Description and crystal structure of nabiasite, BaMn9[(V,As)O-4](6)(OH)(2), a new mineral from the Central Pyrenees (France)

Nabiasite, ideally BaMn9[(V,As)O-4](6)(OH)(2), is a new mineral which occur s in rhodochrosite-, friedelite- and barite-bearing veinlets crosscutting M n ores at the historic Pla de Labasse deposit near the Nabias hamlet, Centr al Pyrenees, France. These syn-genetic exhalative ores are embedded in Lowe r Carboniferous Radiolarian cherts and have acquired a complex mineralogy d uring the Hercynian metamorphism which culminated under upper-green-schist- facies conditions. The fresh metamorphosed ores consist of rhodonite, rhodo chrosite, friedelite, tephroite, spessartine, braunite, hausmannite, locall y abundant tinzenite and pyroxmangite, +/- fluorapatite, strontian barite, hyalophane, rutile, and pyrophanite. Ba and V (+/- As) have probably been e nriched together with Mn by syn-genetic processes. Thus, nabiasite is a pro duct of the hydrothermal remobilization of syn-genetic concentrations, poss ibly by fluids originating from late Hercynian granitic intrusions. Nabiasite is a rare mineral occurring as red anhedral grains up to 100 mu m in diameter. It is cubic with a = 12.832(2) Angstrom and space group Pa (3 ) over bar, Z = 4. Its structure has been determined and refined to R-1 = 0 .0263 for 854 unique reflections with I > 2 sigma(I), collected on a STOE I PDS using MoK alpha X-radiation. The structure of nabiasite contains two sy mmetry-independent Mn2+ octahedrally coordinated by O, one Mn2+ in octahedr al coordination with five O and one OH group, one V5+ in tetrahedral coordi nation with O, and one Bate in twelve-fold coordination with O. The structu re is based on a cubic close-packed anionic framework, with close-packed pl anes parallel to (111). One of every nine close-packed positions in the pla ne passing through the origin is occupied by a Ba atom instead of a O atom. The structure of nabiasite can be described as a three-dimensional framewo rk of edge-sharing Mno(6)-octahedra, with cavities which host Ba(VO4)(6) gr oups.