- © 2013 Mineralogical Society of America
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 R1 = 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) Å, β = 102.090(3)°, and V = 6358.8(18) Å3. 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 [Si0.48As0.29V0.15Mo0.09]. There are in total 21 O sites. The O1–O8 sites belong to the AsO4 and TO4 tetrahedral oxyanions. The other O atoms (O9–O20) are tetrahedrally coordinated by Pb atoms, thus being central for the OPb4 oxocentered tetrahedra. The OPb4 tetrahedra share edges to form the [O21Pb32]22+ layers that can be described as derivatives of the [OPb] layer from the structure of tetragonal PbO (litharge). The [O21Pb32]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 AsO4 and TO4 tetrahedra, respectively. The topology of the [O21Pb32]22+ layer is complex and can be described as a combination of modules extracted from the layers of OPb4 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.