Calcium carbonate Aragonite, Calcite

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless carbonate precipitation is exclusively controlled by biological processes, specifically... 

Calcium Carbonate CaC03 Calcite Aragonite Vaterite Widespread in animals and plants Exoskeleton Gravity, Ca store Eye lens... 

Concerning calcium carbonate polymorphs calcite and aragonite, the experiments of Romanek et al. (1992) at low T (10 to 40 °C) seem to indicate that the enrichment factor of in the solid phase for CaC03-C02 partitioning is reduced by about two units in the aragonite polymorph with respect to the calcite counterpart, r being equal (e.g., at T = 10 °C, 1000 In a 13 for calcite-C02 and 11 for... 

It is of interest that two thermodynamically unstable forms of calcium carbonate, aragonite and vaterite. are found in living organisms as well as the more stable calcite. There appears to be no simple explanation for the distribution of the different forms in the various species. 

Acute scalcntihcdral crystals are sometimes referred to as dogtooth spar. Calcite represents the stable form of calcium carbonate aragonite will go over to calcite at 470 C (K78=F. Calcite is a common constituent of sedimentary rocks, as a vein mineral, and as deposits from hot springs and in eaves as stalactites and stalagmites. 

Although calcium is more metallic in character than magnesium, compounds of the two elements share some similar properties. Calcium carbonate, CaC03, occurs naturally as chalk and limestone. Marble is a dense form of calcium carbonate that can be given a high polish it is often colored by impurities, most commonly iron cations (Fig. 14.26). The two most common forms of pure calcium carbonate are calcite and aragonite. All these carbonates are the fossilized remains of marine life. Calcium carbonate decomposes to calcium oxide, CaO, or quicklime, when heated ... 

We shall study two forms of calcium carbonate CaCO calcite and aragonite. Thus, it is possible to see the behavior of identical chemical entities in different crystalline environments. Both crystals are of an ionic nature and contain CO3 groups as covalent entities with their own internal modes. 

In the oceans, reef and planktonic organisms precipitate calcium carbonate (both calcite and aragonite), which comprises a major component of marine sediments. The precipitation of calcium carbonate is essentially the reverse of reaction (la) above ... 

The most common forms of pure calcium carbonate are calcite and aragonite. All carbonates are the fossilized remains of marine life. 

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless, calcium precipitation is exclusively controlled by biological processes, specifically the formation of hard parts (shells, skeletal parts, etc.). The very few existing amounts of spontaneous inorganic precipitation of CaC03(s) come from the Bahamas region of the Caribbean. 

Yuen et al. determined atomic site charges on carbonate anion by fitting the crystal structures of calcium carbonate polymorphs calcite and aragonite. They obtained c = 0-95(26) and qo = -0.98(9). The 6-31G PD values are = 1-210(11) and qo = - 1.070(4), which are narrowly within the confidence limits of the crystal structure derived charges. Hirshfeld and Mirsky found qQ= - 0.410 e from the observed quadrupole moment of carbon dioxide, in good agreement with the PD value. 

Carbonate minerals form from dissolved HCO3 with only a minor fractionation that is relatively insensitive to temperature. The two most important calcium carbonate minerals—calcite and aragonite— typically have values that are 0.9%o and 2.7%o enriched in 5 C relative to seawater bicarbonate at 25°C (Rubinson and Clayton 1969). The assumption that fractionation factors between seawater HCO3 and calcium carbonate minerals are invariant through time permits the use of stratigraphic variations in carbonate S C values as a proxy for secular changes in seawater 5 X values. 

Calcium carbonate CaCOa Calcite (s), rhombohedral, uniaxial, d = 2.71 Aragonite (m), orthorhombic, biaxial, d = 2.94... 

Calcium carbonate (aragonite) CaC03 471-34-1 100.087 wh orth cry or powder trans calcite 450 2.930 0.00066 s dil acid... 

Synonyms/Trade Names Calcium carbonate. Natural calcium carbonate [Note Calcite aragonite are commercially important natural calcium carbonates.] ... 

Calcium has been recognized as a constituent of gallstones since the earliest chemical studies (see reference 118). It occurs as calcium carbonate (either calcite or aragonite), calcium phosphate (apatite), calcium bilirubinate, or the salts of bile acids and fatty acids. In normal hepatic bile, the calcium concentration is about 21 mg %. It is about 45 mg % in normal gallbladder bile and about twice that in the gallbladder bile of patients with cholesterol stones (119). The concentration of calcium is said to vary with the concentration of bile salts (3). Apparently, calcium in bile is bound in part to bile salt micelles, and under normal conditions this complex remains stable and soluble (120). However, under appropriate conditions of pH, calcium concentration, and anion concentration, calcium salts may precipitate. 

Corrales LR (1999) Dissociative model of water clusters. J Chemical Physics 110 9071-9080 Curtiss LA, Halley JW, Hautman J, Rahman A (1987) Nonadditivity of ab-initio pair potentials for molecular dynamics of multivalent transition metal ions in water. J Chem Phys 86 2319-2327 de Leeuw NH, Parker SC, Catlow CRA, Price GD (2000) Proton-containing defects at forsterite (010) tilt grain boundaries and stepped surfaces. Am Min 85 1143-1154 de Leeuw NH, Parker SC (1998) Surface stracture and morphology of calcium carbonate polymorphs calcite, aragonite, andvaterite An atomistic approach. JPhys ChemB 102 2914-2922... 

The band positions shift with environment and are in different positions in calcium carbonate (polymorphs calcite and aragonite), dolomite (calcium magnesium carbonate), sodium carbonate, and so on, alone or in aqueous solutions. 

Calcium carbonates group Calcium group Aragonite Calcite Chalk Lime Limestone Vaterite White earth Whiting Doerner (1935) 11 Gadbery (1993) Heiz (1990) Laurie (1895) Merrifield (1849) cliii Orton (1992) Palomino (1715-24) 113, 152 Siddall (1997) Vitmvius (1st cent BC/Grainger, 1934) 93... 

Calcium carbonates group Calcium group Aragonite Calcite Coral Cuttlefish bone Eggshell Gofun Mother-of-pearl Pearl white Terrene white Tyrian purple... 

Surface treatment of CaCOj can change its nucleating activity in iPP. In Reference [134] iPP composites with nano-CaCOs were modified with iPP grafted with acrylic acid (PP-g-AA). of iPP in the composites increased with increasing nano-CaCOs content, and it was further increased by the addition of PP-g-AA. Recently, composites of iPP with 1 wt% and 3 wt% of nanosized calcium carbonate, both calcite and aragonite, coated either with PP-g-MA or fatty acids were... 

The carbonate minerals that comprise limestone ate calcite [13397-26-7] (calcium carbonate), which is easily the most abundant mineral type aragonite [14791-73-2] (calcium carbonate) dolomite [17069-72-6] (double carbonate of calcium and magnesium) andmagnesite [13717-31 -5] (magnesium carbonate). Individual limstone types ate further described by many common names (1). Some of this nomenclature is repetitious and overlapping. The following terms ate in common use in Europe and the United States. 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

The choice of selected raw materials is very wide, but they must provide calcium oxide (lime), iron oxide [1309-37-1/, siHca, and aluminum oxide (alumina). Examples of the calcereous (calcium oxide) sources are calcium carbonate minerals (aragonite [14791-73-2] calcite [13397-26-7] limestone [1317-65-3] or mad), seasheUs, or shale. Examples of argillaceous (siHca and alumina) sources are clays, fly ash, mad, shale, and sand. The iron oxide commonly comes from iron ore, clays, or mill scale. Some raw matedals supply more than one ingredient, and the mixture of raw matedals is a function of their chemical composition, as deterrnined by cost and availabiHty. 

The commercial grades of calcium carbonate from natural sources are either calcite, aragonite, or sedimentary chalk. In most precipitated grades aragonite is the predominant crystal stmcture. The essential properties of the two common crystal stmctures are shown in Table 1. 

Of course, freezing of a liquid - or its inverse - are themselves phase transformations, but the scientific study of freezing and melting was not developed until well into the 20th century (Section 9.1.1). Polymorphism also links with metastability thus aragonite, one polymorphic form of calcium carbonate, is under most circumstances metastable to the more familiar form, calcite. 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

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