On the labyrinthine world of arsenites : a single-crystal neutron and X-ray diffraction study of cafarsite

The crystal chemistry of a cafarsite sample from the fengitic orthogneisses of the Mt. Leone-Arbola nappe (Lower Penninic), forming the central body of Mount Cervandone and cropping out both in Switzerland and Italy (Alpe Devero area, Verbano–Cusio–Ossola province), was investigated by electron microprobe analysis in wavelength-dispersive mode (EPMA-WDS), single-crystal Raman spectroscopy, and single-crystal X-ray and neutron diffraction at 293 K. The sample of cafarsite of this study was found experimentally to be anhydrous and the chemical formula obtained on the basis of the EPMA-WDS data and structural refinements is the following: Ca1,Ca2(Ca15.56Na0.44)Σ16Fe1(Na0.53Fe2+0.17REE0.30)Σ1.00Mn1,Ti,Fe2(Ti7.46Fe3+4.47Fe2+3.20Mn2+0.85Al0.11) Σ16.11 As1,As2,As3(AsO3)28 FF, with the general chemical formula Ca16(Na,Fe2+,REE)(Ti, Fe3+,Fe2+,Mn2+,Al)16(AsO3)28F [or Ca16(Na,Fe2+,REE)(Ti,Fe3+,Al)12(Fe2+,Mn)4(AsO3)28F]. Our experimental findings show that fluorine, which was unconsidered in the previous studies, is a key element. The anhydrous nature of this sample is also confirmed by its Raman spectrum, which does not show any evidence of active bands ascribable to the O–H stretching region. The X-ray and neutron structure refinements provide a structure model that is partially in agreement with the previous experimental findings. The space group (i.e. Pn3) and the unit-cell constant [i.e. 15.9507(4) Å] are conform to the literature data, but the structure of cafarsite, here refined, contains the following building units: three independent AsO3 groups (trigonal pyramids), one CaO6F polyhedron, one CaO8 polyhedron, two independent (Ti,Fe)O6 octahedra, one (Na,Fe,REE)O8 polyhedron, and one (Mn,Fe)O6 octahedron. Connections among polyhedra are mainly due to edge- or vertex-sharing; the AsO3 groups are not connected to each other.