Influence of oxygen partial pressure on the Mg/Fe distribution in olivines |
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Authors: | G Will G Nover |
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Institution: | 1. Lehrstuhl für Mineralogie und Kristallographie, Mineralogisches Institut der Universit?t Bonn, Poppelsdorfer Schlo?, 5300, Bonn, Germany (F.R.G.)
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Abstract: | The dependence of Mg/Fe ordering on oxygen partial pressure in natural olivine crystals of volcanic origin has been studied by X-ray diffraction. Two natural crystals with 10% and 12% fayalite have been investigated and the atomic positions, anisotropic temperature factors, extinction coefficients and site occupancies have been refined, reaching R-values of 2.2%. After subjecting the crystals to oxygen partial pressures of 10?16 bar and 10?21 bar the crystals were studied again. In total six crystals were studied and the distribution coefficients K D determined. The natural untreated crystals had K D=1.09 and 1.06, e.g., a slight preference of Fe in (M1). p(O2) of 10?16 bar increased the ordering of Fe in (M1) to K D=1.2, while p(O2)=10?21 bar reversed K D to 0.8 with ordering of Fe in (M2). These experiments suggest that Mg/Fe ordering in olivines is primarily determined by the prevailing oxygen partial pressure. |
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