The Calcite Structure

The calcium ion is six-coordinate, which would be expected based on the similarity of the structure to that of NaCl (the calcium ions are essentially in the octahedral voids). The coordination number of an oxygen in an oxy anion can be easily determined if the coordination number of the cation is known. For example, the calcium ion in CaCOs has a charge of 2+ and is six-coordinate. Every one of the six nearest neighbors therefore must neutralize 2/6 = 1/3 of a positive charge. The neutralizing ability of the carbonate ion 

This determination of the coordination number of the ion is predicated on the electrical neutrality principle given earlier (see page 19). This principle is also called the electrostatic valence principle and is part of Pauling s second rule In a stable ionic structure the valence of each anion, with changed sign, is exactly or nearly equal to the sum of the strengths of the electrostatic bonds to it from the adjacent cations. (Pauling, 1960) 

Try this same reasoning with sphalerite, remembering that the zinc ions are four-coordinate. 

Zinc is 2-1- and because it is four-coordinate, 2/4 =1/2 charge must be neutrahzed by the S anions. Each anion has a full two electron charge and therefore each anion must be coordinated to four cations (-2/4 = -1/2). 

Because the layers have an AB sequence, they are roughly hexagonal closest packed. 

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A unit cell for the calcite structure can be found, on the Web site for this book. From this structure, determine (a) the crystal system and (b) the number of formula units present in the unit cell. 

The structures of CaC03 (calcite) and KjPtClg. The section of the calcite structure shown does not correspond to the unit cell (as can be seen from the orientations of the C03 groups on opposite edges)... 

This model for the system CaC03 MgC03 applies only for ideal ordering of Mg and Ca ions in the dolomite structure. Ideal ordering occurs only in precipitates of dolomite formed at temperatures above about 250°C. Studies in the laboratory (52) show that dolomitization (the development of ordering in the Mg and Ca distribution in the calcite structure) is a very slow process at ordinary temperatures. Therefore, a solid-state chemical model more applicable to precipitated dolomites is ... 

Of the structures with stoichiometry ABO3 that may be derived from that of perovskite (Sect. 2.2), the calcite structure is the simplest in the sense that it requires the fewest parameters to specify it. We earlier described this structure in terms of the parameters obtained with regular BOe octahedra that are rotated (tilted) about their 3 axes from the positions they have in (cubic) perovskite. The coordination of A goes from 12 in the cubic structure to 3 in the rhombohedral calcite structure. 

The octahedron also is adopted in the known carbonates249 (Table 22). MnC03, although thermodynamically unstable in 02 or air, decomposes only slowly, and the pink, almost white, compound occurs naturally as the mineral rhodochrosite. Only one crystalline form is known which has the calcite structure and both CaMn(C03)2 and BaMn(C03)2 have the dolomite structure. The carbonate mineral sidorenkite (Na3MnC03P04) has a bidentate carbonate group, which is somewhat unusual in inorganic structures. 

The carbonate minerals, characterized by the presence of the COj anion unit, are dominated petrologically by calcite (CaCOj) and the isos-tructural phases magnesite (MgCOj) and dolomite [CaMgfCOjlj]. The calcite structure is illustrated in Fig. 5.23, and in this phase, as in all the mineral carbonates, the CO3 groups are linked by the intermediate (Ca +) cations (counterions). The mineral carbonates have been reviewed by Reeder (1983). 

An alternative dating technique that also makes use of the uranium incorporated in speleothems is to treat the speleothem as a dosimeter. The a-particles emitted during uranium decay create defects in the calcite structure. The longer the speleothem has been exposed to the radiation, the greater the number of defects. These defects contain trapped... 

An additional possible reason for differences in behaviour is that CaCOj exists in three crystalline forms. Calcite is the most stable and aragonite and vaterite undergo transitions to calcite at 728 K and between 623 and 673 K, respectively. Consequently, at decomposition temperatures (above 900 K) the reactant CaCOj would normally be in the calcite structure [2]. Salvador et al. [3] concluded that the... 

MgCOj has the calcite structure [2]. Britton et al. [30] concluded that the decomposition of magnesite (MgCOj) was an inter ce process, initiated at boundary surfaces and thereafter advancing inwards. The value of E found (150 kJ mol between 813 and 873 K) was appreciably greater than the enthalpy of dissociation (101 kJ mol ). They considered the possible influences on the reaction rates of factors such as self-cooling, the recombination process at the reaction interface, the restriction of escape of carbon dioxide, and the rate of the nucleation step. 

E3.23 Calcite has a rhombohedral unit cell, whereas NaCl has a cubic one. Both rhombohedral and cubic unit cells have all three dimensions equal (a = fi = c) but they differ in the angles, with cubic unit cell having all angles at 90 and rhombohedral all angles equal but different from 90 . If you look at the arrangement of cations and anions in the calcite structure, you ll see that when all angles are made 90 the two structures are identical. Calcite s rhombohedral unit cell can be obtained if we take cubic NaCl unit cell and pull its opposite comers across the body diagonal apart. 

In soils, the prevalence and importance of solid solution formation in controlling cation and anion solubility has not yet been determined. Certain minerals readily incorporate only those metal ions with radii similar to the radius of the structural metal ion. For calcite, this means that Mn, Cd , and Fe readily enter the calcite structure on precipitation while smaller ions such as Cu " and Zn do not. Even so, it is not clear that metal selectivity demonstrated by pure solid solutions has much control in soils and sediments. A case in point is the lack of evidence for a strong association of Cd and Pb " with calcite in natural sediments, despite the favorable radii of these metals compared with Ca ". On the other hand, both Cd and Pb " are associated with hydroxyapatite in nature because they fit well into the Ca " site of this mineral. This association poses a problem for the long-term use of phosphate fertilizers in soils. 

A further example of a compound in which the geometrical conditions are favourable for dimorphism is calcium carbonate. This compound occurs naturally as both calcite and aragonite, two distinct minerals with quite different structures (see 10.10 and 10.11). The calcite structure is common to a considerable number of carbonates (and nitrates) in which the radius of the cation is less than about i-o A, but when the radius exceeds this value the aragonite structure is found. Calcium carbonate itself is dimorphous because the radius of the calcium ion (0 99 A) is close to this critical value, but dimorphism is not found in those carbonates in which the cation is either appreciably smaller or appreciably larger. The closely analogous case of the dimorphism of CdTi03 has already been described ( 8.52). 

The orthorhombic structure of calcium carbonate in the form of aragonite (fig. 10.04) s related to that of nickel arsenide in the same way as the calcite structure is related to that of sodium chloride it may be regarded as a distorted NiAs structure ( 8.12) with arsenic replaced by calcium and nickel by carbonate groups. As in calcite, all the C03... 

Crystals of UNO3, NaNOs, and KNO3 assume the calcite structure (Sec. 1.31). Nakagawa and Walter [705] carried out normal coordinate analyses on the whole Bravais lattices of these crystals. 

This is due to the calcite structure of InBQs which places Tb on a site with inversion symmetry which forbids the forced electric-dipole transitions (Sect. 2.3.3). An important criterion in the final phosphor selection is their degradation behavior in the tubes under high-density excitation [11]. 

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