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, 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 Fe (super 3+) , replacing each other whereas the octahedral site Mn1 hosts Mn (super 2+) and Fe (super 2+) . The new structure refinements and chemical analyses lead to the simplified chemical formula (Ca (sub 7.8) Na (sub 0.8) Mn (sub 0.5) REE (sub 0.4) ) (sub Sigma 9.5) (Ti (sub 3.9) Fe (super 3+) (sub 2.1) Fe (super 2+) (sub 0.9) Mn (super 2+) (sub 0.1) ) (sub Sigma 7) (AsO (sub 3) ) (sub 14) F (sub 0.5) . According to our findings, there is no evidence for either H (sub 2) O 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 AsO (sub 3) groups per formula unit, not on 12 AsO (sub 3) as previously suggested.