native material. Conversion of calcium oxide and hydroxide produced from the calcination of material such as limestone and the subsequent absorption of atmospheric carbon dioxide furttier accounts for the occurrence of pigments such as those sometimes called lime. 

Forms of Occurrence.—(1) Insoluble Salts.— Tricalcium phosphate calcium carbonate, calcium fluoride. In man, 84-90 per cent, of bone calcium is present as phosphate the rest is chiefly carbonate. Calcium fluoride is the characteristic salt of tooth enamel. Insoluble calcium soaps form most of the fsecal calcium, especially when the diet is rich in fats. 

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