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 Ca14(Fe3+, Al)(Al, Zn, Fe3+, Si, P, Mn, Mg)15O36, 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 CaZn2(OH)6·2H2O (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(SiO4)·H2O 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 Ca7Zn4(Si2O7)2(OH)10·4H2O. To the best of our knowledge, no synthetic analogue is known and therefore, it is a new compound in the system CaO-ZnO-SiO2-H2O. 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 of the Fersman Mineralogical Museum, Leninskiy pr., 18/k2, 115162 Moscow, Russia, catalogue number: 5298/1.

Occurrence and genesis

The mineral khurayyimite, Ca7Zn4(Si2O7)2(OH)10·4H2O, occurs in small cavities and veins in altered spurrite marbles together with calcite, ettringite-thaumasite series minerals, Ca-hydrosilicates like jennite, Ca9(Si3O9)2(OH)6·8H2O and foshagite, Ca4(SiO3)3(OH)2 along with minerals of the tobermorite group. The type locality (N31°24.23′; E36°15.06′) is in Central Jordan, in the northern part of the Siwaqa pyrometamorphic rock area, circa 80 km south of Amman. Daba-Siwaqa is the largest area of the Hatrurim Complex (Mottled Zone) within the Dead Sea rift region, which exhibits twenty fields of pyrometamorphic rocks (Geller et al. 2012; Novikov et al. 2013).

Rock-forming minerals of unaltered dark spurrite marbles are spurrite Ca5(SiO4)2(CO3), calcite CaCO3, fluorapatite Ca5(PO4)3F and cuspidine Ca4Si2O7(F,OH)2. Accessory minerals spinel MgAl2O4—magnesioferrite MgFe23+O4, franklinite ZnFe3+2O4, fluormayenite Ca12Al14O32[☐4F2] —fluorkyuygenite Ca12Al14O32[(H2O)4F2], sphalerite (Zn,Fe)S, pyrite FeS2, chalcocite Cu2S, hematite Fe2O3, clinohedrite CaZn(SiO4)·H2O. Furthermore, barite BaSO4, celestine SrSO4, selenides of Ni, Fe and Cu, greenockite CdS, elbrusite Ca3(Zr1.5U6+0.5)Fe3+3O12, perovskite CaTiO3 and vorlanite (CaU6+)O4 can be found (Galuskin et al. 2011a).

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, CaZn2(OH)6(H2O)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 fine-grained 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).

Results

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 VHN25 = 242 (average of 13 measurement), range 220–264 kg/mm2, 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), 2Vmeas. = 50(10)º and 2Vcalc. = 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).

Fig. 1
figure 1

Khurayyimite and associated minerals: a holotype specimen, contact of unaltered spurrite marble (brown) and altered spurrite marble (light), mainly composed of calcite, minerals of the ettringite-thaumasite series, Ca-hydrosilicates containing cavities with khurayyimite; b, c and d BSE images of typical spherulitic aggregates of khurayyimite: Cal—calcite, Fos—foshagite, Gnk—greenockite, HSi—mixture of Ca-hydrosilicates, Khu—khurayyimite and Spu—spurrite

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Fig. 2
figure 2

Spherulitic aggregates of khurayyimite, from the same area as shown in Fig. 1d, presented in: a plane-polarized transmitted light and b cross-polarized transmitted light. Associated minerals are: Afw—afwillite with finely dispersed Fe hydroxides, Cal—calcite, HSi—mixture of Ca-hydrosilicates, Khu—khurayyimite and Spu—spurrite

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Chemical composition

Quantitative wavelength-dispersive electron-microprobe analyses of khurayyimite and the associated minerals were carried out using a CAMECA SX100 electron probe micro-analyser. 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 analysis of Ca, Si and Zn (all Kα lines). The holotype crystals of khurayyimite show uniform composition. The results based on eleven analyses are summed in the Table 1. The empirical formula, calculated on the basis of 28 O with 10(OH)− and 4H2O is Ca7.070Zn3.894Si4.018O14(OH)10·4H2O. The simplified and ideal formula is: Ca7Zn4(Si2O7)2(OH)10·4H2O, which implies the following weight percentages: CaO 35.03, ZnO 29.05, SiO2 21.45, H2O 14.47. 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, H2O and CO2 contents were not determined by chemical methods. Moreover, absence of CO32− groups and presence of H2O and hydroxyl groups in khurayyimite were confirmed by the structural investigations and Raman spectroscopy.

Table 1 Chemical data for khurayyimite
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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.

Table 2 Parameters for X-ray data collection and crystal-structure refinement for khurayyimite
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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 σ2 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 Uiso = 0.05 Å2 for the H2O 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.

Table 3 Relative atomic coordinates and equivalent isotropic displacement parameters (Å2)
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Table 4 List of selected bond distances (Å) and bond valence sums (BVS, given in valence units) for cations in khurayyimite. Calculated quadratic elongation (l) and bond angle variance (σ2 in °2) are quoted for all SiO4/ZnO4 tetrahedra and CaO6 octahedra. In addition, volume (Å3) is quoted for the tetrahedra
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Table 5 H-bond parameters
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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%) \(\overline{2}13\); 10.3107 (81%) 100;214 2.9519 (68%) 031; 5.455 (59%)\(\overline{1}21\); 2.6607 (57%) 114; 2.9084 (55%) \(\overline{1}31\) and 3.4083 (42 %) \(\overline{2}04\).

Crystal structure

The structure of khurayyimite exhibits dimers of SiO4 tetrahedra, which are connected by ZnO2(OH2)-tetrahedra to form corrugated tetrahedral chains of periodicity six, extending along b. Each of the dimers Si2O7 of this sechser chain is bridged by another Zn1O2(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, 11∞}[6Zn4Si4O21](OH)8.

Fig. 3
figure 3

The structure of khurayyimite: a loop-branched sechser single chains with dreier rings b single chain linking the two clusters of CaOn polyhedra c clusters of CaOn polyhedra with OH¯ groups and H2O molecules d different orientations of clusters in the unit cell e periodicity of crooked chain f crystal structure with chains, clusters and hydrogen bonds between them

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The chains are linking the clusters of seven CaOn polyhedra made of two Ca1O7 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 CaOn polyhedra. Three oxygen atoms shared by Zn-centered tetrahedra and CaOn 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). Ca2 is coordinated by one O anion, four OH¯ groups and one H2O molecule (H13A–O13w–H13B), where Ca4 is coordinated by three O anions, two OH¯ groups and one H2O molecule (H14A–O14w–H14B) (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 Ca2O6 polyhedra and Zn1O4 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 H2O molecules attached to the chains or Ca-clusters.

Fig. 4
figure 4

The strong hydrogen bonds bridging CaOn polyhedra and sechser chains

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The periodicity of crooked loop-branched sechser single chains {lB, 11∞}[6Zn4Si4O21](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 SiO4 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 Zn1O4 has longer average bonds of 1.9642(6) Å than Zn2O4 1.9388(7). Actually, the larger Zn1O4 tetrahedron forms a loop with the Si2O7 groups. Strong repulsive forces between the tetrahedra of the dreier ring are pressing O1 atom as far as possible forming a triangle with longer Zn1O4 edge (∼3.176 Å) and two shorter SiO4 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.

Raman spectroscopy

The Raman spectrum of khurayyimite was recorded using a WITec alpha 300R confocal Raman microscope equipped with an air-cooled solid laser (488 nm) and a charge-coupled device detector operating at -61 °C. The laser radiation was coupled to a microscope through a single-mode optical fibre with a diameter of 3.5 μm. An air Zeiss LD EC Epiplan-Neofluan DIC-100/0.75NA objective was used. Raman scattered light was focused on a broad band single mode fibre with effective Pinhole size about 30 μm and monochromator with a 600 mm−1 grating. The power of the laser at the sample position was ∼30 mW. Integration times of 3 s with accumulation of 15 scans and a resolution 3 cm−1 were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate (Ntziouni et al. 2022). The following bands in the Raman spectra of khurayyimite, Ca7Zn4(Si2O7)2(OH)10·4H2O, were observed (cm−1, Fig. 5): 80, 112, 143, 171, 203, 246, 270, 314, 342, 378, 436, 465, 524, 586, 676, 740, 821, 911, 916, 975, 1624, 2898, 3074, 3109, 3503, 3555, 3580, 3603, 3618, 3633.

Fig. 5
figure 5

Raman spectrum of khurayyimite: a in the region 1800–60 cm−1 b in the region 3800 2600 cm−1

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The main bands correspond to Si-O vibrations in (Si2O7) 6-groups: ν1 911 cm-1, ν4 676 cm-1, ν2 378 cm-1 (Galuskin et al. 2011b) and Zn-O tetrahedra ZnO2(OH)2: ν1 465 cm-1; ν2 + ν4 310cm1, 314 cm-1, 342 cm-1 (Lin et al. 1999; Kesic et al. 2014). The band at 586 cm-1 is related to SiO-Zn vibrations (Chandra Babu and Buddhudu 2014), and band at 524 cm-1 is related to Si-O-Si vibrations. The main bands below 203 cm-1 are due to vibrations of Ca-O in CaO4(OH)3 polyhedra and CaO(OH)4(OH2), CaO4(OH)2, CaO3(OH)2(OH2) octahedra. Bands at 2898, 3074 and 3109 cm-1 are related to O-H vibration in H2O with strong hydrogen bonds, whereas the main bands at 3503, 3580, 3603 and 3618 cm-1 correspond to vibrations of OH groups.

Discussion

The structure of this mineral comprises new and very unusual loop-branched sechser single chains that can be described with the formula {lB, 11∞}[6Zn4Si4O21](OH)8, following the classification of Liebau (1985). This formula denotes a loop-branched (lB) single chain (11∞) with a six-tetrahedra repetition unit (sechser) made of four ZnO4 and four SiO4 corner-sharing tetrahedra (Zn4Si4O21). In the chains, Si2O7 dimers and ZnO2(OH)2 tetrahedra are connected by corners building the loops i.e. dreier ring (Fig. 3a). This combination of the two SiO4 and one ZnO4 tetrahedra in a ring is very rare due to the strong repulsive forces in this formation. The volume of the ZnO4 tetrahedra with ∼3.97 Å3 is two times larger than the volume of SiO4 tetrahedra (∼ 2.05 Å3) (Abrahams and Bernstein 1969; Kato and Nukui 1976).

So far, only a few compounds are reported with the same ring formation but utterly different structures (see Fig. 6). One of them is a structure of synthetic K2ZnSi2O6 (Hogrefe and Czank 1995), where two layers with three-membered rings (2 × SiO4, 1 × ZnO4) are forming the loop-branched zweier framework {lB,3∞}[2(Zn1Si2)O6]. In the structure of K2ZnSi4O10 (Kohara and Kawahara 1990), a tectosilicate framework is built by ten-membered rings of SiO4 tetrahedra interconnected with five-member rings (4 × SiO4, 1 × ZnO4), four-membered rings (3 × SiO4, 1 × ZnO4) and three-membered rings (2 × SiO4, 1 × ZnO4).

Fig. 6
figure 6

Dreier rings in: a K2ZnSi2O6 (Hogrefe and Czank 1995) and b K2ZnSi4O10 (Kohara and Kawahara 1990). SiO4 tetrahedra are shown in yellow and ZnO4 in blue. K atoms were omitted for clarity

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Quite the contrary, three-membered rings comprising two ZnO4 and one SiO4 are widespread among zinc silicates, like in the minerals willemite Zn2SiO4 (Simonov et al. 1977), hemimorphite Zn4Si2O7(OH)2·H2O (Li and Bass 2020), hodgkinsonite Zn2MnSiO4(OH)2 (Rentzeperis 1963), junitoite, CaZn2Si2O7·H2O (Hamilton and Finney 1985), K2Zn2Si8O19 (Kohara and Kawahara 1990) etc.

Different combinations of chains and frameworks made of ZnO4 and SiO4 tetrahedra are known. In ZnSiO3 (Morimoto et al. 1975), with Zn atoms in six- and four-fold coordination, two pyroxene-like chains are branched with two ZnO4 tetrahedra forming four-member loops. The crystal structures of LT and HT forms BaZn2Si2O7 (Lin et al. 1999) have a disilicate group Si2O7 linked via corners with ZnO4 tetrahedra in a three-dimensional framework, which exhibits sixmember rings (2 × Si2O7, 2 × ZnO4), four member-rings (2 × SiO4, 2 × ZnO4) and three-membered rings (1 × SiO4, 2 × ZnO4).

Loop-branched sechser single chains are observed in vlasovite Na2ZrSi4O11 (Sokolova et al. 2006; Voronkov and Pyatenko 1962). However, chains of vlasovite {lB, 11∞}[6Si4O22] exhibit four-membered rings of SiO4-tetrahedra linked to form a chain (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 cTr = 2T4 3T4 and topological repeat unit cVr = 2V2 3V2 as the mineral vlasovite and the three synthetic compounds HNb(H2O)[Si4O11](H2O), Cs0.66H0.33Nb(H2O)[Si4O11] and Na2H(NbO)[Si4O11](H2O)1.25 reported by Salvadó et al. (2001), and therefore they belong to the same group. This new structural hierarchy is based on the connectedness of one-dimensional polymerization of the (TO4)n− tetrahedra.

The geometrical repeat unit has ng = 6 tetrahedra. Its connectivity is denoted as cTr = 2T4 3T4; i.e. contains four ZnO4 tetrahedra with connectivity of two (2T4) and four SiO4 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 cVr = 2V2 3V2 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 Na2ZrSi4O11 (Sokolova et al. 2006) with chains made of four-membered rings of SiO4 tetrahedra belongs to the same group (Fig. 7d-f). Another member of the group is synthetic compound HNb(H2O)[Si4O11](H2O) with chains made of six-membered rings.

Fig. 7
figure 7

a Khurayyimite chain [Zn4Si4O21] b khurayyimite geometrical repeat unit ng = 6 cTr = 2T4 3T4 c khurayyimite topological repeat unit cVr = 2V2 3V2 d vlasovite chain [Si8O22] e vlasovite geometrical repeat unit ng = 6 cTr = 2T4 3T4 f vlasovite topological repeat unit cVr = 2V2 3V2

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