Site preference energies

The affinity of different cations to the octahedral coordination can be estimated based on the Octahedral Site Preference Energy [279-281]. The above-defined ratio characterizes the way in which the octahedrons are linked [282]. Table 41 illustrates the correlation between the value of the X Me ratio and the way in which the octahedrons are linked. 

The physical properties of ferrospinels are sensitive to the iron-ion distribution on the two cation subarrays. Normally the Fe ion has a stronger A-site preference than the Fe " " ion, but the difference in site-preference energy depends upon the character of the counter cation, and in some spinels the Fe valence state competes with the Fe " valence state for tetrahedral-site occupation. 

Whether a solute cation substitutes for A-site or B-site iron depends upon the relative site-preference energies of the ions. Moreover, a solute atom may lower the energy A of Fig. 3 to where significant concentrations of FeA ions are stabilized in the presence of Fcb" ions at room temperature. Temperature-dependent cation distributions can be expected where the relative site-preference energies differ by a Ae < kT. 

Co(II) in chloroform (40), In this latter case, the AF difference is about 5 kcal/mole, so the Ni-Co AF difference is smaller. Compare this with the 20 kcal/mole site-preference energy for octahedral Ni(II) over Co(II), according to crystal-field theory [for example, (56)], The fundamental assumption of crystal-field theory, of course, is that the radial factors in cation-coordination-sphere relations are constant, which is tantamount to saying that bonding does not change. As we have seen in deriving Eq. (24), binding factors are very important (8) and, as the above numerical relations confirm, play by far the dominant role. 

No. of d electrons Ian ground state Octahedral field configuration Tetrahedral field configuration (cm-1) (cm-1) CFSE (Idmol- ) OchAedral site preference energy (Id hkF )... 

Octahedral site preference energy not calculated because CofHjObf is low spin. 

Common ions in enzyme systems are those that have low site preference energies (from LFSE) such as Co2". Zn2, and Mn2 rather than Fe Nr2, Or CuJ+. Discuss this phenomenon in terms of the enialk hypothesis.w... 

Measurements of absorption spectra of oxides, glasses and hydrates of transition metal ions have enabled crystal field stabilization energies (CFSE s) in tetrahedral and octahedral coordinations to be estimated in oxide structures (see table 2.5). The difference between the octahedral and tetrahedral CFSE is called the octahedral site preference energy (OSPE), and values are summarized in table 6.3. The OSPE s may be regarded as a measure of the affinity of a transition metal ion for an octahedral coordination site in an oxide structure such as spinel. Trivalent cations with high OSPE s are predicted to occupy octahedral sites in spinels and to form normal spinels. Thus, Cr3, Mn3, V3+... 

Table 6.3. Octahedral site preference energies of transition metal ions in oxide structures...

The octahedral site preference energy parameter listed in table 6.3, applied originally to spinel crystal chemistry, has had a profound influence in transition metal geochemistry following its introduction into earth science literature in 1964 (Bums and Fyfe, 1964 Curtis, 1964). The use of such site preference energies to explain distribution coefficients of transition metal ions in coexisting minerals and phenocryst/melt systems are described in 7.6, 7.8 and 8.5.3. 

Crystal chemistry of spinels. A classic example showing that transition metal ions display distinct site preferences in oxides stems from studies of spinel crystal chemistry. The spinel structure contains tetrahedral and octahedral sites normal and inverse forms exist in which divalent and trivalent ions, respectively, fill the tetrahedral sites. The type of spinel formed by a cation is related to its octahedral site preference energy (OSPE), or difference between crystal field stabilization energies in octahedral and tetrahedral coordinations in an oxide structure. Trivalent and divalent cations with large site preference energies (e.g., Cr3 and Ni2+) tend to form normal and inverse spinels, respectively. The type of spinel adopted by cations with zero CFSE (e.g., Fe3+ and Mn2+) is controlled by the preferences of the second cation in the structure. 

Figure 7.8 Relationship between the octahedral site preference energy and distribution coefficient of divalent transition metal ions partitioned between olivine or pyroxene crystals and the basaltic groundmass (modified from Henderson Dale, 1969 Henderson, 1982, p. 147).

A relationship between octahedral site preference energies (table 6.3) and distribution coefficients has been demonstrated for transition metal ions partitioned between olivine or pyroxene crystals and the groundmass of oceanic basalts, which is assumed to represent the composition of the magma from which the ferromagnesian silicates crystallized (Henderson and Dale, 1969 Dale and Henderson, 1972). Plots of In D against OSPE, such as those illustrated in fig. 7.8, show linear trends between the two parameters. 

Because of the very large octahedral site preference energies of Ni2+ and Cr3+, their partitioning would be strongly biased in favour of the crystals. Hence, equilibria in eq. (8.22) were increasingly shifted to the right for Ni2+ and Cr3, with the result that these cations were preferentially expelled from the magma at similar rates. The Ni2+ ions entered the olivine structure whereas the Cr3 effectively controlled the precipitation of chromite. Irvine (1974, 1975) demonstrated experimentally that the fractionation path of a silicate melt... 

Bums, R. G. Fyfe, W. S. (1964) Site preference energy and selective uptake of transition metal ions during magmatic crystallization. Science, 144,1001-3. 

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