Structure of a new upper-mantle, magnetoplumbite-type phase, Ba[Ti₃Cr₄Fe₄Mg]O₁₉

The new phase, ideally Ba[Ti₃Cr₄Fe₄Mg]O₁₉, is found in metasomatized lithosphere-derived harzburgite, in the Kimberley district of South Africa. It has hexagonal symmetry, space group P6₃/mmc, with a = 5.871(2) and c = 23.06(2) Å. The crystal structure has been refined to R = 0.044 for 368 independent reflections [I > 3_σ(I)] collected on a Siemen's aed diffractometer using MoKα radiation. The new mineral has the magnetoplumbite-type structure, with a statistical disorder of the cations occupying the bipyram-idal site, in positions displaced by 0.2 Å on either side of the mirror plane. Cation ordering in the structure is consistent with a genesis model in which the new mineral derives from magnesiochromite spinel by periodic replacement of the mixed octahedral, tetrahedral (11l)_sp layer with a segment of hexagonal BaTiO₃.