Plagioclase 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. 

Fig. 22. The regular variation of mineral/melt distribution coefficients for plagioclase with ionic radius, showing the effect of lattice size and valency. Eu enters the Ca " (r = 1.12 A) site more readily than the trivalent lanthanides, the largest (La) of which have preference over the smaller (Yb).

The second point of interest is the relationship between ionic radius and distribution coefficients (Onuma et al. 1968, Jensen 1973, Philpotts 1978). This is demonstrated in fig. 22 for plagioclase which shows the regular variation in with ionic radius, and the dramatic effect due to the presence of europium as the divalent ion. 

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