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

Abstract

The crystal structure of the most K-rich natural pyroxene ever reported, a chromian diopside with 1.5 wt% K20, has been refined (diffractometer data, filtered MoKa radiation, by least-squares using XTAL to Ru = 3.2%) to examine the effect of K on the average structure. The crystal with structural formula Cao.8oKo.o73Nao.o23M&>.95Feo.o6Cro.Q7Alo.o2Si206 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 A 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 M2-0 distances (2.504 A) is slightly larger than for diopside (2.498 A). The effect of K on the average M2-0 distance can be seen by calculating the average cation radius of atoms at M2 (+ M2') from the occupancy: 0.798.1.12 A ([8ICa)+ 0.073 .1.51 (K) + 0.023.1.18 (Na) + 0.036.0.92 (Fe) + 0.070'0.89 (Mg) = 1.127 -?:.R ([8ICa).The large size of K is mostly offset by Mg + Fe in M2' in the diopside-like structure; this sizedistortion 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.