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The mineral khurayyimite Caenter code here Zn$_4$(Si$_2$O$_7$)$_2$(OH)$_{10}$·4H$_2$O, (IMA 2018-140) was found in small cavities in altered spurrite marbles, in the northern part of the Siwaqa pyrometamorphic rock area, Central Jordan. It is a low-temperature, hydrothermal mineral and it forms at a temperature of ca. 100 °C. It builds nearly 50 μm white or colourless, platy crystals arranged in up to 200-300 μm big spherulitic aggregates.
Single-crystal X-ray diffraction experiments at ambient conditions were performed at the X06DA beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). Khurayyimite crystallises in space group P21/c, with unit cell parameters (a=11.2450(8), b=9.0963(5), c=14.0679(10)Å, β = 113.237(8)º, V = 1322.25(17)Å$^3$ and Z = 2. The average structure was solved using direct methods. All H-sites are located by difference Fourier analysis. The resulting structure model was refined up to R1= 0.02. The crystal structure of khurayyimite consists of sheets perpendicular to a. Each sheet is built by very unusual loop-branched sechser single chains {lB,${1_∞^1}$}[$^6$Zn$_4$Si$_4$O$_{21}$]. Voids between chains are filled by blocks of five Ca-octahedra and two CaO$_7$ polyhedra with additional OH groups and water molecules.
The loop-branched $\textit{sechser}$ single chains {lB,${1_∞^1}$}[$^6$Zn$_4$Si$_4$O$_{21}$ ] are made of dimers of Si$_2$O$_7$ and two types of ZnO$_2$(OH)$_2$ tetrahedra connected by corners. Loops of the chain contain three tetrahedra, analogous to $\textit{dreier}$ ring. Strong repulsive forces between the tetrahedra in $\textit{dreier}$ ring are pressing connecting O atoms as far as possible forming equilateral triangle with longer ZnO$_4$ edge (≈ 3.176 Å) and two shorter two SiO$_4$ edges (≈ 2.67). These repulsive forces are, according to Libau (1985), a possible reason for why such loops have not been observed more frequent.
Different combinations of chains and frameworks made of ZnO$_4$ and SiO$_4$ tetrahedra are known. In ZnSiO$_3$ (Morimoto et al. 1975), with Zn atoms in six and four-fold coordination, two pyroxene-like chains running along the c-direction are branched with two ZnO$_4$ tetrahedra forming four-membered loops. The crystal structure of the LT and HT forms of BaZn$_2$Si$_2$O$_7$ (Lin et al. 1999) exhibit a disilicate group Si$_2$O$_7$ linked via corners with ZnO$_4$ tetrahedra in a three-dimensional framework. In such a manner are created six member rings (2×Si$_2$O$_7$, 2×ZnO$_4$), four member-rings (2×SiO$_4$, 2×ZnO$_4$) and three-membered rings (1×SiO$_4$, 2×ZnO$_4$). Still, three-membered loops, made of Si$_2$O$_7$ and ZnO$_4$, like in khurayyimite, have not been observed yet.
Libau, F. (1985) Structural chemistry of the silicates. Springer‐Verlag Berlin, Heidelberg, New York, Tokyo, 347 S.
Morimoto, N., Nakajima, Y., Syono, S., Akimoto, S. and Matsui, Y. (1975) Crystal structure of pyroxene-type ZnSiO$_3$ and ZnMgSi$_2$O$_6$. Acta Crystallographica Section B, 31, 1041-1049.
Lin, J.H., Lu, G.X., Du, J., Su, M.Z., Loong, C.-K. and Richardson. J.W (1999) Phase transition and crystal structures of BaZn$_2$Si$_2$O7. Journal of Physics and Chemistry of Solids, 60(7), 975-983.
