Structure and cation order in manganilvaite: A combined X-ray-diffraction, neutron-diffraction and Mossbauer study

The crystal structure of manganilvaite, CaFe2+Fe3+(Mn2+,Fe2+)(Si2O7)O(OH), from the Ossikovo Pb-Zn skarn deposit, Rhodope Mountains, Bulgaria, was refined in space group P2(1)/a, with a 13.0250(7), b 8.8514(5), c 5.8486(3) angstrom, beta 90.167(1)degrees, V 674.28(7) angstrom(3), Z = 4, using combined neutron and X-ray powder diffraction as well as single-crystal X-ray-diffraction data. The empirical chemical formula proposed on the basis of the Rietveld refinements is: (Ca0.9Mn0.12+)(Fe0.832+Fe0.173+)(Fe-0.17(2+) Fe-0.83(3+))(Mn0.512+Fe0.492+)(Si2O7)O(OH). The occupancy of the M11 site (0.83Fe(2+) + 0.17Fe(3+)) corresponds to an order parameter Q of 0.65(10). The high Mn content leads to strong enhancement (14%) of Fe2+ -> Fe3+ charge transfer in comparison to Mn-free ilvaite from Seriphos, Greece, as observed by MOssbauer spectroscopy.