Aragonite polymorphism

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... 

A key question is then the relationship between the isotopic composition of the C02(g) in the soil and the CO2 dissolved in the mineral species growing in the soils. The equilibrium carbon isotope fractionations between C02(g) and carbonate minerals (CaC03 (both calcite and aragonite polymorphs), MgC03 (magnesite)) are known experimentally and can be accurately calculated from quantum chemistry (75). Overall, carbon in carbonate minerals is isotopically heavier than carbon in C02(g) by about... 

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. 

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. 

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

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. 

The two polymorphs of CaC03, e. g. the rhombohedral calcite (space group R 3c) and the orthorhombic aragonite (space group Pmen) are most common in organisms. 

This method of analysis is particularly valuable when chemical methods are inadequate or inapplicable. For instance, for complex mixtures where the different elements or ions may be associated in many different ways, all compatible with the analytical figures or for mixtures of polymorphous forms of the same substance, such as the three crystalline forms of CaC03 (calcite, aragonite, and vaterite) or the three crystalline forms of FeO(OH) (goethite, lepidocrocite, and P FeO(OH)—see Bunn, 1941)—mixtures for which chemical analytical methods are irrelevant. 

Paramorphism results when the internal crystal structure of a mineral is changed In a polymorphous form, yet retaining the external crystal form of the original mineral, e.g.. aragonite after caleite. 

Aragonite A calcium carbonate mineral (CaCCF) that is a polymorph of calcite. 

Polymorph Two or more solid crystalline substances with the same chemistry, but different crystalline structures. Calcite (rhombohedral CaCCE) and aragonite (orthorhombic CaCCE) are polymorphs. 

Figure 2.1. Generalized phase relations in the unary system CaCC>3. "A" -aragonite "I" through "V - calcite polymorphs with metastable fields indicated by (). Dash-dot line at 800°C represents transition encountered on cooling runs solid line at lower T represents transition encountered on heating runs. (After Carlson, 1980.)...

Falini, G., Albeck, S., Weiner, S., and Addadi, L. "Control of aragonite or calcite polymorphism by mollusk shell macromolecules". Science 271(5245), 67-69 (1996). 

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