Structure refinement of a natural K-rich diopside; the effect of K on the average structure Available to Purchase

The crystal structure of the most K-rich natural pyroxene ever reported, a chromian diopside with 1.5 wt% K₂O, has been refined (diffractometer data, filtered MoKα radiation, by least-squares using XTAL to R_u = 3.2%) to examine the effect of K on the average structure. The crystal with structural formula Ca_0.80K₀₀₇₃Na_0.023Mg_0.95Fe_0.06Cr_0.07Al_0.02Si₂O₆ was found as an inclusion in a Koffiefontein diamond. The refined structure is typical of clinopyroxene on the diopside-enstatite join: Mg in M2 leads to distortion that is modeled by site splitting, with M2′ (Mg + Fe) displaced 0.33 Å from M2 (Ca + K + Na). Assignment of K to M2 is required to account for the electron density at that site. The average of eight M2-O distances (2.504 Å) is slightly larger than for diopside (2.498 Å). The effect of K on the average M2-O distance can be seen by calculating the average cation radius of atoms at M2 (+ M2′) from the occupancy: 0.798-1.12 Å (⁽⁸⁾Ca) + 0.073 1.51 (K) + 0.023-1.18 (Na) + 0.036-0.92 (Fe) + 0.070-0.89 (Mg) = 1.127 ∼ ≥R (^[8]Ca). The large size of K is mostly offset by Mg + Fe in M2′ in the diopside-like structure; this size-distortion balancing may facilitate K uptake in mantle clinopyroxene in K-rich environments. Large apparent thermal motion parameters of most atomic sites indicate sizable local distortions of the structure from substitution of K into M2.