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

Abstract

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 formulaA2M(T1O4)(T2O4)(OH, H2O), withA= Pb2+, Ba, Ca, Sr;M= Cu2+, Zn, Fe2+, Fe3+, Mn3+, Al;T1 = As5+, P, V5+; andT2 = 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 VO4tetrahedra (each located on a special position 2e, site symmetrym). One type of VO4tetrahedra is linked with the1∞[MnO4/2O2/1] chain by one common vertex, alternating with H atoms along the chain, while the other type of VO4tetrahedra 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. The1∞[Mn3+(VO4)2OH] chains run parallel to [010] and are held together by two types of irregular [PbOx] polyhedra (x= 8, 11), both located on special position 2e(site symmetrym). The magnitude of the libration component of the O atoms of the1∞[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.