Abstract

The crystal structures of natural liebenbergite—(Ni1.52Co0.05Fe0.09Mg0.34)SiO4—from Barberton, South Africa, and of synthetic liebenbergite—(Ni1.16Mg0.84)SiO4—synthesized at 500°C, have been studied in order to determine the intracrystalline Ni–Mg distribution. The natural liebenbergite is fully ordered, with M1 occupied only by Ni, whereas the synthetic sample is only partially ordered with KD=[Mg(M2)Ni(M1)]/[Ni(M2)Mg(M1)]=9.9(4)ΔGex=3.5 kcal/mole. Comparison with the results of Rajamani et al. (1975), who found KD = 9.2(2) and ΔGex=6.9 kcal/mole for a sample synthesized at 1280°C, implies that the 500°C synthetic sample in the present study crystallized metastably in a disordered or partially ordered state and ordering proceeded slowly.

Although site size effects are small in Fe–Mg, Ni–Mg, and Co–Mg olivines, the crystal field stabilization energy is important in determining the observed cation distribution in Ni–Mg and Co–Mg olivines. Electronegativity or covalency effects are known only qualitatively, but there is a preference of less electronegative ions (Mg, Ca) for M2.

Ordering of Ni into the M1 site of olivine should appreciably affect Ni partitioning between olivine and melt, and activity–composition relations have been examined assuming ideal solution behavior. Deviations from Raoult’s law increase with increasing order, but variations in activity coefficients are less than 10 percent below 10 mole% Ni.

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