New data on cafarsite: reinvestigation of its crystal structure and chemical composition

The crystal structure of cafarsite from Wanni glacier, Monte Cervandone, was re-investigated by single-crystal X-ray diffraction (space group Pn (3) over bar, R1= 0.022) and new electron-microprobe analyses. The REE (mainly Ce and Y) were found to substitute Ca. A new anion site, four-coordinated by Ca, was located by difference Fourier analysis and refined with F scattering factors. Two octahedral sites (Ti1 and Fe2) are occupied by mainly Ti and Fe3+, replacing each other whereas the octahedral site Mnl hosts Mn2+ and Fe2+. The new structure refinements and chemical analyses lead to the simplified chemical formula (Ca7.8Na0.8Mn0.5REE0.4)(Sigma)(9.)(5)(Ti3.9Fe2.13+Fe0.92+Mn0.12+)(Sigma 7)(AsO3)(14)F-0.5. According to our findings, there is no evidence for either H2O or OH in the structure. In contrast to the original study of 1977, the structure has no significant cation vacancies and is based on 14 AsO3 groups per formula unit, not on 12 AsO3 as previously suggested.