Structural chemistry of copper and zinc minerals. VI. Bayldonite, (Cu,Zn)3Pb(AsO4)2(OH)2: a complex layer structure

Abstract

Bayldonite, (Cu,Zn)3Pb(AsOa)2(OH)2 from Tsumeb, S.W. Africa, is monoclinic, space group C2/c, with cell dimensions: a = 10.147(2), b = 5.892(1), c = 14.081 (2) A, fl = 106.05 (1) ° and Z = 4. The crystal structure has been determined by the heavy-atom method and refined by the full-matrix least-squares method to an R factor of 0.053 for 1057 observed Xray reflections. The crystal structure of bayldonite consists of interconnected Cu octahedral layers and lead arsenate polyhedral-tetrahedral layers which alternate along the c axis, giving rise to complex pseudohexagonal layers parallel to (001). The Cu octahedral sheet, with six-membered octahedral tings, is formed by three crystallographically indep_endent [CuO4(On)2] octahedra with point symmetry 1, showing Jahn-Teller distortion; the square-planar Cu-O (or OH) distances vary from 1.878 to 2.087 A (average 1.970 A), whereas the apical Cu-O distances vary from 2.272 to 2.474 A. The PbO s polyhedron is a square antiprism with an average Pb--O bond length of 2.722/k. The tetrahedral As-O bond lengths vary from 1.66 to 1.724 A (average 1-696 A). The H atom appears to be disordered between the (OH) group and a highly charge-deficient O atom, which is bonded to one Pb atom, in addition to the As atom. The observed twinning, cleavage and crystal habit result from the pseudohexagonal nature of the layer structure.

Theoretical and experimental study of the vibrational spectra of liroconite, Cu2Al(AsO4)(OH)4∙4H2O and bayldonite, Cu3Pb[O(AsO3OH)2(OH)2]

“…2e). The presence of the copper octahedrons, CuO 4 (OH) 2 in the mineral structure [5,7] and two structural different OH groups resulted in two OH stretchings bands. However, we have recollected the spectra using KBr RT and LNT pellets and Fluorolube mull (Fig.…”

“…In olivinite (545 cm À1 ) [31] and liroconite (517 cm À1 , see forthcoming discussion), Cu adopts octahedral coordination and the CuÀ ÀO bands occur at lower wavenumbers compared to conichalcite (573 cm À1 ) [31] where Cu is in tetrahedral coordination. Although de Portilla et al [8], on the basis of the higher (CuÀ ÀO) wavenumbers, postulated tetrahedral Cu coordination as in conichalcite, the Cu in bayldonite lies in tetragonally distorted [CuO 4 (OH) 2 ] octahedron consisted of two O atoms and two OH ions in the square plane, with two O atoms at the apices [7]. The lowest bands in the IR spectrum observed at 529, 507 and 462 cm À1 are in close similitude to the adamite bands at 536, 503, 473 cm À1 [30] and represent the three component of the 4 (AsO 4 ) mode ( Table 1).…”

“…Ghose and Wan [7] were first to determine the crystal structure of bayldonite using heavy atom method and found the presence of copper octahedrons, CuO 4 (OH) 2 , which form a chain parallel to the b-axis. Pb is coordinated by eight O atoms, forming a square antiprism, PbO 8 .…”

Structure de la demesmaekerite, Pb2Cu5(SeO3)6(UO2)2(OH)6.2H2O

“…11H20, la famille des uranyl-vanadates avec la carnotite K 2- (Ghose & Wan, 1979) bien que les octa~dres y soient moins distordus car les angles varient entre 82 et 98 °.…”

The Chemistry of Formation of Some Secondary Arsenate Minerals of Cu(II), Zn(II) and Pb(II)

“…Three simple arsenates of Pb(II) with or without necessary copper occur in nature. These are schultenite (PbHAsO4), bayldonite (Pbeu3(AsO4)2(OH)2 or Pb(Cu,Zn)3(AsO4)2 (OH)2; Ghose and Wan, 1979), and duftite (PbCuAsO,OH), the Ca-and Zn-free end-member of a complex solid-solution series. Of these, the last two are found in small amounts at a number of localities around the world.…”