Experimental methods

The crystal morphology, optical properties and chemical composition of qeltite and associated minerals were studied using an optical microscope, a Phenom XL analytical scanning electron microscope (Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Sosnowiec, Poland), and an electron microprobe analyser (Cameca SX100, Micro-Area Analysis Laboratory, Polish Geological Institute – National Research Institute, Warsaw, Poland). The microprobe chemical analyses were performed in wavelength-dispersive spectroscopy mode at acceleration voltage 15 kV, beam current 20 nA and beam diameter 1 μm. The following lines and standards were used: MgKα – diopside; SiKα and ZrLα – zircon; AlKα and KKα – orthoclase; CaKα – wollastonite; SrLα – celestine; NbLα – metallic Nb; BaLβ – baryte; TiKα – rutile; VKα – metallic V; CrKα – Cr₂O₃; MnKα – rhodonite; FeKα – pentlandite; NiKα – nickeline; CuKα – chalcopyrite; and ZnKα – ZnS.

Raman spectra of qeltite were recorded on a WITec alpha 300R Confocal Raman Microscope (Department of Earth Science, University of Silesia, Poland) equipped with an air-cooled solid laser (488 nm) and a CCD camera 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 by a broad-band single mode fibre with effective pinhole size of ~30 μm and a monochromator with 1800 gr/mm. The power of the laser at the sample position was ~20 mW. Integration times of 3 s with an accumulation of 20 scans and a resolution of 2 cm^–1 were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm^–1).

Single-crystal X-ray diffraction (SCXRD) studies were carried out with a four-circle SuperNova diffractometer with AgKα radiation (λ = 0.56087Å), equipped with an Eos CCD detector (Agilent). The detector-to-crystal distance was 66.0 mm. AgKα radiation (λ = 0.0560 Å) was used at 65 kV and 0.6 mA. Crystals were attached to a non-diffracting MiTeGen micromount support. A frame-width of 1° in ω scans and a frame time of 90 s were used for data collection. Reflection intensities were corrected for Lorentz, polarisation and absorption effects and converted to structure factors using CrysAlisPro 1.171.40.67a (Rigaku Oxford Diffraction, 2019) software. Observed unit-cell parameters are consistent with trigonal symmetry. The statistical tests on the distribution of |E| values (|E ²–1| = 0.729) suggested space group symmetry was P321. The crystal showed significant systematic absences, violations of the glide planes and screw axes. Further examination of the structural model of lower, P3 symmetry led to a model equivalent to a P321 structure. Scattering curves for neutral atoms were taken from the International Tables for Crystallography (Prince, Reference Prince2004). The following curves were used: Ca at the A site; Ti at the B site; Fe vs. Si at the C site; Si at the D site and O at the O1–O3 sites. The A, B, D and O sites were found to be fully occupied by Ca, Ti, Si and O, respectively. The C site has a mixed (Fe,Si) occupancy. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material.

Powder X-ray diffraction data for qeltite could not be measured, therefore we present a calculated powder pattern (CuKa radiation, Debye–Scherrer geometry), based on the obtained structure model in Supplementary Table S1.

Qeltite occurrence and description

Qeltite was found in gehlenite–rankinite paralava within the pyrometamorphic Hatrurim Complex, which is a unique geological object described in numerous publications by different authors (Bentor et al., Reference Bentor, Gross and Heller1963a, Reference Bentor, Gross and Heller1963b; Gross, Reference Gross1977; Vapnik et al., Reference Vapnik, Sharygin, Sokol and Shagam2007; Geller et al., Reference Geller, Burg, Halicz and Kolodny2012; Novikov et al., Reference Novikov, Vapnik and Safonova2013; Galuskina et al., Reference Galuskina, Vapnik, Lazic, Armbruster, Murashko and Galuskin2014), so here we provide only a brief characterisation.

Rocks of the Hatrurim Complex are represented mainly by spurrite and fluorapatite marbles, gehlenite, larnite and spurrite rocks, which form large areas in the immediate surroundings of the Dead Sea Rift on the territories of Israel, Palestine and Jordan (Bentor et al., Reference Bentor, Gross and Heller1963a, Reference Bentor, Gross and Heller1963b; Gross, Reference Gross1977; Burg et al., Reference Burg, Starinsky, Bartov and Kolodny1991, Reference Burg, Kolodny and Lyakhovsky1999; Novikov et al., Reference Novikov, Vapnik and Safonova2013; Khoury et al., Reference Khoury, Sokol, Kokh, Seryotkin, Nigmatulina, Goryainov, Belogub and Clark2016). Paralavas of different composition occur within pyrometamorphic rocks of the Complex (Vapnik et al., Reference Vapnik, Sharygin, Sokol and Shagam2007), among them gehlenite–wollastonite–rankinite oxidised paralavas, in which qeltite was discovered and which contain only Fe³⁺-bearing minerals (Galuskin et al., Reference Galuskin, Galuskina, Kamenetsky, Vapnik, Kusz and Zieliński2022). Rarely, reduced phosphide-bearing gehlenite and diopside paralavas are encountered (Britvin et al., Reference Britvin, Murashko, Vapnik, Polekhovsky and Krivovichev2015; Galuskin et al., 2023). The genesis of the Hatrurim Complex rocks remains enigmatic and is considered an unsolved problem (Galuskina et al., Reference Galuskina, Vapnik, Lazic, Armbruster, Murashko and Galuskin2014). Two proposed hypotheses – the ‘classic’ hypothesis, assuming that pyrometamorphic transformation was driven by dispersed organic matter in a sedimentary protolith (Burg et al., Reference Burg, Starinsky, Bartov and Kolodny1991, Reference Burg, Kolodny and Lyakhovsky1999), and the ‘mud volcanos’ hypothesis, proposing the participation of methane in the activation of the combustion processes (Sokol et al., Reference Sokol, Novikov, Zateeva, Vapnik, Shagam and Kozmenko2010; Novikov et al., Reference Novikov, Vapnik and Safonova2013) – cannot explain a number of geological particularities of the Complex, such as thick almost homogeneous beds of pyrometamorphic rocks extending across a dozen square kilometres. Pyrometamorphic rocks of the Hatrurim Complex are characterised by an extraordinary variety of minerals caused by the reactions of combustion by-products (gases, fluids and melts) with earlier minerals of the clinker association and altered country rocks (Galuskin et al., Reference Galuskin, Galuskina, Gfeller, Krüger, Kusz, Vapnik, Dulski and Dzierżanowski2016).

The qeltite-type locality ‘Nabi Musa’, near the Palestinian village Nabi Musa, lies close to a historical place with the same name (probably the Tomb of Moses), situated in the Judean Desert, West Bank, Palestine (31°48′N, 35°25′E) (Fig. 1). Nabi Musa is one of several localities of the Hatrurim Complex located in the Judean Desert in the vicinity of the Jerusalem–Jericho highway, and most of the outcrops are at the road truncation (Fig. 1a). According to Sokol et al. (Reference Sokol, Novikov, Zateeva, Vapnik, Shagam and Kozmenko2010), the Nabi Musa locality is a huge crater-like structure. A massive, brecciated fragment of pyrometamorphic rocks, mainly larnite, gehlenite, spurrite, are embedded in altered rock represented by zeolitic and calcium silicate hydrated rocks. Small paralava bodies form veins and nests up to 0.15 m long (Fig. 1b). Paralava containing qeltite is composed of rankinite, gehlenite, rarer wollastonite, Ti-bearing andradite and kalsilite. Minerals of the khesinite–dorrite series, barioferrite, minerals of magnesioferrite–magnetite–maghemite series, hematite, Si-bearing perovskite, Si–V-bearing fluorapatite, gurimite, hexacelsian and an unidentified Ca–U-silicate are accessory minerals (Fig. 2). Baryte, hydrated calcium silicates such as tobermorite, afwillite, tacharanite and a fabrièsite-like mineral are later, hydrothermal minerals.

Figure 1. (a) Nabi Musa locality along the Jerusalem–Jericho highway truncation. (b) Gehlenite–rankinite–wollastonite paralava nest in altered hydrogrossular-bearing rock.

Figure 2. (a) Thin section made from paralava of the type specimen (5695/1) containing qeltite; yellow – gehlenite, brown – Ti-bearing andradite, and white and transparent – rankinite, wollastonite and hydrated calcium silicates. (b) Qeltite is in small enclaves inside rankinite grains; the fragment magnified in Fig. 2c is shown in the frame. (c) Flattened qeltite crystals. (d, e) Typical mineral association containing qeltite crystals. Qeltite often contains fluorapatite inclusions (d) and very small inclusions of hematite (e). (b–e) Back-scattered electron spectroscopy (BSE) images. Key: Adr – Ti-bearing andradite; Baf – barioferrite; Brt – baryte; Hem – hematite; Hsi – calcium hydrated silicate; Fap – fluorapatite; Gh – gehlenite; Khe – khesinite; Qlt – qeltite; Rnk – rankinite; Wo – wollastonite. The abbreviations are after Warr (Reference Warr2021).

Later, qeltite was detected in paralava at two localities in the Hatrurim Basin in the Negev Desert in Israel. The first locality is in the upper reaches of a tributary of the Halamish Wadi. Here, qeltite with composition (Ca_2.95Sr_0.02Ba_0.01Mn_0.01)_Σ2.99Ti⁴⁺(Fe³⁺_1.55Si_0.57Al_0.46Ti⁴⁺_0.42Cr_0.01)_Σ3.01Si₂O₁₄ was found in gehlenite–wollastonite–Ti-bearing andradite paralava, which also contains a significant amount of fluorapatite–fluorellestadite-group minerals. Andradite and åkermanite are minor minerals, and khesinite, barioferrite, magnesioferrite, dorrite and perovskite are accessory minerals in this rock. Another locality with rankinite–gehlenite–Ti-bearing andradite paralava containing qeltite, (Ca_2.96Sr_0.03Ba_0.01)_Σ3Ti⁴⁺(Fe³⁺_1.44Al_0.58Si_0.55Ti⁴⁺_0.44)_Σ3.01Si₂O₁₄, and its Ti-analogue, Ca₃Ti⁴⁺(Fe³⁺_1.27Al_0.76Ti⁴⁺_0.55Si_0.43)_Σ3.01Si₂O₁₄, is located 700 m to the left of road no. 31 Arad–Dead Sea. Wollastonite, kalsilite and åkermanite are occasionally observed in this paralava. Barioferrite, magnesioferrite, perovskite, khesinite, fluorapatite, aradite, gurimite, Ba–U-perovskite are accessory minerals.

Qeltite generally forms aggregates of flattened crystals up to 40–50 μm in length and less than 5 μm in thickness. These aggregates occur in small enclaves 100–200 μm in size in rankinite (Fig. 2a–c). Rarely, tabular qeltite crystals with inclusions of fluorapatite (Fig. 2d) and hematite (Fig. 2e) more than 100 μm in length and ~10 μm in thickness are noted. In optical images it is clear that qeltite exhibits a light–brown colour with a red hue (Fig. 3b). It has a yellowish–white streak and a vitreous to subadamantine lustre. Its microhardness VHN₂₅ is 708(17) kg/mm²; average of 22 measurements; range 683–738 kg/mm². It has a hardness of ~6 on the Mohs scale. Cleavage and parting are not observed. The mineral is brittle. It displays an uneven and conchoidal fracture. It is not magnetic. Qeltite is uniaxial (+), its refractive indexes are ω ≈ 1.85, ε ≈ 1.90, Δ ≈ 0.05 and its mean calculated refractive index is 1.871, ε = С (λ = 589 nm). It exhibits pleochroism, as it is light coloured, pink along Z and intensively coloured, red–brown along X/Y (Fig. 3b,c). The density of qeltite was not measured because of the small size of its crystals. Its calculated density is 3.48 g⋅cm^–3 based on the empirical formula and unit cell volume refined from the SCXRD data. The Gladstone–Dale compatibility index is 1 – (K _P/K _C) = 0.030 (excellent) (Mandarino, Reference Mandarino1989).

Figure 3. (a) BSE image of a qeltite crystal. (b–c) Optical images of the same qeltite crystal showing pleochroism changing from light brown (~ ‖ Z) to dark brown with a red hue (~⊥ Z). Key: Baf – barioferrite; Fap – fluorapatite; Gh – gehlenite; Qlt – qeltite; Rnk – rankinite. The abbreviations are after Warr (Reference Warr2021).

The results of the electron microprobe analyses of qeltite are given in Table 1.

Table 1. Chemical data (in wt.%) for qeltite.

n.d. – not detected; S.D. – standard deviation.

The qeltite studied is characterised by significant Ti and Al content (Table 1). The empirical formulas of qeltite, (Ca_2.96Sr_0.02Mn²⁺_0.01)_Σ2.99(Ti⁴⁺_0.99Cr³⁺_0.01)_Σ1.00(Fe³⁺_1.59Si_0.60Al_0.43Ti⁴⁺_0.39)_Σ3.01 (Si_1.99P_0.01)_Σ2.00O₁₄ (grain used for SCXRD) and (Ca_2.96Sr_0.02Ba_0.01Mn²⁺_0.01)_Σ3.00(Ti⁴⁺_0.96Cr³⁺_0.03Zr_0.01)_Σ1.00(Fe³⁺_1.53Si_0.63Al_0.46Ti⁴⁺_0.38)_Σ3.00 (Si_1.98P_0.02)_Σ2.00O₁₄ (Table 1, taking into account the dominant valence rule and possibility of double occupation at one structural site, can be simplified to Ca₃Ti(Fe³⁺₂Si)Si₂O₁₄. The content of the paqueite end-member, Ca₃Ti(Al₂Ti)Si₂O₁₄, in qeltite varies in the range 21–23%.

Raman spectroscopy

The features of the Raman spectra of qeltite depend on the crystal orientation (Fig. 4). The Raman spectrum of qeltite differs from the spectra of typical nesosilicates (for example, minerals of the garnet and schorlomite groups) by the fact that the strongest band in the qeltite spectrum at 611–613 cm^–1 is related to the symmetric stretching vibration of Ti–O in the ^B(TiO₆)^8– octahedron (Frank et al., Reference Frank, Zukalova, Laskova, Kürti, Koltai and Kavan2012; Vásquez et al., Reference Vásquez, Maestre, Cremades and Piqueras2017; Su et al., Reference Su, Balmer and Bunker2000; Heyns et al., Reference Heyns, Harden and Prinsloo2000). The bands of lower intensities are complex and are mainly connected with vibrations of Si–O, Fe³⁺–O, Al–O and Ti⁴⁺–O bonds at the tetrahedral sites C and D. The main bands in the qeltite Raman spectrum are as follows (Fig. 4, cm^–1, ~⊥Z/‖Z): 166/172, 218/215 related to Ca–O vibrations and/or ν₂^B(TiO₆)^8–; 244/252, 331/~353 related to the vibrations R(TO₄), ν₂^C(FeO₄)^5–; 437/448 – ν₄^B(TiO₆)^8–, ν₄^C(FeO₄)^5–, ν₄^D(SiO₄)^4–; 611/613 – ν₁^B(TiO₆)^8–, ν(^BTi–O–^DTi); 713/718 – ν₁^D(FeO₄)^5–; 766 – ν₁^C(TiO₄)^4–, ν₁^C(AlO₄)^5–; 855 – ν₁^D(SiO₄)^4–; 978/986 – ν₃^D(SiO₄)^4–, ν(^CSi–O–^DSi). The interpretation of bands was on the basis of Raman data obtained by different authors for TiO₂ polymorphs, Ti-bearing garnets, titanite and other titanosilicates (Galuskina et al., Reference Galuskina, Galuskin, Dzierżanowski, Armbruster and Kozanecki2005; Frank et al., Reference Frank, Zukalova, Laskova, Kürti, Koltai and Kavan2012; Vásquez et al., Reference Vásquez, Maestre, Cremades and Piqueras2017; Su et al., Reference Su, Balmer and Bunker2000; Heyns et al., Reference Heyns, Harden and Prinsloo2000).

Figure 4. Raman spectra of qeltite on different orientations of a crystal.

Crystallography

Structural data were obtained for a 0.04 × 0.02 × 0.01 mm crystal at 295.5(4) K. Experimental data and the results of structure refinement are given in Tables 2–5. Bond valence sum (BVS) calculations are shown in Table 6.

Table 2. The crystal information and details of X-ray diffraction data collection and refinement for qeltite.

* – including Al.

Table 3. Site occupation factor (s.o.f.), atomic coordinates and isotropic displacement parameters (Ų) for qeltite.

Table 4. Anisotropic displacement parameters (Ų).

Table 5. Selected interatomic distances (Å) for qeltite.

Table 6. Bond valence calculations for qeltite (valence units). Bond valence parameters were taken from (Gagné and Hawthorne, Reference Gagné and Hawthorne2015).

Qeltite, Ca₃Ti(Fe³⁺₂Si)Si₂O₁₄ [P321, a = b = 8.0077(5), c = 4.9956(4) Å], belongs to the langasite structural type – a family of synthetic compounds with the general formula A ₃BC ₃D ₂O₁₄ (Mill, Reference Mill2009; Marty et al., Reference Marty, Bordet, Simonet, Loire, Ballou, Darie, Kljun, Bonville, Isnard, Lejay, Zawilski and Simon2010; Lyubutin et al., Reference Lyubutin, Naumov, Mill, Frolov and Demikhov2011; Markina et al., Reference Markina, Mill, Pristáš, Marcin, Klimin, Boldyrev and Popova2019). Paqueite, Ca₃Ti(Al₂Ti)Si₂O₁₄ (there are only electron back-scatter diffraction data; the structural model is the langasite-type synthetic phase Ca₃Ti(Al,Ti,Si)₃Si₂O₁₄: P321, a = b = 7.943, c = 4.930 Å; Scheuermann et al., Reference Scheuermann, Kutoglu, Schosnig and Hoffer2000), is known to exist in Nature, as it has been described in meteorites (Paque et al., Reference Paque, Beckett, Barber and Stolper1994; Ma and Beckett, Reference Ma and Beckett2016). Qeltite, Ca₃Ti(Fe³⁺₂Si)Si₂O₁₄, is an Fe³⁺-analogue of paqueite, at the D tetrahedra of which Si > Ti⁴⁺ (Table 5). The qeltite structure belongs to the trigonal non-centrosymmetric P321 space group. In qeltite, CaО₈ polyhedra and TiО₆ octahedra form a layer in which the central TiO₆ octahedron shares three edges with three CaO₈ polyhedra. These CaO₈ polyhedra are further connected to other CaO₈ polyhedra by corner sharing (Fig. 5, 6). The CaO₈ polyhedra are distorted, with bond lengths ranging from 2.358(5) to 2.868(4) Å. In fact, Ti at coordination 6 is at the centrum of a truncated trigonal trapezohedron with the distances Ti–O(3) = 1.954(5). In adjacent layers, SiO₄ tetrahedra share three corners (O1 atoms) with larger [(Fe³⁺,Al)₂(Si,Ti)]O₄ C tetrahedra. The base of the SiO₄ tetrahedron has three longer bond lengths of 1.638(4) Å to O1 atoms and one shorter bond length of 1.584(8) Å to O2, which connects the SiO₄ tetrahedra to three CaO₈ polyhedra from the next layer. These relatively weak Ca–O2 bonds of 2.645(3) Å contribute to the underbonding of O2 (Table 6). The limited degree of positional freedom (O2 lies on a three-fold axis, 2d Wyckoff position) prevents this underbonding from being relieved. Thus, the bonding deficiency is relieved by the remaining O atoms that show a little overbonding (Table 6). The bond valence sum for all anions per formula unit averages ideally to 2.00 valence units.

Figure 5. Structure of qeltite. (a) projection on (001), (b) projection on (100). The unit cell is shown by a black line. Key: Ca polyhedra – blue; Ti octahedra – pink; Si tetrahedra – navy blue; Fe tetrahedra – light brown. Drawn using CrystalMaker® software for Windows 2.7.

Figure 6. Projection of qeltite structure on (001). Two layers – polyhedral (z = 0) and tetrahedral (z = ½) in the qeltite structure are shown. Unit cell is shown by the black line. Key: Ca polyhedra – blue; Ti octahedra – pink; Si tetrahedra – navy blue; Fe tetrahedra – light brown. Drawn using CrystalMaker® software for Windows 2.7.

The [(Fe³⁺,Al)₂(Si,Ti)]O₄ tetrahedron has two shorter bonds of 1.791(5) to O3 atoms, connecting this tetrahedron to the TiO₆ octahedra, and two longer bonds of 1.883(4) to O1 atoms, connecting this tetrahedron to the CaO₈ polyhedra. We included two additional weak interactions of C-site cations to O3, with a distance of 2.564(6) Å, which contribute to O2 underbonding compensation. The obtained structural formula of qeltite Ca_3.00Ti(Fe_1.75Si_1.25)_Σ3.00Si_2.00O₁₄, which is charge balanced due to the substitution of part of Fe³⁺ and Si by Ti⁴⁺ and Al at the C site, and the empirical formula, (Ca_2.96Sr_0.02Mn²⁺_0.01)_Σ2.99(Ti⁴⁺_0.99Cr³⁺_0.01)_Σ1.00(Fe³⁺_1.59Si_0.60Al_0.43Ti⁴⁺_0.39)_Σ3.01(Si_1.99P_0.01)_Σ2.00O₁₄, are well-matched. The number of electrons for the C tetrahedron in the structural formula is 20.96 and 21.30 electrons in the qeltite empirical formula calculated on the basis of microprobe analyses.