Cation ordering in Ni-Mg olivine

V Rajmani; Gordon E. Brown; C. T. Prewitt

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Research Article| April 01, 1975

Cation ordering in Ni-Mg olivine

V Rajmani;

V Rajmani

Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794

¹ Present Address: Department of Geology, University of Toronto, Toronto, Ontario, Canada.

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Gordon E. Brown;

Gordon E. Brown

Department of Geology, Stanford University, Stanford, California 94305

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C. T. Prewitt

Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794

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Author and Article Information

V Rajmani

Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794

¹ Present Address: Department of Geology, University of Toronto, Toronto, Ontario, Canada.

Gordon E. Brown

Department of Geology, Stanford University, Stanford, California 94305

C. T. Prewitt

Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794

Publisher: Mineralogical Society of America

Received: 12 Aug 1974

Accepted: 08 Nov 1974

First Online: 11 Jul 2018

Online ISSN: 1945-3027

Print ISSN: 0003-004X

Mineralogical Society of America

American Mineralogist (1975) 60 (3-4): 292–299.

Abstract

A synthetic crystal of Ni-Mg olivine-Ni_1.09Mg_0.97SiO₄; a = 4.7366(4); b =10.1716(13); c = 5.9374(4) Ă—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; σ = ± 0.002 Ni) and of Mg on M2 (0.737 Mg + 0.263 Ni). The intracrystalline distribution coefficient, K_D = (Mg[M2]/Ni[M2])/(MgM1]/NiM1]), equals 9.22. This amount of Ni-Mg ordering is surprising in view of the small size difference [Δr = 0.03 Å] between the two cations and the slight ordering observed in some Fe-Mg olivines[K_D = 1.00 - 1.30] and suggests that crystal field effects strongly favor Ni on the M1 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, ζ, at these sites [Z_M1(ol) = Z_M2(ol) = ζ_M2(pxy) = 0.0; ζ_M1(pxy) -0.66 v.u.] enhances the ordering of transition metal cations onthe M2 site of pyroxenes relativeto Mg.