Abstract

Nabiasite, ideally BaMn 9 [(V,As)O 4 ] 6 (OH) 2 , is a new mineral which occurs in rhodochrosite-, friedelite- and barite-bearing veinlets crosscutting Mn ores at the historic Pla de Labasse deposit near the Nabias hamlet, Central Pyrenees, France. These syn-genetic exhalative ores are embedded in Lower Carboniferous Radiolarian cherts and have acquired a complex mineralogy during the Hercynian metamorphism which culminated under upper-green-schist-facies conditions. The fresh metamorphosed ores consist of rhodonite, rhodochrosite, friedelite, tephroite, spessartine, braunite, hausmannite, locally abundant tinzenite and pyroxmangite, + or -fluorapatite, strontian barite, hyalophane, rutile, and pyrophanite. Ba and V (+ or -As) have probably been enriched together with Mn by syn-genetic processes. Thus, nabiasite is a product of the hydrothermal remobilization of syn-genetic concentrations, possibly 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) Aa and space group Pa 3, Z = 4. Its structure has been determined and refined to R 1 = 0.0263 for 854 unique reflections with I > 2sigma (I), collected on a STOE IPDS using MoKalpha X-radiation. The structure of nabiasite contains two symmetry-independent Mn (super 2+) octahedrally coordinated by O, one Mn (super 2+) in octahedral coordination with five O and one OH group, one V (super 5+) in tetrahedral coordination with O, and one Ba (super 2+) in twelve-fold coordination with O. The structure is based on a cubic close-packed anionic framework, with close-packed planes parallel to (111). One of every nine close-packed positions in the plane 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 framework of edge-sharing MnO 6 -octahedra, with cavities which host Ba(VO 4 ) 6 groups.

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