• journal_title：American Mineralogist
• Contributor：Oleg I. Siidra ; Sergey V. Krivovichev ; Rick W. Turner ; Mike S. Rumsey ; John Spratt
• Publisher：Mineralogical Society of America
• Date：2013-01-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4241
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：1
• firstpage：248
• section：Articles

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) ų. 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_0.48As_0.29V_0.15Mo_0.09]. 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.