Redetermination of brackebuschite crystal structure and formula

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Marco E. Ciriotti
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Redetermination of brackebuschite crystal structure and formula

Messaggio da Marco E. Ciriotti » lun 29 feb, 2016 12:34

Referenza:
▪ Lafuente, B. & Downs, R.T. (2016): Redetermination of brackebuschite, Pb2Mn3+(VO4)2(OH). Acta Crystallographica, E72, 293-296.

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 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∞[MnO4/2O2/1] 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 [PbOx] 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.

CIF:
space_group_crystal_system monoclinic
_space_group_name_H-M_alt 'P 21/m'
_space_group_symop_operation_xyz
'x, y, z'
'-x, y+1/2, -z'
'-x, -y, -z'
'x, -y-1/2, z'
_cell_length_a 7.64920(10)
_cell_length_b 6.12620(10)
_cell_length_c 8.9241(2)
_cell_angle_alpha 90
_cell_angle_beta 112.1950(10)
_cell_angle_gamma 90
_cell_volume 387.202(12)
_cell_formula_units_Z 2
Marco E. Ciriotti

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