Determination of site-occupancies in silicates using ALCHEMI

12 Determination of site-occupancies in silicates using ALCHEMI 

Many minerals contain minor elements whose distribution among possible sites can be ordered or disordered. Olivine, for example, in addition to Mg, Si, and O usually contains Fe and smaller amounts of Mn and Ni, which substitute for Mg atoms in the two nonequivalent Ml and M2 sites. The distribution of Mg, Fe, Mn, and Ni in Ml and M2 sites in a specimen of San Carlos olivine was determined by Tafto and Spence (1982) using ALCHEMI, and more recently, McCormick, Smyth, and Lofgren (1987) have used the same technique to determine the site-occupancies of these minor elements in synthetic olivines as a function of equilibration temperature. Recall from Section 7.4 that ALCHEMI is not fully quantitative for the determination of site-occupancy in some structures because of the so-called localization effect. However, because these studies of olivine indicate that planar ALCHEMI can be used with confidence in this important mineral, it is therefore appropriate to consider this particular application of the technique as an example. 

The results of McCormick et al. (1987) indicate that the degrees of ordering at different equilibration temperatures differ by less than the analytical errors, although there may be significant differences in ordering between olivines that have been quenched from temperatures of 300° C (or higher) 

ALCHEMI has also been successfully used for determining Al/Si ordering in tetrahedral sites in K-feldspars by Tafto and Buseck (1983) and McLaren and Fitz Gerald (1987). 

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