Calcite, structure

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 ... 

The only possible position for Pb in calcite structure is instead of Ca " with a coordination number of 6, where ionic radii of Ca + and Pb are 114 and 133 pm. 

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. 

We now turn to the formation of some of these hydride structures. The majority of them are based on a fee array of metal atoms, as shown by the open circles in Figure 2. The dihydride structure comes from filling the tetrahedral interstice (large solid circles) in the lattice with hydrogens and gives the well known CaF2 or calcite structure. Similarly, if one fills the octahedral interstice (small solid circles), one gets the NaCl or rocksalt structure found in nickel hydride and palladium hydride, which we will discuss near the end of this chapter. 

Table XXIII-9.—Substances Crystallizing in Calcite Structure...

Fig. 1. Plot of the rhombohedral lattice parameters aR of a variety of binary and ternary Carbonates of calcite structure (e.g. Ca-M, Ca-M-M, Mg-M, M-M where M,M = Mn, Fe, Co, Cd, etc.) against the mean cation radius.

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. 

Figure 1 Calcite structure of MBO3. Small filled circles represent B atoms, large shaded circles represent O atoms, and stippled circles represent M atoms, here, and in Figures 2, 3, and 8...

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... 

Calcite structure liNOa, NaN03 MgC03, CaC03, FeC03 InB03, YBO3... 

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). 

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