The cation distribution of major and minor elements over the two non-equivalent sites in olivine and its temperature dependence.

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The cation distribution coefficients K_D (=(Fe/Mg)_<M1>/(Fe/Mg)_<M2>) were calculated from the X-ray site occupancy refinements for olivines. The K_D values of Mg, Feolivines obtained are 0.95-1.13 in basalt, 1.01-1.12 in ultramafic nodule, 1.18 in rhyolite, 0.95 in metamorphic rocks atid 0.91 in H6-chondolite. As a whole, the volcanic olivines have the slight preference of Fe for the Ml site, and the metamorphic olivines for the M2 site, as the previous investigators reported.In-situ high temperature X-ray experiments showed that the K_D values of the heated olivines increased with increasing temperature, and that values above 600 ﾟC increase with increasing fayalite content. Ttiese results suggest that tile cation exchange in olivine is not sluggish at high temperature and the cation distribution at high temperature is not preserved on cooling. Ttierefdre the K_D values at room temperature for olivines are riot obviously distinguished by the modes of occurrence.Using ALHEMI (Atomic Location by CHanneling Enhanced MIcroanalysis) technique, with an Akaslii EM-002B TEM having EDAX9100 and double tilt specimen holder, the distribution of cations over the two cation sites in olivine is determined. A suitable set of planes for electron clianrie. ling is(002)in Pbnm setting. Here, the new equation for the calculation to determine the site occupancies, using Si atom as reference atom, is derived. Mg and Fe show a slight preference for the M2 and M1 sites respectively. Minor elements Ni and Mn show a strong preference for the M1 site and the M2 site respectively.