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

The crystal structure of a technogeneous Fe3+-rich diopside from the C helyabinsk coal basin, Southern Urals, has been refined from X-ray pow der diffraction data (CoKalpha radiation) using the Rietveld method (R -wp = 0.049). The refined monoclinic cell dimensions (Z = 4, space gro up C2/c) are a = 9.786(2) Angstrom, b = 8.860(1) Angstrom, c = 5.338(1 ) Angstrom and beta = 105.87(1)degrees. This technogeneous mineral has previously been described as esseneite according to the empirical for mula Ca1.00Fe0.513+Ti0.02Al0.60Si1.43O6, but in contrast to ideal esse neite (end member: CaFe3+AlSiO6); Fe3+ ions are not restricted to the octahedral M(1) site, but are partitioned between the M(1) and the tet rahedral 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+ a nd 0.20(3) Al, and the T site by 0.70(2) Si, 0.20(2) AI and 0.08(2) Fe 3+.