Qeltite – the first terrestrial high-temperature mineral with a langasite-type structure from the pyrometamorphic rocks of the Hatrurim Complex

Abstract

Qeltite (IMA2021–032), ideally Ca₃Ti(Fe₂Si)Si₂O₁₄, was found in gehlenite–rankinite–wollastonite paralava from a pyrometamorphic rock of the Hatrurim Complex at Nabi Musa locality, Judean Desert, West Bank, Palestine. It generally occurs as light-brown flattened crystals up to 40–50 μm in length and less than 5 μm in thickness. Its aggregates reach 100–200 μm in size. Its empirical crystal chemical formula based on 14 O is: (Ca_2.96Sr_0.02Mn_0.01)_Σ2.99Ti⁴⁺(Fe³⁺_1.59Si_0.60Al_0.43Ti⁴⁺_0.38Cr_0.01)_Σ3.01(Si_1.99P_0.01)_Σ2O₁₄. The strongest reflections in its calculated X-ray diffraction pattern are [d, Å, (I, %), hkl]: 3.12, (100), 111; 2.85, (61), 201; 2.85, (48), 021; 2.32, (45), 211; 6.93, (31), 100; and 1.81, (30), 212. Qeltite is trigonal and crystallises in the noncentrosymmetric P321 space group, with a = 8.0077(5) Å, c = 4.9956(4) Å, V = 277.42(4) ų and Z = 1. Its microhardness VHN₂₅ is 708(17) kg/mm² and its hardness on the Mohs scale is ~6. Its calculated density is 3.48 g/cm³. It was found in fine-grained mineral aggregates within coarse-grained main minerals of rankinite–gehlenite paralava with subordinate wollastonite, Ti-bearing andradite and kalsilite. In these aggregates, the mineral is associated with khesinite, paqueite and pseudowollastonite, indicating a high-temperature genesis (~1200°C). Its crystallisation can be compared with the crystallisation of minerals containing refractory inclusions in meteorites.

Introduction

In gehlenite paralava of the Hatrurim Complex, spreading across the territories of Israel and Palestine, a whole series of new minerals with a langasite-type structure (Belokoneva et al., 1980; Mill et al., 1982; Kaminskii et al., 1983; Mill and Pisarevsky, 2000; Andreev, 2006) has been recently discovered (Galuskina et al., 2023), some of which have a composition close to garnet of the andradite–schorlomite series, which has complicated their correct identification. One of the first, qeltite, Ca₃Ti(Fe₂Si)Si₂O₁₄, has been studied by us and approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021–032, Galaskina et al., 2021). Qeltite is the Fe–Si analogue of paqueite, Ca₃Ti(Al₂Ti)Si₂O₁₄ (IMA2013–53), recently described from the Allende CV3 carbonaceous chondrite (Ma et al., 2022) and later detected in contact facies of phosphide-bearing gehlenite paralava of the Hatrurim Complex in the wadi Zohar, Hatrurim Basin, Israel (Galuskin et al., 2022).

Langasite phases comprise synthetic family compounds with the general formula A ₃BC ₃D ₂O₁₄, where A = Ba, Sr, Ca, Pb²⁺, Na and K; B = Ti⁴⁺, Sb⁵⁺, Nb⁵⁺, Ta⁵⁺ and Te⁶⁺; C = Fe³⁺, Co²⁺, Mn²⁺, Ga, Al and Ti⁴⁺; D = Si, Ge⁴⁺, P⁵⁺, V⁵⁺ and As⁵⁺ (Mill, 2009; Lyubutin et al., 2011; Markina et al., 2019; Scheuermann et al., 2000). The name of this family comes from the names of the chemical elements in lantanium gallium silicate La₃Ga₅SiO₁₄ – one member of the family (Andreev, 2004). The first phase of the langasite-type structure with composition Ca₃Ga₂Ge₄O₁₄ was synthesised in 1979 (Mill and Pisarevsky, 2000). Industry requests for piezoelectric materials for middle-band monolithic BAW (bulk acoustic wave) devices prompted the synthesis of these compounds at scale (Mill and Pisarevsky, 2000). The high piezoelectric and electromechanical constants of these materials (higher than those of quartz), and the absence of phase transformation up to the melting point (e.g. 1470°C) make these materials attractive for practical applications (Tichý et al., 2010). At present, more than 200 synthetic compounds belonging to the langasite family are known (Markina et al., 2019). These compounds are interesting not only for basic investigations but also for many applications due to functional properties such as piezoelectricity, optical nonlinearity and multiferroicity. The langasite family phases have been examined intensively in the context of their applications in bulk and surface acoustic wave devices, as well as in the field of lasers, photorefractive media, nonlinear optics and electrooptics (Markina et al., 2019).

In Nature, until now only one phase with the langasite-type structure that was formed at high temperatures – paqueite – had been found, and that mineral was found in a meteorite (Ma et al., 2022). Some low-temperature mineral phases belonging to the langasite family, such as the dugganite-group minerals (trigonal, P321) dugganite, Pb₃Zn₃(AsO₄)₂(TeO₆), a = 8.460(2) Å, c = 5.206(2) Å (Williams, 1978; Lam et al. 1998); joëlbruggerite, Pb₃Zn₃(Sb⁵⁺,Te⁶⁺)As₂O₁₃(OH,O), a = 8.4803(17) Å, c = 5.2334(12) Å (Mills et al., 2009); and kuksite, Pb₃Zn₃(PO₄)₂(TeO₆), a = 8.39 Å, c = 5.18 Å, formed in oxidised ore, pyrite-bearing, metasomatites with gold–telluride mineralisation (Kim et al., 1990) or in the oxidation zone of silver–lead and silver–polymetallic ores (Williams, 1978; Mills et al. 2009). The structure of cheremnykhite, Pb₃Zn₃(VO₄)₂(TeO₆), which belongs to the dugganite group, needs re-investigation, as along with the structure of kuksite, it was defined as orthorhombic (a = 8.58(3) Å, b = 14.86(5) Å and c = 5.18(3) Å, Kim et al., 1990). The structure of joëlbruggerite should also be clarified, as the OH position was incorrectly determined as a result of a bond valence sum miscalculation (Mills et al., 2009). By analogy with the formula of langasite, which was adapted to paqueite and qeltite, the crystal chemical formula of minerals of the dugganite group should be written as follows: (for example, for dugganite) Pb₃Te⁶⁺Zn₃As⁵⁺₂O₁₄.

There is another interesting mineral, taikanite, Sr₂BaMn³⁺₂(Si₄O₁₂)O₂, which was found in oxidised manganese ore (Kalinin et al., 1985). The structure of taikanite is monoclinic (C121, a = 14.600(2) Å, b = 7.759(4) Å, c = 5.142(1) Å, β = 93.25(2)°) and derived from structures of the langasite type A ₃BC ₃D ₂O₁₄, where the A site is split and occupied by Ba and Sr, and one of the tetrahedral sites D changes into an octahedral one and is occupied by Mn³⁺ (Armbruster et al., 1993).

In this paper we described qeltite – one of the terrestrial minerals with a langasite-type structure which has been discovered in paralava of the Hatrurim pyrometamorphic Complex. The mineral is named after the Wadi Qelt in the close vicinity of the qeltite type locality Nabi Musa, Judean Desert, Palestine. Type material was deposited in the mineralogical collection of the Fersman Mineralogical Museum, Leninskiy pr., 18/k2, 115162 Moscow, Russia, catalogue numbers 5695/1.

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, 2004). 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., 1963a, 1963b; Gross, 1977; Vapnik et al., 2007; Geller et al., 2012; Novikov et al., 2013; Galuskina et al., 2014), 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., 1963a, 1963b; Gross, 1977; Burg et al., 1991, 1999; Novikov et al., 2013; Khoury et al., 2016). Paralavas of different composition occur within pyrometamorphic rocks of the Complex (Vapnik et al., 2007), among them gehlenite–wollastonite–rankinite oxidised paralavas, in which qeltite was discovered and which contain only Fe³⁺-bearing minerals (Galuskin et al., 2022). Rarely, reduced phosphide-bearing gehlenite and diopside paralavas are encountered (Britvin et al., 2015; Galuskin et al., 2023). The genesis of the Hatrurim Complex rocks remains enigmatic and is considered an unsolved problem (Galuskina et al., 2014). Two proposed hypotheses – the ‘classic’ hypothesis, assuming that pyrometamorphic transformation was driven by dispersed organic matter in a sedimentary protolith (Burg et al., 1991, 1999), and the ‘mud volcanos’ hypothesis, proposing the participation of methane in the activation of the combustion processes (Sokol et al., 2010; Novikov et al., 2013) – 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., 2016).

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. (2010), 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 (2021).

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, 1989).

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 (2021).

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., 2012; Vásquez et al., 2017; Su et al., 2000; Heyns et al., 2000). 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., 2005; Frank et al., 2012; Vásquez et al., 2017; Su et al., 2000; Heyns et al., 2000).

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, 2015).

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, 2009; Marty et al., 2010; Lyubutin et al., 2011; Markina et al., 2019). 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., 2000), is known to exist in Nature, as it has been described in meteorites (Paque et al., 1994; Ma and Beckett, 2016). 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.

Discussion

In the last few years, a number of different mineral phases with the langasite-type structure and the general formula A ₃BC ₃D ₂O₁₄, where A = Ca and Ba; B = Ti, Nb, Sb and Zr; C = Ti, Al, Fe and Si; and D = Si, have been detected in pyrometamorphic rocks of the Hatrurim Complex. Among them are minerals close in composition to Ti-rich garnets of the andradite–schorlomite series – for example, qeltite, Ca₃Ti⁴⁺(Fe³⁺₂Si)Si₂O₁₄, as described in this article – and minerals with exotic composition such as Ba₃Nb⁵⁺Fe³⁺₃Si₂O₁₄. We consider that the systematics and nomenclature of minerals with a langasite-type structure (dugganite supergroup) should be elaborated on after the full study of the minerals found in the Hatrurim Complex rocks (Galuskina et al., 2023).

Following the discovery of qeltite in paralava of Nabi Musa locality, Palestine, it transpired that isostructural minerals with the common composition Ca₃Ti(Fe³⁺,Al,Si,Ti⁴⁺)₃Si₂O₁₄ are widely distributed in the paralavas of the Hatrurim Basin, Israel and often associate with garnets of the andradite–schorlomite series, which are similar by composition. In Ti-bearing garnets, Ca₃(Fe³⁺,Al…,Ti⁴⁺)₂(Si,Fe³⁺,Al)₃O₁₂ an entry of Ti⁴⁺ at the octahedral site is facilitated by the substitution of some Si by trivalent cations at the tetrahedral site: ^VI(Fe³⁺, Al…)^IVSi → ^VI(Ti⁴⁺)^IV(Fe³⁺,Al). In qeltite, Ca₃Ti(Fe³⁺₂Si)Si₂O₁₄, Ti⁴⁺ occupies the octahedral site and can enter the tetrahedral site, replacing Si. Increasing Fe³⁺ and Al content at the tetrahedral site to more than 2 atoms per formula unit can be related to the presence of Nb⁵⁺ or Sb⁵⁺ at the octahedral site according to the isomorphic scheme ^VI(Ti⁴⁺)^IV(Si,Ti⁴⁺)→^VI(Nb⁵⁺,Sb⁵⁺)^IV(Fe³⁺,Al) (Galuskina et al., 2023). The appearance of Ti⁴⁺ at the tetrahedral coordination is an exceedingly rare phenomenon, which has also been noted in Si,Al-deficient pyroxenes and amphiboles (Carbonin et al., 1989; Oberti et al., 1992). Titanium in qeltite shares a site coordinated by O1×2 (M–O = 1.883 Å) and O3×2 (M–O = 1.791 Å) with Al, Si and Fe³⁺, for which the tetrahedral coordination is usual. The C site can be considered 4+2 coordinated, as two O3 atoms are at 2.564 Å from the centre of the tetrahedron (Table 5). The effect of these two additional oxygens on the cation at the C site is relatively insignificant in the case of Si and Al, but in the case of Fe³⁺ and Ti⁴⁺ the effect is noticeable (Table 6). A unique aspect of the qeltite structure (close to the composition of isostructural minerals) is the wide isomorphism of cations at the C tetrahedral site, including Ti⁴⁺, for which this coordination is atypical. We consider that the entry of Ti⁴⁺ (and other large cations) at the C site is simplified by a change of its coordination to 4+2 (octahedral). It is likely that qeltite has a domain structure due to the significant differences among the cation sizes of the C site.

The Ti⁴⁺ site coordinated by six oxygens is usually called the octahedral site. However, a more accurate description would be a trigonal trapezohedron truncated by a pinacoid, which has left and right forms. Comparison of the structures of paqueite and qeltite shows that they are right and left forms of the archetypal langasite structure (Galuskina et al., 2023). It is interesting that the qeltite space group Р321 does not have screw axes, which are a necessary condition for the appearance of enantiomorphic forms (Fecher et al., 2022). Chirality in phases with the langasite-type structure is related to the effects of specific structural disordering in the distribution of electron density, which leads to the formation of a pseudoscrew axis with the period 3c (Dudka and Mill, 2014).

Qeltite in paralava is usually confined to small oval aggregates of fine-grained minerals against a coarse-grained background of rock-forming minerals (Fig. 2b,c). As a rule, these aggregates are enriched in elements that are incompatible with rock-forming minerals, such as Ti, Fe, Ba, U, V and Nb (Galuskina et al., 2017a). In similar aggregates, a series of new minerals and varieties characterised by unusual composition and structure, such as hexacelsian, BaAl₂Si₂O₈, zadovite, BaCa₆[(SiO₄)(PO₄)](PO₄)₂F, aradite, BaCa₆[(SiO₄)(VO₄)](VO₄)₂F, gurimite, Ba₃(VO₄)₂, mazorite, Ba₃(PO₄)₂, bennesherite, Ba₂Fe²⁺[Si₂O₇], uranium-bearing cuspidine, Ca₈(Si₂O₇)₂F₄, vorlanite, Ca(U⁶⁺)O₄, and khesinite, Ca₄Mg₂Fe³⁺₁₀O₄[(Fe³⁺₁₀Si₂)O₃₆], has been described. Previously, we interpreted similar aggregates as a crystallisation of minerals from residual melt (liquid) enriched with incompatible elements, which remained between crystals of pre-existing rock-forming minerals. The paralavas have no flow structures and are completely crystallised, and the size of rock-forming minerals reaches more than 1 cm. These aggregates rather resemble pegmatites and veins, in which minerals crystallise in a particular direction from the walls of a cavity. We also cannot exclude the possibility that the enrichment of small fragments of paralava is conditional on the inhomogeneity of protolith and weakly homogenised melt. If such an assumption is valid, the aggregates with Ba, U, Fe–Ti and V mineralisation should be interpreted as refractory inclusions, like those which occur in meteorites. The Ti–Al-analogue of qeltite – paqueite, Ca₃Ti(Al₂Ti)Si₂O₁₄ – has been described in such inclusions (Ma et al., 2022). Khesinite, a mineral analogue of the SFCA phase (silico-ferrite of calcium and aluminium), which appears in products of the calcination of iron ore at temperatures above 1200°C (Galuskina et al., 2017b), occurs in association with qeltite. The temperature of crystallisation of paqueite in association with iron phosphides, osbornite and pseudowollastonite from explosive breccia of the Hatrurim Complex in Israel was higher than 1250°C (Galuskin et al., 2022). Similarly, the temperature of qeltite crystallisation in paralava was ~1200°C. High-temperature and near-surface conditions of qeltite genesis probably defines its absence in terrestrial magmatic and metamorphic rocks, which contain widely distributed Ti-rich garnets close to it in composition. Nevertheless, intimate intergrowths of qeltite with Ti-garnets in paralavas of the Hatrurim Complex indicate that the conditions for its crystallisation can be realised in high-temperature magmatic systems of defined composition.