Khurayyimite Ca7Zn4(Si2O7)2(OH)10·4H2O: a mineral with unusual loop-branched sechser single chains

Abstract

The new mineral khurayyimite Ca₇Zn₄(Si₂O₇)₂(OH)₁₀·4H₂O occurs in colorless spherulitic aggregates in small cavities of altered spurrite marbles located in the northern part of the Siwaqa pyrometamorphic rock area, Central Jordan. It is a low-temperature, hydrothermal mineral and is formed at a temperature lower than 100 °C. Synchrotron single-crystal X-ray diffraction experiments have revealed that khurayyimite crystallizes in space group P2₁/c, with unit cell parameters a = 11.2171(8), b = 9.0897(5), c = 14.0451(10) Å, β = 113.297(8)º, V = 1315.28(17) ų and Z = 2. The crystal structure of khurayyimite exhibits tetrahedral chains of periodicity 6. The sequence of SiO₄ and ZnO₂(OH)₂-tetrahedra along the chain is Si–Si-Zn. The neighboring SiO₄-tetrahedra of the corrugated chains are bridged by additional ZnO₂(OH)₂-tetrahedra to form 3-connected dreier rings. The chains can be addressed as loop-branched sechser single chains {lB, 1¹_∞}[⁶Zn₄Si₄O₂₁]. The chains are linked by clusters of five CaO₆ and two CaO₇ polyhedra with additional OH groups and H₂O molecules in the coordination environment. Based on the connectedness and one-dimensional polymerisations of tetrahedra (TO₄)ⁿ⁻, chains of khurayyimite belong to the same group as vlasovite Na₂ZrSi₄O₁₁, since they can be described with geometrical repeat unit ^cT_r = ²T₄ ³T₄ and topological repeat unit ^cV_r = ²V₂ ³V₂.