Crystal chemistry of synthetic pyroxenes on the join CaNiSi₂O₆-CaMgSi₂O₆ (diopside); a Rietveld structure refinement study

A series of monoclinic C2/c pyroxenes was synthesized along the join CaNiSi₂O₆-CaMgSi₂O₆ (diopside). The crystal structures of these pyroxenes were refined with CuKα X-ray powder data using the Rietveld method; R_B indices were in the range 2.7–3.7%. Cell parameters and mean bond lengths vary linearly as a function of composition along the join. However, the derived trends differ significantly from those of the CaM²⁺Si₂O₆ (M²⁺ = Mg, Co, Fe, Mn) C2/c pyroxenes. In particular, the slope of the 〈M(l)-O〉 vs. 〈r_M(1)〉 trend is 0.20, contrasting strongly with the slopes of the analogous relationship in the other calcic pyroxenes (slope = 0.89) and the hard-sphere model (slope = 1.0). Similar deviations occur at the large-cation end of the series NaM³⁺Si₂O₆ (M³⁺ = Al, Cr, Fe, Sc, In) and CaM³⁺SiM³⁺O₆ (M³⁺ = Al, Fe, Sc). This type of anomalous behavior may be characteristic of anisodesmic structures close to their limits of chemical stability.