This paper describes the synthesis and characterization of potassium richterite containing variable amounts of Ni, Mg, and Co in the octahedral strip. Monophase amphibole products were obtained at 750 °C, 1 kbar (H2O) for all compositions in the system studied. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final R(Bragg) indices in the range 2–4%. The trends for individual cell dimensions along the Ni-Mg-Co join show a marked discontinuity at the ordered Mg composition, indicating that the behavior of the individual cell dimensions is not controlled just by the size of the substituent cations, whereas cell volume and grand mean 〈M-O〉 bond length are linear with constituent cation size. Refined site occupancies show that both Ni, Mg and Mg, Co distributions are ideal. The partition coefficients for Ni and Co over M1,3 and M2, defined as KM2+ = (M2+/Mg)M1,3,/(M2+/Mg)M2, with M2+ = Ni2+ or Co2+, are KNi = 4.26 ± 0.56 and KCo = 1.92 ± 0.29. Both Kd values are >1.0, indicating that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The Rietveld method proves to be an extremely powerful method for the characterization of fine-grained synthetic products.

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