Redetermination of brackebuschite, Pb₂Mn³⁺(VO₄)₂(OH)

The crystal structure of brackebuschite, ideally Pb2Mn³⁺(VO₄)₂(OH) [dilead(II) manganese(III) vanadate(V) hydroxide], was redetermined based on single-crystal X-ray diffraction data of a natural sample from the type locality Sierra de Cordoba, Argentina. Improving on previous results, anisotropic displacement parameters for all non-H atoms were refined and the H atom located, obtaining a significant improvement of accuracy and an unambiguous hydrogen-bonding scheme. Brackebuschite belongs to the brackebuschite group of minerals with general formula A₂M(T1O₄)(T₂O₄)(OH, H₂O), with A = Pb²⁺, Ba, Ca, Sr; M = Cu²⁺, Zn, Fe²⁺, Fe³⁺, Mn³⁺, Al; T1 = As⁵⁺, P, V⁵⁺; and T2 = As⁵⁺, P, V⁵⁺, S⁶⁺. The crystal structure of brackebuschite is based on a cubic closest-packed array of O and Pb atoms with infinite chains of edge-sharing [Mn³⁺O6] octahedra located about inversion centres and decorated by two unique VO4 tetrahedra (each located on a special position 2e, site symmetry m). One type of VO4 tetrahedra is linked with the ¹ ∞[MnO₄/2O_2/1] chain by one common vertex, alternating with H atoms along the chain, while the other type of VO₄ tetrahedra link two adjacent octahedra by sharing two vertices with them and thereby participating in the formation of a three-membered MN₂V ring between the central atoms. The ¹ ∞[Mn³⁺(VO₄)₂OH] chains run parallel to [010] and are held together by two types of irregular [PbO x[ polyhedra (x = 8, 11), both located on special position 2e (site symmetry m). The magnitude of the libration component of the O atoms of the ¹ ∞[Mn³⁺(VO₄)₂OH] chain increases linearly with the distance from the centerline of the chain, indicating a significant twisting to and fro of the chain along [010]. The hydroxy group bridges one Pb²⁺ cation with two Mn³⁺ cations and forms an almost linear hydrogen bond with a vanadate group of a neighbouring chain. The O⋯O distance of this interaction determined from the structure refinement agrees well with Raman spectroscopic data.