Structure refinement of a natural K-rich diopside: the effect of K on the average structure

The crystal structure of the most K-rich natural pyroxene ever reported, a chromian diopside with 1.5 wt% [K.sub.2]O, has been refined (diffractometer data, filtered MoK[Alpha] radiation, by least-squares using XTAL to [R.sub.u] = 3.2%) to examine the effect of K on the average structure. The crystal with structural formula [Ca.sub.0.80][K.sub.0.073][Na.sub.0.023][Mg.sub.0.95][Fe.sub.0.06 ][Cr.sub.0.07][Al.sub.0.02][Si.sub.2][O.sub.6] 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[prime] (Mg + Fe) displaced 0.33 [Angstrom] 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 [Angstrom]) is slightly larger than for diopside (2.498 [Angstrom]). The effect of K on the average M2-O distance can be seen by calculating the average cation radius of atoms at M2 (+ M2[prime]) from the occupancy: 0.798-1.12 [Angstrom] ([8]Ca) + 0.073[center dot!1.51 (K) + 0.023[center dot]1.18 (Na) + 0.036[center dot]0.92 (Fe) + 0.070[center dot]0.89 (Mg) = 1.127 [similar to] [greater than or equal to]R ([8]Ca). The large size of K is mostly offset by Mg + Fe in M2[prime] 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.