Minerals aragonite, CaCO

Sedimentary calcium carbonates are formed as the shells of marine plants and animals. Biologically produced CaCOs consists primarily of two minerals aragonite and calcite. Shallow-water carbonates, primarily corals and shells of benthic algae (e.g. Halimeda) are heterogeneous in their mineralogy and chemical composition but are composed mainly of aragonite and magnesium-rich calcite (see Morse and Mackenzie (1990) for a discussion). Carbonate tests of microscopic plants and animals, most of which hve in the surface ocean (there are also benthic animals that produce carbonate shells), are primarily made of the mineral calcite, which composes the bulk of the CaCOs... 

The most important carbonate mineral is calcium carbonate, CaCOs. This substance occurs in beautiful colorless hexagonal crystals as the mineral calcite. Marble is a microcrystalline form of calcium carbonate, and limestone is a rock composed mainly of this substance. Calcium carbonate is the principal constituent also of pearls, coral, and most sea shells. It also occurs in a second crystalline form, as the orthorhombic mineral aragonite. 

As summarized in Table 14.1, teeth, bones, shells, etc. are indispensable components, consisting of inorganic mineral crystals and protein film, with sizes, morphologies, and textures suitable to fulfil the function of the particular organs involved. In this section we will look at hydroxyapatite, aragonite and calcite (two polymorphs of CaCO ), and magnetite in greater detail. 

Calcium carbonate minerals (CaCOs) are ubiquitous in biomineralizing systems, since they are in the skeletons of organisms ranging from bacteria and algae to humans, most commonly as calcite (rhomboedral) or aragonite (orthorhombic). Thus, their structures and compositions are highly diversified. Here, only some examples are discussed. 

Several mineral phases are produced by phytoplankton and exported to depth in the ocean, associated with and analogous to the production and export of organic matter. First among these is CaCOs, comprised of two mineral phases calcite and the more soluble aragonite (Milliman, 1974 Mucci, 1983). The solubility of calcite depends also on the concentration of magnesium higher... 

Biogenous oozes are either calcareous or siliceous. Calcareous oozes are predominantly the calcitic tests of coccolithophores and/or foraminifera, or the aragonitic tests of pteropods. The solubility of CaCOs increases with decreasing temperature and increasing pressure, and thus with increasing depth in the oceans. Aragonite is 1.45 times more soluble than calcite (Morse and Mackenzie, 1990), so aragonitic oozes are confined to shallower depths than the calcitic oozes. The compensation depth for each mineral is defined as the depth at which the rates of... 

Carbonate minerals. Calcite (left) and aragonite (middle) are both CaCOs, and smithsonite (right) isZnCOs. 

Because minerals are defined both by composition and stracture, minerals can have the same composition but different structures and thus are distinctly different minerals. Diamond and graphite are both pure carbon but have different atomic arrangements and thus are different materials, with quite different properties. Likewise calcium carbonate, CaCO, has a definite composition but this is not a mineral name because calcium carbonate can occur as aragonite or as calcite, each with a unique arrangement of atoms. 

Carlson, W.D. (1983) The polymorphs of CaCOs and the aragonite- calcite transformation, in R.J. Reeder (ed.),Carbonates Mineralogy and Chemistry, Rev. Mineral. vol. 11, Mineralogical Society of America, Washington, pp. 191-225. 

Catti, M., Pavese, A., and Price, G.D. (1993) Thermodynamic properties of CaCOs calcite and aragonite a quasi-harmonic calculation, Phys. Chem. Minerals 19,472-479. 

Pavese A, Catti M, Price GD, Jackson RA (1992) Interatomic potentials for the CaCOs polymorphs (calcite and aragonite) fitted to elastic and vibrational data. Phys Chem Miner 19 80-87 Perram JW, Petersen HG, Leenw SW de (1988) An algorithm for the simulation of condensed matter which grows as the 3/2 power of the nurnber of particles. Mol Phys 65 875-893 Petersen HG, Soelvason D, Perram JW, Smith ER (1994) The very fast multipole method. J Chem Phys 101 8870-8876... 

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