SUBSILICIC FERRIAN ALUMINIAN DIOPSIDE FROM THE CHELYABINSK COAL BASIN (SOUTHERN URALS) - AN UNUSUAL CLINOPYROXENE

Abstract

The crystal structure of a technogeneous Fe3+-rich diopside from the Chelyabinsk coal basin, Southern Urals, has been refined from X-ray powder diffraction data (CoKα, radiation) using the Rietveld method (Rwp = 0.049). The refined monoclinic cell dimensions (Z = 4, space group C2/c) are a = 9.786(2) Å, b = 8.860(1) Å, c = 5.338(1) Å and β = 105.87(1)°. This technogeneous mineral has previously been described as esseneite according to the empirical formula Ca1.00Fe3+0.51Ti0.02Al 0.60Si1.43O6, but in contrast to ideal esseneite (end member: CaFe3+AlSiO6), Fe3+ ions are not restricted to the octahedral M(1) site, but are partitioned between the M(1) and the tetrahedral T site. Moessbauer spectroscopy indicates a Fe3+ distribution over the M(1) and T sites with a 2:1 ratio. Site occupancy refinement confirms that the M(1) site is occupied by 0.44(3) Mg, 0.35(3) Fe3+ and 0.20(3) Al, and the T site by 0.70(2) Si, 0.20(2) Al and 0.08(2) Fe3+.