Occurrence and description of fluoralforsite

Fluoralforsite Ba₅(PO₄)₃F was found in coarse-grained rankinite paralava in the Hatrurim Basin (Negev Desert), Hatrurim Complex in Israel (Fig. 1). The unique Hatrurim Complex is composed of pyrometamorphic rocks and their alteration products, which are distributed on both sides of the Dead Sea Transform Fault in Israel, the West Bank, and Jordan (Fig. 2; Gross, Reference Gross1977; Burg et al., Reference Burg, Starinsky, Bartov and Kolodny1991; Geller et al., Reference Geller, Burg, Halicz and Kolodny2012). The pyrometamorphic rocks were formed as a result of the high-temperature metamorphism of a sedimentary protolith at sanidinite facies (>600°C and low pressure). They are represented mainly by spurrite marble, larnite pseudoconglomerates, gehlenite hornfelses and different types of paralavas (Vapnik et al., Reference Vapnik, Sharygin, Sokol and Shagam2007; Novikov et al., Reference Novikov, Vapnik and Safonova2013; Galuskina et al., Reference Galuskina, Vapnik, Lazic, Armbruster, Murashko and Galuskin2014). In the Hatrurim Basin, the common type of paralava occurs as lenses within the hornfelses mainly composed of gehlenite, andradite and larnite. The lenses range in size from about ten centimetres to a few millimetres. The occurrence of the paralava in hornfels suggests that the high temperature of the pyrometamorphic processes caused partial protolith melting on a local scale. Moreover, rock-forming minerals of coarse-grained paralava contain melt inclusions with 1200–1250°C homogenisation temperature (Sharygin et al., Reference Sharygin, Vapnik, Sokol, Kamenetsky and Shagam2006). Andradite–flamite, gehlenite–flamite and walstromite–kalsilite intergrowths are present in the paralava, which also indicate crystallisation of rocks from melt (Gfeller et al., Reference Gfeller, Widmer, Krüger, Galuskin, Galuskina and Armbruster2015; Krzątała et al., Reference Krzątała, Krüger, Galuskina, Vapnik and Galuskin2020). The characteristic feature of rankinite paralavas is the occurrence of the Ba, V-bearing enclaves, which are a source of new minerals species such as zadovite, BaCa₆[(SiO₄)(PO₄)](PO₄)₂F, aradite, BaCa₆[(SiO₄)(VO₄)](VO₄)₂F, hexacelsian, BaAl₂Si₂O₈, gurimite Ba₃(VO₄)₂, bennesherite Ba₂FeSi₂O₇, pliniusite Ca₅(VO₄)₃F and mazorite Ba₃(PO₄)₂.

Figure 2. (a) Schematic map of the Middle East with outcrops of pyrometamorphic rocks of the Hatrurim Complex (dark yellow). The framed part is magnified in b. (b) Schematic map of the Hatrurim Basin. The sampling areas of rankinite paralava are marked by a red pin (an unnamed tributary of wadi Zohar) and a blue pin (Gurim Anticline) (modified after Britvin et al., Reference Britvin, Murashko, Vapnik, Polekhovsky and Krivovichev2015).

Fluoralforsite has been found in the rankinite-rich paralava (the holotype locality 31°11.3'N, 35°16.6'E) from the upper reaches of the tributary of the wadi Zohar. Additionally, it was found in a paralava sample from Gurim Anticline (Fig. 2). It occurs in small enclaves in association with the following Ba-minerals: walstromite, zadovite–aradite series, gurimite, bennesherite, barioferrite and baryte (Fig. 3). The rock-forming minerals of those paralava are mainly represented by rankinite, Ti-rich andradite, members of the gehlenite–alumoåkermanite series, members of the fluorapatite–fluorellestadite series, kalsilite and wollastonite (Fig. 3). Additionally, the following minerals occur in the association: pliniusite, fresnoite, celsian, members of the magnetite–magnesioferrite–trevorite series, U–Nb-bearing cuspidine, nepheline, native copper and hashemite. The paralava voids are filled by low-temperature mineralisation represented by zeolites, afwillite and tobermorite-like Ca-hydrosilicates.

Figure 3. Back-scattered electron (BSE) images of fluoralforsite in paralava near the wadi Zohar tributary (a–d) and Gurim Anticline (e,f). Crystals of fluoralforsite occur between rock-forming minerals in small enclaves enriched in barium. The white frames correspond to the magnified fragments shown on the right side. Mineral abbreviations after Warr (Reference Warr2021): Aåk – alumoåkermanite, Adr – andradite, Baf – barioferrite, Bnh – bennesherite, Brt – baryte, Csp – cuspidine, Fafr – fluoralforsite, Fap – fluorapatite, Fmt – flamite, Gh – gehlenite, Gur – gurimite, HSi-Ca – hydrosilicate of calcium, Kls – kalsilite, Mag – magnetite, Mzo – mazorite, Rnk – rankinite, Wo – wollastonite, Ws – walstromite, Zad – zadovite.

The crystals of fluoralforsite are colourless and transparent up to 50 μm in length and characterised by a white streak and a vitreous lustre. The cleavage is not observed. The tenacity is brittle and the fracture is uneven. Fluoralforsite is uniaxial (–) with refractive indices (589 nm) n _ω = 1.689(3) and n _ɛ = 1.687(3). Based on the average chemical composition and unit cell parameters, the calculated density is 4.57 g/cm³. Micro-hardness measurements gave a mean VHN₂₅ value of 320.4 kg/mm² (ranging from 301 to 340 kg/mm²) based on 12 measurements. This value corresponds to a Mohs hardness of ca. 4–4½.

Experimental methods

The morphology and composition of the minerals were studied using optical microscopy, a scanning electron microscope (Phenom XL and FEI Quanta 250, Institute of Earth Science, Faculty of Natural Sciences, University of Silesia in Katowice, Poland) and an electron microprobe analyser (EMPA, Cameca SX100, Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw). Chemical analyses were carried out (wavelength dispersive spectroscopy mode, 15 kV, 10 nA and ~1 μm beam diameter) using the following lines and standard materials: FKα – synthetic fluorophlogopite; ClKα – sodalite; NaKα – albite; SiKα and CaKα – diopside; KKα – orthoclase; FeKα – Fe₂O₃; BaLα and SKα– baryte; SrLα – celestine; VKα – V₂O₅; and PKα – YPO₄.

The Raman spectra of fluoralforsite were recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Earth Science, Faculty of Natural Sciences, University of Silesia in Katowice, Poland) equipped with an air-cooled solid-state laser (488 nm) and a CCD camera operating at –61°C. The laser radiation was coupled with 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 broadband single-mode fibre with an effective pinhole size of ~30 μm and a monochromator with a 600 mm^–1 grating. The power of the laser at the sample position was ~40 mW. An integration time of 5 s, an accumulation of 20 scans and a resolution of 3 cm^–1 were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm^–1). Spectra processing, such as baseline correction and smoothing, was performed using the SpectraCalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band fitting was performed using a Gauss–Lorentz cross-product function, with a minimum number of component bands used for the fitting process.

Single-crystal X-ray diffraction data were collected with ω scans at different phi-kappa-theta settings using a four-circle Rigaku Synergy R diffractometer (University of Bern, Switzerland), equipped with a PhotonJet-R source, featuring a Mo-rotating anode (MoKα = 0.71073 Å) and a Hypix Arc 100° detector. The measurement was carried out at room temperature. The investigated sample consisted of a grain (~0.05 × 0.04 × 0.03 mm) from the wadi Zohar tributary. The analysis of the collected reflections showed that the selected grain consisted of several phases. Approximately 40% of the total reflections were indexed with a hexagonal unit cell: a = 10.00 Å, c =7.54 Å; V = 654 ų, corresponding to fluoralforsite. The second set of reflections (~30%) was indexed by the rhombohedral cell a = 7.083 Å, c = 25.498 Å; and V =1107.5 ų, ascribed to the mineral zadovite (BaCa₆[(SiO₄)(PO₄)](PO₄)₂F; R $\bar{3}$m; a = 7.097 Å and c = 25.73 Å) (Galuskin et al. Reference Galuskin, Gfeller, Galuskina, Pakhomova, Armbruster, Vapnik, Włodyka, Dzierżanowski and Murashko2015b). No specific phase could be identified in the remaining diffraction spots. Experimental details are summarised in Table 2. The data processing was performed using the software package CrysalisPro 42.89a. The data reduction was carried out by using an empirical absorption correction. The fluoralforsite structure was solved in the P6₃/m space group by direct methods using the software SHELXS (Sheldrick, Reference Sheldrick2008) and then refined using SHELXL (Sheldrick, Reference Sheldrick2015) in the WinGX package (Farrugia, Reference Farrugia2012). The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below). Nevertheless, the integration, solution, and refinement attempts were carried out for test purposes in the following space groups P6₃, P $\bar{3}$, and P2₁/m (Supplementary Tables S1–S6). All of them converged to inferior to P6₃/m refinement. For better comparison with literature data, the final structural model was refined by using the atomic coordinates reported by Hata et al. (Reference Hata, Marumo, Iwai and Aoki1979) for alforsite (P6₃/m).

Table 2. X-ray data collection and crystal structure refinement parameters of fluoralforsite.

Chemical composition

The empirical formulas of fluoralforsite were calculated on the basis of 8 cations (Table 3). The following crystal chemical formulas have been obtained on the crystals from the wadi Zohar tributary and Gurim Anticline locality, respectively: $ ( {{\rm B}{\rm a}_{3.81}{\rm C}{\rm a}_{0.97}}$ $ {{\rm N}{\rm a}_{0.07} {\rm K}_{0.05}{\rm S}{\rm r}_{0.05}{\rm F}{\rm e}_{0.05}} ) _{\Sigma 5} ( {{\rm P}^{5 + }_{2.32} {\rm V}^{5 + }_{0.29} {\rm S}^{6 + }_{0.22} {\rm S}{\rm i}^{4 + }_{0.17} } ) _{\Sigma 3}{\rm O}_{12}( {{\rm F}_{0.85}{\rm C}{\rm l}_{0.13}} ) _{\Sigma 0.98} $  $ ( {{\rm B}{\rm a}_{3.92}{\rm C}{\rm a}_{0.88}{\rm N}{\rm a}_{0.08}{\rm K}_{0.02}{\rm S}{\rm r}_{0.11} ) _{\Sigma 5.01}} ( {{\rm P}^{5 + }_{2.17} {\rm V}^{5 + }_{0.41} {\rm S}^{6 + }_{0.23} {\rm S}{\rm i}^{4 + }_{0.11}{\rm A}{\rm l}_{0.07}} ) _{\Sigma 2.99} $ $ {{\rm O}_{12.02}( {{\rm F}_{0.92}{\rm C}{\rm l}_{0.02}} ) _{\Sigma 0.94.}}$ The analyses of fluoralforsite crystals reveal a significant substitution of Ca²⁺, nearly equal to one apfu. Thus, the simplified and ideal formula can be written as (Ba,Ca)₂Ba₃(PO₄)₃F and Ba₅(PO₄)₃F, respectively.

Table 3. Chemical data (in wt. %) of fluoralforsite from wadi Zohar tributary and Gurim Anticline*.

* Note: S.D. – standard deviation; n.d. – not detected.

Raman investigation

The Raman spectra of phosphate minerals of the apatite group are characterised by strong ν₁ symmetric stretching vibrations of (PO₄)³⁻ tetrahedra at 950–990 cm^–1. The weak bands of ν₂ symmetric bending vibrations, ν₃ antisymmetric stretching vibrations and ν₄ antisymmetric bending vibration are observed in 400–500 cm^–1, 1000–1150 cm^–1 and 550 and 660 cm^–1 regions, respectively. Bands below 300 cm^–1 are related to the lattice vibrations (Litasov and Podgornykh, Reference Litasov and Podgornykh2017). In the Raman spectra of fluoralforsite (Fig. 4), the main band of (PO₄)³⁻ groups vibrations occur at ~942/941 cm^–1. The bands at ~410–430 cm^–1 are related to ν₂ symmetric bending vibrations. The weak bands at ~1008–1048 cm^–1 can be assigned to ν₃(PO₄)^3– antisymmetric stretching vibrations, whereby ν₄ antisymmetric bending vibration occurs at ~550–590 cm^–1 (Levitt and Condrate, Reference Levitt and Condrate1970; Meegoda et al., Reference Meegoda, Bonner, Loutts, Stefanos and Miller1999). The bands at 840/830 cm⁻¹ can be attributed to asymmetric stretching vibrations (ν₃) of VO₄^3– or symmetric stretching vibration (ν₁) of SiO₄^4– (Schulte et al., Reference Schulte, Buchter, Chai, Scheps and Kokta1995; Comodi et al., Reference Comodi, Liu, Stoppa and Woolley1999; Frost et al., Reference Frost, Palmer, Čejka, Sejkora, Plášil, Bahfenne and Keeffe2011; Galuskin et al., Reference Galuskin, Gfeller, Galuskina, Pakhomova, Armbruster, Vapnik, Włodyka, Dzierżanowski and Murashko2015b). Low-intensity bands at ~995/991 cm^–1 can be assigned to the vibrations of (SO₄)²⁻ groups. The bands from moderate to weak intensity in the 80–250 cm^–1 region can be connected with lattice or Ba(Ca)–O modes. The remaining bands in the Raman spectrum of fluoralforsite from both localities and their assignments are [cm⁻¹]: 349, 365/350 (ν₂VO₄^3–), 379/385 (ν₄VO₄^3–), 625/626 (ν₄SO₄^2–), 840/830 (ν₃VO₄^3– or ν₁SiO₄^4–), 860/857 (ν₁VO₄^3–), 995/991 (ν₁SO₄^2–) (Fig. 4). Accurate band assignment in the Raman spectra of fluoralforsite is hampered by overlapping bands corresponding to vanadate, sulfate and silicate anionic-group vibrations. The spectra analysis from 3600 cm^–1 to 3700 cm^–1 indicates the absence or very low content of hydroxyl groups in the fluoralforsite channel.

Figure 4. The Raman spectrum of fluoralforsite from (a) the type locality and (b) Gurim Anticline Hatrurim Basin, Hatrurim Complex, Israel.

X-ray crystallography and structure refinement

The structure of fluoralforsite was solved and refined in space group P6₃/m. During the first refinement cycles, M1, M2 and tetrahedral (P1) sites were located (Table 4). Initially, M1 and M2 sites and tetrahedral position (P1) were refined using barium and phosphorus scattering curves, respectively. The next refinement cycle allowed for the location of the anion position (F site, Wyckoff position 4e), which was refined with fluorine scattering factors. However, a strong peak in the difference-Fourier maps was observed along the c-axis at 1.0 Å from the F site. This residual electron density was modelled as a low-occupied site (2b position), corresponding to the Cl position in alforsite (Table 4; Hata et al. Reference Hata, Marumo, Iwai and Aoki1979; Newberry et al., Reference Newberry, Essene and Peacor1981). In order to model the residual electron density close to the F site, a new low-occupied site F1 (at ~0.6 Å from F) was inserted in the model. Similarly, the residual peak close to M2, was modelled by an additional site, M2A, refined with barium scattering factors (Tables 4, 5). A calculation of the mean electron number of the M1 site gave values of 38.56 e ^–, indicating the significant substitution of a lighter cation. Owing to the complex chemistry of the crystal investigated, M1 and P1 sites were described with mixed scattering curves according to the chemical microanalyses. M1 occupancy was refined with Ba vs. Ca curves and finally converged to 0.501(3) Ba and 0.498(3) Ca. The tetrahedral position was refined using a constrained mixed-scattering curve (2.8 P + 0.2 V). The refined site-scattering values are compatible with the chemical formula calculated from the EMPA data (Tables 2, 3). The location of Ca at the M1 position is in agreement with the suggestion of Pasero and co-authors guidelines for the apatite supergroup, according to which smaller cations fill the M1 site and larger cations the M2 site (Pasero et al., Reference Pasero, Kampf, Ferraris, Pekov, Rakovan and White2010). All sites, except low-occupied Cl, F, F1 and M2A, were refined using anisotropic displacement parameters (Table 5). The pronounced displacement of O3 was modelled by splitting the site over O3 and O3A (12i positions) positions, 0.5 Å apart (Table 4, Table 5). The O2 site was located at the 12i position, whereas O1 was at the 6h special position. The final R-value converged to 1.92% for 1009 unique reflections and 64 refined parameters.

Table 4. Atom coordinates, equivalent isotropic or isotropic (*) displacement parameters and site occupancies of fluoralforsite.

Table 5. Anisotropic displacement parameters (Ų).

Structure description

Fluoralforsite has an apatite structure type and crystallises in the space group P6₃/m with the following unit cell parameters: a =10.0031(2), c = 7.5382(2) Å and V = 653.23(3) ų (Table 2). The M1 site, building columns along the c-axis, is occupied equally by Ba and Ca (Figs 5, 6). The M1–O bond distances vary from 2.60Å (M1–O1) to 2.90Å (M1–O3). It is worth noting that two different M1–O2 bond distances (2.40Å and 2.79 Å) result as a consequence of the O2 position, which is shifted with respect to the mirror plane. However, the two equivalent O2 positions (at z and – z + ½) cannot be simultaneously present in the coordination sphere of M1 (Table 6, Fig. 6). Hence, the nine-fold coordination is maintained, similar to the archetype structure (Table 6). As mentioned above, the O2 site lies slightly above the mirror plane (z = 0.21, Table 4), resulting in two possibilities of tetrahedra tilting (Fig. 5b). The tetrahedron connecting M1 columns, is occupied mainly by phosphorus with slight impurities of vanadium. The average <T–O> bond distance is 1.55Å, as for the pure phosphorus tetrahedron (Mathew et al., Reference Mathew, Mayer, Dickens and Schroeder1979; Newberry et al., Reference Newberry, Essene and Peacor1981). This value also agrees with the estimated bond distance (1.56 Å) considering the V, S and Si substitutions at the P1 site, as shown by chemical analyses (Nyfeler and Armbruster, Reference Nyfeler and Armbruster1998; Hawthorne et al., Reference Hawthorne, Krivovichev, Burns, Alpers, Jambor and Nordstrom2000; Baikie et al., Reference Baikie, Schreyer, Wei, Herrin, Ferraris, Brink, Topolska, Piltz, Price and White2014). The cation position in the inter-column space is partially disordered along the c-axis and split over two barium sites, M2 (Occupancy 0.93) and M2A (Occ. 0.037) (Table 4; Fig. 7). The channel contains three partially occupied anionic sites (Fig. 5c). Fluorine occupies two 4e positions, F and F1, respectively. In turn, the 2b position is a low-occupied (Occ. = 0.112(10)) Cl site corresponding to the Cl position in the alforsite (Fig. 7; Hata et al., Reference Hata, Marumo, Iwai and Aoki1979; Newberry et al., Reference Newberry, Essene and Peacor1981). The anionic arrangement is described in detail in the discussion session.

Figure 5. (a) Arrangement of M1 and P polyhedra in the crystal structure of fluoralforsite projected along [110]. M2 and anionic sites are not shown for the sake of clarity. The circle marks the part magnified in b; (b) phosphorus tetrahedron with split O2 and O3 sites; (c) crystal structure of fluoralforsite projected along [001]. The triangle highlights the part of the structure magnified in Fig. 7. Turquoise and light violet polyhedra represent the M1 prism and P tetrahedra, respectively. M2 sites are shown as a turquoise sphere, red spheres correspond to oxygens and dark blue and green spheres represent fluorine and chlorine, respectively. The dotted line indicates the unit cell. Drawn using CrystalMaker® software.

Figure 6. (a) Two variants of an M1 metaprism coordinated by O1 and O2, displaced off the mirror plane; (b) ordered M1 column arrangement along [001]. The radii proportions of Ca/Ba have been preserved. Drawn using CrystalMaker® software.

Table 6. Selected interatomic distances (Å).

Figure 7. Anion positions (c-axis vertical) with respect to the mirror plane in fluoralforsite. Atoms are represented as thermal ellipsoids. Colour code as in Fig. 5. The dark grey triangle represents the mirror plane. The dotted line indicates c-axis.

Genesis of fluoralforsite

Fluoralforsite is a novel barium mineral from rankinite-rich paralavas that occurs in intergranular space and enclaves. Studies on the pyrometamorphic rocks of the Hatrurim Basin have distinguished two main types of enclaves, ferrite and silicate–phosphate. The origin of them is related to the crystallisation of residual melt between large crystals of paralava rock-forming minerals. The first type is dominated by minerals such as khesinite, Fe-spinels (magnetite, magnesioferrite, trevorite and cuprospinel), harmunite, barioferrite, perovskite and a nataliakulikite-like phase (Galuskina et al., Reference Galuskina, Galuskin, Pakhomova, Widmer, Armbruster, Krüger, Grew, Vapnik, Dzierażanowski and Murashko2017b; Krzątała et al., Reference Krzątała, Panikorovskii, Galuskina and Galuskin2018). Walstromite or zadovite minerals can be dominant in the second type – silicate–phosphate enclaves enriched in elements such as Ba, K, V, Nb, U and Sr. The walstromite-type enclaves, which co-occur with wollastonite and cuspidine have been described by Krzątała et al. (Reference Krzątała, Krüger, Galuskina, Vapnik and Galuskin2020). Fluoralforsite was found in zadovite enclaves, which are usually associated with fluorapatite (Fig. 3). The formation of a Ba-bearing enclave is a result of non-equilibrium between the primary minerals and residual melt, indicated by the melting of fluorapatite rims (Fig. 3d). It led to the enrichment of the silica-rich residual melt in phosphorus. Consequently, zadovite and fluoralforsite were formed at the final crystallisation stage of the paralava.

Fluoralforsite is a novel barium mineral from rankinite-rich paralavas that occurs in intergranular space and enclaves. Studies on the pyrometamorphic rocks of the Hatrurim Basin have distinguished two main types of enclaves, ferrite and silicate–phosphate. The origin of them is related to the crystallisation of residual melt between large crystals of paralava rock-forming minerals. The first type is dominated by minerals such as khesinite, Fe-spinels (magnetite, magnesioferrite, trevorite and cuprospinel), harmunite, bario ferrite, perovskite and a nataliakulikite-like phase (Galuskina et al., Reference Galuskina, Galuskin, Pakhomova, Widmer, Armbruster, Krüger, Grew, Vapnik, Dzierażanowski and Murashko2017b; Krzątała et al., Reference Krzątała, Panikorovskii, Galuskina and Galuskin2018). Walstromite or zadovite minerals can be dominant in the second type – silicate–phosphate enclaves enriched in elements such as Ba, K, V, Nb, U and Sr. The walstromite-type enclaves, which co-occur with wollastonite and cuspidine have been described by Krzątała et al. (Reference Krzątała, Krüger, Galuskina, Vapnik and Galuskin2020). Fluoralforsite was found in zadovite enclaves, which are usually associated with fluorapatite (Fig. 3). The formation of a Ba-bearing enclave is a result of non-equilibrium between the primary minerals and residual melt, indicated by the melting of fluorapatite rims (Fig. 3d). It led to the enrichment of the silica-rich residual melt in phosphorus. Consequently, zadovite and fluoralforsite were formed at the final crystallisation stage of the paralava.

In summary, rock-forming minerals such as melilites of the alumoåkermanite–gehlenite series, Ti-rich andradite, fluorapatite and calcium silicates such as rankinite and/or wollastonite crystallised from the primary melt. Subsequently, residual fluid/melt rich in halides, mainly fluorine and chlorine and incompatible elements reacted with early-formed minerals. These changes enhanced the mineral variety and contributed to the formation of new minerals such as fluoralforsite. It is noteworthy that the mineralisation of both ferrite and silicate–phosphate enclave types originate from similar processes – high-temperature alteration between pre-existing pyrometamorphic minerals and residual melt (Galuskina et al., Reference Galuskina, Galuskin, Pakhomova, Widmer, Armbruster, Krüger, Grew, Vapnik, Dzierażanowski and Murashko2017b; Galuskin et al., Reference Galuskin, Galuskina, Gfeller, Krüger, Kusz, Vapnik, Dulski and Dzierżanowski2016, Reference Galuskin, Gfeller, Galuskina, Armbruster, Krzątała, Vapnik, Kusz, Dulski, Gardocki, Gurbanov and Dzierżanowski2017).