Redetermination of brackebuschite, Pb(2)Mn(3+)(VO(4))(2)(OH)

The crystal structure of brackebuschite, ideally Pb(2)Mn(3+)(VO(4))(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 A (2) M(T1O(4))(T2O(4))(OH, H(2)O), with A = Pb(2+), Ba, Ca, Sr; M = Cu(2+), Zn, Fe(2+), Fe(3+), Mn(3+), Al; T1 = As(5+), P, V(5+); and T2 = As(5+), P, V(5+), S(6+). The crystal structure of brackebuschite is based on a cubic closest-packed array of O and Pb atoms with infinite chains of edge-sharing [Mn(3+)O(6)] octa­hedra located about inversion centres and decorated by two unique VO(4) tetra­hedra (each located on a special position 2e, site symmetry m). One type of VO(4) tetra­hedra is linked with the (1) (∞)[MnO(4/2)O(2/1)] chain by one common vertex, alternating with H atoms along the chain, while the other type of VO(4) tetra­hedra link two adjacent octa­hedra by sharing two vertices with them and thereby participating in the formation of a three-membered Mn(2)V ring between the central atoms. The (1) (∞)[Mn(3+)(VO(4))(2)OH] 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) (∞)[Mn(3+)(VO(4))(2)OH] 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 Pb(2+) cation with two Mn(3+) 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.