Crystal chemistry of layered Pb oxychloride minerals with PbO-related structures: Part I. Crystal structure of hereroite, [Pb_(32)O _(20)(O,□)](AsO_4)_2[(Si,As,V,Mo)O _4]_2Cl_(10)

摘要

The crystal structure of hereroite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R_1 = 0.054 for 6931 unique observed reflections. The mineral is monoclinic C2/c, a = 23.139(4), b = 22.684(4), c = 12.389(2) ?, β = 102.090(3)°, and V = 6358.8(18) ?~3. The structure contains 16 independent Pb sites in strongly asymmetric coordination by O and Cl atoms. There are two tetrahedral sites, from which one (As) is occupied solely by As, whereas the second (T) has the mixed occupancy of [Si _(0.48)As_(0.29)V_(0.15)Mo_(0.09)]. There are in total 21 O sites. The O1-O8 sites belong to the AsO_4 and TO _4 tetrahedral oxyanions. The other O atoms (O9-O20) are tetrahedrally coordinated by Pb atoms, thus being central for the OPb_4 oxocentered tetrahedra. The OPb_4 tetrahedra share edges to form the [O _(21)Pb_(32)]~(22+) layers that can be described as derivatives of the [OPb] layer from the structure of tetragonal PbO (litharge). The [O_(21)Pb_(32)]~(22+) layer in hereroite can be obtained from the [OPb] layer by removal of blocks of oxocentered tetrahedra, which results in formation of double-square sevenfold and square fourfold cavities. The cavities are occupied by the AsO_4 and TO_4 tetrahedra, respectively. The topology of the [O_(21)Pb _(32)]~(22+) layer is complex and can be described as a combination of modules extracted from the layers of OPb_4 tetrahedra present in the structures of kombatite and symesite. The topological functions of tetrahedra within the layer are analyzed using the square lattice method, which shows that each symmetry-independent tetrahedron has its own topological function in the layer construction. The structure of hereroite belongs to the 2:1 type of layered Pb oxyhalides and consists of alternating PbO-type layers and Cl sheets oriented parallel to the (010) plane.