Calcium ionic crystal radius

In deriving theoretical values for inter-ionic distances in ionic crystals the sum of the univalent crystal radii for the two ions should be taken, and corrected by means of Equation 13, with z given a value dependent on the ratio of the Coulomb energy of the crystal to that of a univalent sodium chloride type crystal. Thus, for fluorite the sum of the univalent crystal radii of calcium ion and fluoride ion would be used, corrected by Equation 13 with z placed equal to y/2, for the Coulomb energy of the fluorite crystal (per ion) is just twice that of the univalent sodium chloride structure. This procedure leads to the result 1.34 A. (the experimental distance is 1.36 A.). However, usually it is permissible to use the sodium chloride crystal radius for each ion, that is, to put z = 2 for the calcium... 

Calcium, strontium and barium each have slightly different impacts on the properties of the perovskite. This is due, in part, to the difference in ionic radii of the ions. Calcium has a radius of 1.36 A, whereas the radii of strontium and barium are 1.44 and 1.65, respectively. Thus barium-containing perovskites have a more open crystal lattice and thus have a higher ionic conductivity compared to strontium- and calcium-based materials. However, barium is more reactive towards C02-containing gas mixtures and forms barium carbonate very readily. 

Cadmium. Cadmium appears to be compatible or very mildly incompatible, similar to zinc. Almost nothing is known about which minerals it prefers. From a crystal-chemical view, cadmium has similar ionic radius and charge to calcium, but a tendency to prefer lower coordination due to its more covalent bonding with oxygen (similar to zinc and indium). Cadmium in spinel Uierzolites varies from 30 ppb to 60 ppb (BVSP) and varies in basalts from about 90 ppb to 150 ppb (Hertogen et al., 1980 Yi et al., 2000). Cd/Zn is —10 in peridotites (BVSP) and the continental cmst (Gao etal., 1998), and —1.5 X 10 in basalts (Yi etai, 2000). We adopt the mean of these ratios (1.2 X 10" ). 

Given Equation (16) we derived, by nonlinear least-squares fitting, values of r from a total of 82 experiments in which three or more clinopyroxene-liquid REE partition coefficients were measured and in which the compositions of crystal and liquid phases were well constrained. Adopted radii values for individual REE were those of Shannon (1976) for 3+ ions in VIII coordination. Resulting values of range from 0.979 A to 1.055 A. We performed stepwise linear regression of the derived values of r against all major compositional parameters, pressure and temperature. The result was that the only important parameters appeared to be the octahedral (Ml) A1 content and the calcium content of the M2-site. Fitting Tq to these parameters yields an equation which reproduces the 82 points with a standard deviation of 0.009 A (i.e., less than the difference in ionic radius between adjacent REE) ... 

Minasgeraisite-(Y) was described by Foord et al. (1986) as a new mineral from Minas Gerais, Brazil. X-ray powder diffraction data showed that the mineral is isostructural with gadolinite-(Y). However, minasgeraisite-(Y) has a peculiar chemical composition and includes significant amounts of Bi and Ca (Foord et al. 1986). It is unusual from the viewpoint of crystal chemistry that the calcium atom with a large ionic radius occupies the relatively small octahedral site which is occupied by Fe in the case of gadolinite-(Y). 

Numerous attempts have been made by geochemists to account for the ways in which trace elements distribute themselves. It has not been possible to define the energies of different possible locations of trace elements so that their equilibrium distributions can be accurately predicted. There are primitive notions, e.g., that the lanthanides merely follow calcium, that are too crude to be of much use, although there is a grain of truth in them. The most useful rule is that lanthanides (and other trace cations) readily enter those sites in crystals normally occupied by more common cations of approximately equal ionic radius (e.g., Neumann et al., 1966). Effects on lanthanide behavior of balancing charge in the event that the cation substituted for was not 3-1- seem to be second order for chemically complex natural systems as long as lanthanide concentrations are low (tens of ppm or less). 

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