Cation ordering in Ni-Mg olivine

Abstract

A synthetic crystal of Ni-Mg olivine-Ni,.*MgorSiO,;a = 4.7366(4); b : l0.l7l6(13); c: 5.9374(4) A-has been investigated by X-ray methods in order to determine the intracrystalline distribution of Ni and Mg. Anisotropic least squares refinement of the structure, including cation occupancies, to R = 0.029 indicates significant ordering of Ni on Ml (0.767 Ni + 0.233 Mg; o = +0.002 Ni) and of Mg on M2 (0.737 Mg + 0.263 Ni). The intracrystalline distribution coefficient, Kp : (Mg[M21/N|IM2])/(M\IMll/ Ni[Ml]), equals 9.22. This amount of Ni-Mg ordering is surprising in view of the small size difference [Ar = 0.03 A] between the two cations and the slight ordering observed in some Fe-Mg olivines [Ko = 1.00 - 1.30] and suggests that crystal field effects strongly favor Ni on the Ml site of olivine, in agreement with the predictions of Burns (1970a) and others. Comparison of the available site occupancy data for transition metal olivines and pyroxenes indicates that site preferences of divalent Fe, Co, Ni, and Mg depend on a balance between such crystal chemical factors as cation size, charge, electronegativity, crystal field effects, and the nature of the two octahedral sites in each structure. In particular, the differences in residual charge, f, at these sites [Zyr,o', : Zar<oy: (vzrpy't = 0.0] frupv'r = -0.66 v.u.] enhances the ordering of transition metal cations onthe M2 site of pyroxenes relativeto Mg.