Calcium calcite aragonite

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 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. 

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

Some of the important compounds containing the group IA and IIA metals are the carbonates, nitrates, sulfates, and phosphates. We have already mentioned the mineral trona as the source of sodium carbonate. Calcium carbonate is found in many forms that include chalk, calcite, aragonite, and marble, as well as in egg shells, coral, and seashells. In addition to its use as a building material, calcium phosphate is converted into fertilizers in enormous quantities (see Chapter 14). 

Calcium carbonate (calcite, aragonite, Exoskeletons (mollusc and egg shells ... 

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. 

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. 

L. N. Plummer and E. Busenberg, The Solubilities of Calcite, Aragonite, and Valerite in Carbon Dioxide-Water Solutions Between 0 and 90 °C, and an Evaluation of the Aqueous Model for the System Calcium Carbonate-Carbon Dioxide-Water , Geochim. Cosmochim. Acta., 46, 1011-1040 (1982). 

In the oceans half of the DIC riverine input accumulated in nearshore (coastal zones, oceanic banks and atolls, shelves) and open ocean (hemipelagic and pelagic) sediments as calcium and magnesium carbonate minerals (calcite, aragonite, magnesian calcite), and half eventually returned to the atmosphere owing to precipitation (e.g.) ... 

Some of the Group IA and IIA metals are found in nature in the form of carbonates, silicates, nitrates, and phosphates. For example, calcium carbonate is one of the most important naturally occurring compounds, and it is found in several forms. The most common form of calcium carbonate is limestone, which is used extensively as a building stone as well as the source of lime. Other forms include chalk, calcite, aragonite, Iceland spar, marble, and onyx. Many other materials such as egg shells, coral, pearls, and seashells are composed predominantly of calcium carbonate. Thus, it is one of the most widely occurring compounds in nature. 

Part of the difficulty encountered in searching and interpreting the literature on polymorphic behaviour of materials is due to the inconsistent labelling of polymorphs. In many cases, the inconsistency arises from lack of an accepted standard notation. However, often, and perhaps more important, it is due to the lack of various authors awareness of previous work or lack of attempts to reconcile their own work with earlier studies (see, for instance. Bar and Bernstein 1985). While many polymorphic minerals and inorganic compounds acmally have different names (e.g. calcite, aragonite and vaterite for calcium carbonate or rutile, brookite, and anatase for titanium dioxide) this has not been the practice for molecular crystals, which have been labelled with Arabic (1, 2, 3,...) or Roman (I, II, III,...) numerals, lower or upper case Latin (a, b, c,... or A, B, C,...) or lower case Greek a, P,y, ) letters, or by names descriptive of properties (red form, low-temperature polymorph, metastable modification, etc.). 

Calcium hydroxide (slaked lime) calcium oxide (quicklime) calcium carbonate (calcite, aragonite, marble, limestone) calcium sulfate (gypsum, plaster of Paws). 

Calcium carbonate minerals are deposited in a wide variety of bacteria, protozoa, algae, higher plants, and invertebrates (Table I). They are also formed, although not as frequently, in vertebrates. The major structural polymorphs identified in biological systems are calcite, aragonite, and vaterite, although there is also evidence for monohydrate, amorphous, and a range of Ca/Mg carbonate phases. 

Carbonates Phosphates Silica Calcite Aragonite Vaterite Monohydrocalcite Amorphous Dahllite Francolite Amorphous calcium phosphate hydrogel Amorphous ferric phosphate hydrogel Opal Iron oxides Sulfates Halides Oxalates Magnetite Goethite Lepidocrocite Amorphous hydrates Celestite Barite Gypsum Fluorite Weddellite Whewellite... 

Polymorphism occurs when different structures can occur for the same chemical formula. The atoms are the same but their atomic arrangement differs between the structures. Although ultimately thermodynamics (the minimum Gibbs free energy) dictates which of the probable structures is formed at a particular temperature and pressure, other factors such as electrostatic interaction mean that a variety of different structures is possible. Polymorphism applies not only to elements, e.g. black and red phosphorus, but also to compounds such as calcium carbonate, which can exist in a number of forms including calcite, aragonite and vaterite. 

X-ray diffraction analysis of the products of the benefici-ation of Dow-Colony shale is shown in Table I. It is clear that the oleophilic extract (pellets) retains or concentrates the calcium and magnesium carbonates (dolomite, calcite, aragonite), particularly the dolomite. The hydrophilic gangue consists mainly of feldspar and quartz. Since silica, silicates and Ca/Ca-Mg carbonates, in a clean condition, are water wettable... 

The isotopic composition of calcium in aragonite and calcite precipitated from seawater, shows a lower content of the heavier " " Ca relative to Ca of some — 0.6%o to — 0.7%o as compared with the ratio in seawater (Gussone et al. 2003). Fewer measurements of Mg relative to Mg show a lowering of — 1.0 to — l.l%o relative to the ratio in seawater (Chang et al. 2004). This difference is consistent with the difference in absolute masses of Mg and Ca. We need to account for the negative values. 

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