Description and crystal structure of nabiasite, BaMn9[(V,As)O4]6(OH)2, a new mineral from the Central Pyrenees (France)
Brugger, Joel; Bonin, Michel; Schenk, Kurt J.; Meisser, Nicolas; Berlepsch, Peter; Ragu, Alain
European Journal of Mineralogy Volume 11 Number 5 (1999), p. 879 - 890
published: Sep 30, 1999
manuscript accepted: Mar 30, 1999
manuscript received: Mar 24, 1998
ArtNo. ESP147051105008, Price: 29.00 €
Abstract Nabiasite, ideally BaMn9[(V,As)O4]6(OH)2, is a new mineral which occurs in rhodochrosite-, friedeliteand 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-greenschist- facies conditions. The fresh metamorphosed ores consist of rhodonite, rhodochrosite, friedelite, tephroite, spessartine, braunite, hausmannite, locally abundant tinzenite and pyroxmangite, ± fluorapatite, strontian barite, hyalophane, rutile, and pyrophanite. Ba and V (± 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 μm in diameter. It is cubic with a = 12.832(2) A and space group Pa 3̄, Z = 4. Its structure has been determined and refined to R1 = 0.0263 for 854 unique reflections with I > 2σ(I), collected on a STOE IPDS using MoKa X-radiation. The structure of nabiasite contains two symmetry-independent Mn2+ octahedrally coordinated by O, one Mn2+ in octahedral coordination with five O and one OH group, one V5+ in tetrahedral coordination with O, and one Ba2+ 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 MnO6-octahedra, with cavities which host Ba(VO4)6 groups.