Clinopyroxenes ionic radius

This expression is relatively imprecise because of the scarcity of data. Also, the oxidation state of Pb in these experiments is not known. However, it is interesting that for those experiments in which both Dpb and Dsr have been determined, the Dpb/Dsr ratio is consistently less than would be predicted from the 2+ lattice strain model using parameters presented above. As in the case of clinopyroxene, increasing the effective Vlll-fold ionic radius of Pb in plagioclase, to 1.38 A, does retrieve the observed ratios. Thus one can... 

Most trace elements have values of D< C 1, simply because they differ substantially either in ionic radius or ionic charge, or both, from the atoms of the major elements they replace in the crystal lattice. Because of this, they are called incompatible. Exceptions are trace elements such as strontium in plagioclase, ytterbium, lutetium, and scandium in garnet, nickel in olivine, and scandium in clinopyroxene. These latter elements acmally fit into their host crystal structures slightly better than the major elements they replace, and they are therefore called compatible. Thus, most chemical elements of the periodic table are trace elements, and most of them are incompatible only a handful are compatible. 

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) ... 

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