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

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) Å3 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 cTr = 2T43T4 and topological repeat unit cVr = 2V23V2.


Introduction
Spurrite marbles of the Siwaqa area, a pyrometamorphic complex in Central Jordan, have anomalously high contents of Zn (Khoury et al. 2016;Sokol et al. 2017;Vapnik et al. 2019). Most common Zn-bearing minerals are sulphides, but Zn can be found in selenides and oxides, too. In the northern part of the Siwaqa region, the mineral tululite Ca 14 (Fe 3+ , Al)(Al, Zn, Fe 3+ , Si, P, Mn, Mg) 15 O 36 , was found in medium-temperature (800 -850 °C) combustion metamorphic (CM) rocks i.e. Zn-rich marbles with high Ca:Al ratio (Khoury et al. 2016). A natural equivalent of CaZn 2 (OH) 6 ·2H 2 O (Stahl and Jacobs 1997), named qatranaite (Vapnik et al. 2019), was recently discovered in the same area. Qatranaite was found in a single outcrop within cuspidine veins cutting spurrite marbles. This mineral is a product of low-temperature (< 70 °C) alteration of pyrometamorphic rocks by hyper-alkaline solutions (Vapnik et al. 2019). Furthermore, the mineral clinohedrite CaZn(SiO 4 )·H 2 O was reported to replace sphalerite in the bleaching zones cutting through dark spurrite marbles from the same type locality (Khoury et al. 2016).
In the same area, we have found the new low-temperature hydrothermal mineral khurayyimite (IMA 2018-140), with ideal chemical formula Ca 7 Zn 4 (Si 2 O 7 ) 2 (OH) 10 ·4H 2 O. To the best of our knowledge, no synthetic analogue is known and therefore, it is a new compound in the system CaO-ZnO-SiO 2 -H 2 O. The name khurayyimite is given after Mount Khurayyim (Jabal al Khurayyim), Siwaqa pyrometamorphic rock area, central Jordan. Khurayyimite was found in the immediate vicinity of this mountain. Type material was deposited in the mineralogical collection 1 3 of the Fersman Mineralogical Museum, Leninskiy pr., 18/k2, 115162 Moscow, Russia, catalogue number: 5298/1.
The formation of low-temperature zinc-bearing hydrated minerals in spurrite rock of the Hatrurim Complex was discussed in detail by Vapnik et al. (2019) in a publication on the mineral qatranaite, CaZn 2 (OH) 6 (H 2 O) 2 . The authors describe dark and fractured spurrite rocks, where cm-sized white zones are visible along the cracks. Within these zones re-crystallization of finegrained spurrite, occurrence of metacrysts (up to 0.5 cm in size), and local enrichments in cuspidine are observed. The occurrence of qatranaite is restricted to cuspidine zones, whereas clinohedrite and khurayyimite are associated with the hydrated fragments of re-crystallized spurrite rock. Sphalerite is a widespread mineral in spurrite rock and it is considered to be a source of the zinc for the low-temperature minerals (Khoury et al. 2016). The stability of thaumasite (Jallad et al. 2003;Matschei and Glasser 2015) indicates that qatranaite, khurayyimite and clinohedrite are formed from highly alkaline solutions at ∼70 °C, after the crystallization of thaumasite and calcite veins (Vapnik et al. 2019).

Physical and optical properties
Khurayyimite forms colorless spherulitic aggregates up to 200-300 µm in size (Figs. 1,2). Individual elongated platy crystals in the spherules are nearly 50 µm long, 20 µm wide and up to 10 µm thick. Crystals show white streak and white vitreous lustre. The measured micro-indentation hardness of khurayyimite gave Vickers Hardness VHN 25 = 242 (average of 13 measurement), range 220-264 kg/mm 2 , which corresponds to a value of 3.5-4 on the Mohs scale. Cleavage or parting were not observed. Tenacity is brittle and fracture is splintery. Because of the small size of the crystals, the density could not be measured. Instead, we calculated the density on the basis of the empirical formula and unit cell volume, as refined from single-crystal X-ray diffraction data. The calculated density is 2.806 g·cm −3 . The mineral dissolves in 10% HCl. Khurayyimite is optically negative, α = 1.603(2), β = 1.607(2), γ = 1.610(2) (at λ = 589 nm), 2V meas. = 50(10) º and 2V calc. = 40.9º. Dispersion of the optical axes is very weak; the optical orientation is: Z = b, X^c = 20(5)°, and it is non-pleochroic. Gladstone-Dale's compatibility factor is superior (1-(KP/KC) = -0.012).

Chemical composition
Quantitative wavelength-dispersive electron-microprobe analyses of khurayyimite and the associated minerals were carried out using a CAMECA SX100 electron probe microanalyser. A beam diameter of 10 µm was used. A counting time for peaks was 30 s and 15 s for the background. Diopside and sphalerite were used as reference materials for the  The total sum is quite low with ∼96.28 wt%, because the measurement was done with a broad beam of 10 µm. Using a narrower beam the total wt% was higher, but the ratio of (Ca + Zn)/Si was worse. Because of the small size of the khurayyimite spherulites and difficulties to select pure material, H 2 O and CO 2 contents were not determined by chemical methods. Moreover, absence of CO 3 2− groups and presence of H 2 O and hydroxyl groups in khurayyimite were confirmed by the structural investigations and Raman spectroscopy.

X-ray crystallography
Single crystal diffraction experiments at ambient conditions were performed at the X06DA beamline of the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The beamline was equipped with an Aerotech one-axis goniometer and a PILATUS 2 M detector.
Data collection was carried out at ambient conditions using the DA + acquisition software (Wojdyla et al. 2018). The radiation source was a SLS super-bending magnet (2.9 T). A wavelength of 0.70849 Å was obtained using a Bartels monochromator. The detector was placed 80 mm from the sample, resulting in a maximum resolution of 0.7 Å. A total of 1800 frames were recorded using fine-sliced (0.1°) ω-scans at 0.2 s per frame. Experimental details are given in Table 2.
Determination of lattice parameters, data reduction and absorption correction were processed with the program CrysAlisPro (Rigaku 2020). The average structure was solved using SIR2004 (Burla et al. 2005). The least-squares refinements were performed using the program Shelxl97 (Sheldrick 2008). Bond valence sum calculations were done with the BondStr program (Brown and Altermatt 1985;Rodríguez-Carvajal 2005). For the analysis of the chains in the structure of khurayyimite the Crystana software was employed (Klein and Liebau 2014). Figures of the crystal structure and deviations of the polyhedra from their ideal geometries expressed with the quadratic elongation l and the angle variance σ 2 as defined by (Robinson et al. 1971) were calculated using Vesta3 (Momma and Izumi 2011). All H-sites were located by difference Fourier analysis. The resulting structure model was refined using 223 parameters and 4676 independent reflections. All of the atoms, except H, were described using anisotropic displacement parameters. Hydrogen positions were refined at a fixed value of U iso = 0.05 Å 2 for the H 2 O molecules and OH groups bonded to cations. OH distances were constrained to 0.90(5) Å. Refinement details are summarized in Table 2. Table 3 lists atomic coordinates. In Table 4 selected bond distances, bond valence sums, quadratic elongation and bond angle variance are given. In the Table 5 parameters for H-bonds (D-A) are listed. A CIF is available in the Supplement.
As khurayyimite occurs only in tiny amounts X-ray powder diffraction data were not collected. Instead we calculated the powder pattern with Jana2006 (Petříček et al. 2014) using the structural data obtained from the single-crystal structure refinements. The seven strongest powder X-ray diffraction lines, d in Å (I %) hkl are: 3.8333 (100%) 213 ; 10.3107 (81%)

Crystal structure
The structure of khurayyimite exhibits dimers of SiO 4 tetrahedra, which are connected by ZnO 2 (OH 2 )-tetrahedra to form corrugated tetrahedral chains of periodicity six, extending along b. Each of the dimers Si 2 O 7 of this sechser chain is bridged by another Zn1O 2 (OH) 2 -tetrahedron resulting in dreier rings of two Si-and one Zn-centered tetrahedra (Fig. 3a). According to the silicate nomenclature of Liebau (1985), these chains can be addressed as loop-branched sechser single chains {lB, 1 1 ∞ }[ 6 Zn 4 Si 4 O 21 ](OH) 8 .
The chains are linking the clusters of seven CaO n polyhedra made of two Ca1O 7 and five octahedra with Ca2, Ca3 and Ca4 atoms in the center (Fig. 3b and c). The clusters occur in two different orientations in the unit cell (Fig. 3d). The sechser chains are twisting around clusters sharing corners and edges with the CaO n polyhedra. Three oxygen atoms shared by Zn-centered tetrahedra and CaO n polyhedra are connected to additional H atoms (O8-H8, O9-H9, O10-H10 and O12-H12). Another hydrogen atom, H11 is attached to O11, an apical oxygen between Ca1-, Ca2-and Ca3-centered polyhedra (Fig. 3c) Table 5). The chains extend infinitely along b. Along the a-direction the corrugation of the chains creates ∼11 Å thick sheets (Fig. 3d, e). These sheets are interconnected by oxygen (O12) shared by Ca2O 6 polyhedra and Zn1O 4 tetrahedra (Fig. 4). In addition, the narrow gaps between the sheets, formed parallel to [001], are filled by strong hydrogen bonds formed between five OH¯ groups two H 2 O molecules attached to the chains or Ca-clusters. The periodicity of crooked loop-branched sechser single chains {lB, 1 1 ∞ }[ 6 Zn 4 Si 4 O 21 ](OH) 8 along b is 9.0897 Å, which corresponds to the b lattice parameter of the cell. The stretching factor of the chain is rather small. Both SiO 4 tetrahedra within the chain show average bond lengths of 1.6329(6) and 1.6367(7) and low measures of distortion (Table 4). Two Zn-centered tetrahedra are equally distorted (see Table 4). Still Zn1O 4 has longer average bonds of 1.9642(6) Å than Zn2O 4 1.9388(7). Actually, the larger Zn1O 4 tetrahedron forms a loop with the Si 2 O 7 groups. Strong repulsive forces between the tetrahedra of the dreier ring are pressing O1 atom as far as possible forming a triangle with longer Zn1O 4 edge (∼3.176 Å) and two shorter SiO 4 edges (2.66 and 2.68 Å). These repulsive forces are, according to Liebau (1985), a possible reason why such loops are only rarely observed.

Discussion
The structure of this mineral comprises new and very unusual loop-branched sechser single chains that can be described with the formula {lB, 1 1 ∞ }[ 6 Zn 4 Si 4 O 21 ](OH) 8 , following the classification of Liebau (1985). This formula denotes a loopbranched (lB) single chain (1 1 ∞ ) with a six-tetrahedra repetition unit (sechser) made of four ZnO 4 and four SiO 4 cornersharing tetrahedra (Zn 4 Si 4 O 21 ). In the chains, Si 2 O 7 dimers and ZnO 2 (OH) 2 tetrahedra are connected by corners building the loops i.e. dreier ring (Fig. 3a). This combination of the two SiO 4 and one ZnO 4 tetrahedra in a ring is very rare due to the strong repulsive forces in this formation. The volume of the ZnO 4 tetrahedra with ∼3.97 Å 3 is two times larger than the volume of SiO 4 tetrahedra (∼ 2.05 Å 3 ) (Abrahams and Bernstein 1969;Kato and Nukui 1976).
Loop-branched sechser single chains are observed in vlasovite Na 2 ZrSi 4 O 11 (Sokolova et al. 2006;Voronkov and (Liebau 1985). According to the structural hierarchy for chain, ribbon and tube silicates established by Day and Hawthorne (2020), khurayyimite has the same geometrical repeat unit c T r = 2 T 4 3 T 4 and topological repeat unit c V r = 2 V 2 3 V 2 as the mineral vlasovite and the three synthetic compounds HNb ( Salvadó et al. (2001), and therefore they belong to the same group. This new structural hierarchy is based on the connectedness of onedimensional polymerization of the (TO 4 ) n− tetrahedra.
The geometrical repeat unit has n g = 6 tetrahedra. Its connectivity is denoted as c T r = 2 T 4 3 T 4 ; i.e. contains four ZnO 4 tetrahedra with connectivity of two ( 2 T 4 ) and four SiO 4 tetrahedra with connectivity of three (Fig. 7b). The topological repeat unit is denoted by degree of vertex (r) and the number of vertices (c) in the topological repeat unit. All the branches are moved to the one side of the chain. The topological repeat unit c V r = 2 V 2 3 V 2 in khurayyimite is half of the size of geometrical repeat unit ( Fig. 7c). According to the classification of Day and Hawthorne (2020) the mineral vlasovite Na 2 ZrSi 4 O 11 (Sokolova et al. 2006) with chains made of four-membered rings of SiO 4 tetrahedra belongs to the same group ( Fig. 7d- (Hogrefe and Czank 1995) and b K 2 ZnSi 4 O 10 (Kohara and Kawahara 1990). SiO 4 tetrahedra are shown in yellow and ZnO 4 in blue. K atoms were omitted for clarity SC XRD investigation at the synchrotron. BK solved and analyzed the structure.BK and IOG wrote the paper with help from all coauthors.
Funding Open access funding provided by University of Innsbruck and Medical University of Innsbruck.

Competing interests
The authors declare no competing interests.
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