Methods of investigation

The morphology and composition of ferrodimolybdenite and associated minerals were studied using optical microscopy, scanning electron microscopes (Phenom XL and Quanta 250, 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). Chemical analyses of sulfide nodule minerals were conducted in WDS mode (wavelength-dispersive spectroscopy, settings: 15 kV, 20 nA and ∼1 μm beam diameter) using the following lines and standards: MoLβ – Mo; CuKα – Cu; NiKα – pentlandite; FeKα – FeS₂; SKα – FeS₂, ZnS; PKα – InP; SiKα, MgKα – diopside; AlKα – orthoclase; SeLβ – In₂Se₃; AsLβ, CoKα – CoNiAs₃; CaKα – apatite; SbLα – Sb₂Te₃; PbMβ – pyromorphite; ZnKα – ZnS; MnKα – rhodonite; CrKα – Cr₂O₃; VKα – V; and TiKα – rutile. Other chemical elements were below the detection limit.

The Raman spectra of ferrodimolybdenite and molybdenite 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. The Raman scattered light was focused onto a multi-mode fibre and monochromator with a 1800 gr mm^–1 grating. The laser power at the sample position was 5–7 mW. Twenty scans with an integration time of 3 s and a resolution of 2 cm^–1 were collected and averaged. The spectrometer monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm^–1).

Single-crystal X-ray diffraction (XRD) studies of ferrodimolybdenite were carried out using a SuperNova diffractometer with a mirror monochromator (MoKα and λ = 0.71073 Å) and an Atlas CCD detector (formerly Agilent Technologies, currently Rigaku Oxford Diffraction) at the Institute of Physics, University of Silesia, Poland. Single-crystal XRD data were collected using a ferrodimolybdenite crystal fragment 40×30×10 μm in size. The ferrodimolybdenite structure was refined using the SHELX-2019/2 program (Sheldrick, Reference Sheldrick2015). The crystal structure was refined starting from the atomic coordinates of synthetic FeMo₂S₄ (Vaqueiro et al., Reference Vaqueiro, Kosidowski and Powell2002).

Background information

The pyrometamorphic rocks of the Hatrurim Complex, represented by spurrite, larnite and gehlenite rocks, form large fields along the Dead Sea Rift in Israel, Palestine and Jordan. Descriptions of the Hatrurim Complex can be found in numerous publications (Bentor, Reference Bentor1960; 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; Khoury et al., Reference Khoury, Salameh and Clark2014; Galuskina et al., Reference Galuskina, Vapnik, Lazic, Armbruster, Murashko and Galuskin2014). The genesis of the Complex remains an unsolved problem, but all researchers agree that the rocks of the Hatrurim Complex are products of combustion metamorphism. Two genetic hypotheses have been proposed, the first of which relates pyrometamorphic transformations of the sedimentary protolith to the combustion of dispersed organic fuels (Kolodny and Gross, Reference Kolodny and Gross1974; Matthews and Gross, Reference Matthews and Gross1980; Geller et al., Reference Geller, Burg, Halicz and Kolodny2012). The second hypothesis suggests that fires were activated by methane combustion originating from the tectonically active Dead Sea rift zone (Sokol et al., Reference Sokol, Novikov, Zateeva, Vapnik, Shagam and Kozmenko2010; Novikov et al., Reference Novikov, Vapnik and Safonova2013). Locally, melting of rocks of the protolith of the Hatrurim Complex can be observed, along with the formation of paralavas and slag-like rocks of different composition, forming bodies of different sizes from thin veins, ranging from gehlenite hornfels with a thickness of a few centimetres to large oval fields of diopside-bearing paralava with a diameter of tens of metres. The near-surface character of the paralava generation processes determines the predominantly oxidised mineral composition of the paralavas, with minerals containing only Fe³⁺ (Galuskina et al., Reference Galuskina, Galuskin, Pakhomova, Widmer, Armbruster, Krüger, Grew, Vapnik, Dzierażanowski and Murashko2017). In relatively rare cases, black reduced paralavas enriched with pyrrhotite are formed. The rarest are paralavas containing native iron and phosphides, as well as osbornite – an indicator of super-reduced conditions (Btitvin et al., Reference Britvin, Murashko, Vapnik, Polekhovsky and Krivovichev2015; Galuskin et al., Reference Galuskin, Galuskina, Kamenetsky, Vapnik, Kusz and Zieliński2022, Reference Galuskin, Kusz, Galuskina, Książek, Vapnik and Zieliński2023b, Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2025; Futrzyński et al., Reference Futrzyński, Juroszek, Skrzyńska, Vapnik and Galuskin2023). The hypothesis of phosphide and native iron formation as a result of reducing carbothermal reactions occurring at the boundary of hot paralava and thermally altered sedimentary rock enriched with graphitised and phosphoritised organic residues has been proposed elsewhere (Galuskin et al., Reference Galuskin, Galuskina, Kamenetsky, Vapnik, Kusz and Zieliński2022, Reference Galuskin, Kusz, Galuskina, Książek, Vapnik and Zieliński2023b, Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2025). One of the rare examples of a reduced paralava is from Daba-Siwaqa, Jordan. Along with numerous aggregates of phosphides in the contact facies, this paralava features a sulfide nodule with inclusions of tetrataenite and nickelphosphide, containing ferrodimolybdenite crystals. It was found in the central part of the body (Galuskin et al., Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2025).

Occurrence and composition

Ferrodimolybdenite was found in a small quarry (prospecting for phosphorite deposits) in the Daba-Siwaqa complex within the Transjordan Plateau, Jordan (31°22’01’’N, 36°11’10’’E), where a basalt-like paralava body ∼30 metres in diameter is exposed in pyrometamorphically altered carbonate rocks of the Muwaqqar Chalk-Marl Formation. The paralava consists of diopside, anorthite, wollastonite, tridymite and small amounts of glass. Cristobalite is observed growing on tridymite crystals (Trd), exhibiting specific cracking patterns reminiscent of fish scales, indicative of the transition Trd_cub → Trd_tetr and a paralava generation temperature exceeding 1400°C (Galuskin et al., Reference Galuskin, Stachowicz, Galuskina, Woźniak, Vapnik, Murashko and Zieliński2023a). Fluorapatite, titanite and spinel of the magnesioferrite–magnetite–chromite series are the accessory minerals of the paralava. Ferrodimolybdenite was identified in a sulfide nodule ∼7 mm in size (Fig. 1) found in the central part of the paralava body (Galuskin et al., Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2025). The sulfide nodule comprises troilite and pentlandite zones and gaseous bubbles (Figs 1b, 2a). Inclusions of tetrataenite, nickelphosphide, molybdenite, (rarely) galena and rudashevskyite are observed. The presence of crystals of the new mineral karwowskiite, with a composition of Ca₉Mg(Fe²⁺_0.5□_0.5)(PO₄)₇ (IMA2023–080), was detected on the boundary between the nodule and the paralava (Galuskin et al., Reference Galuskin, Galuskina, Kusz, Książek, Vapnik and Zieliński2024). These crystals were absent in the groundmass of the rock. An analysis of the nodule minerals is presented in Galuskin et al. (Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2025). The composition of tetrataenite varies in the range Fe_0.59Ni_0.39Cu_0.01 – Ni_0.53Fe_0.44Cu_0.02 with a tendency for the Ni content to increase towards the edge of the grain. The composition of nickelphosphide also varies significantly, so the smallest crystals are characterised by the highest Ni content (up to 2.57 Ni per formula unit), while in the relatively large crystals in the centre the Ni content decreases to 2.06 Ni pfu, with a tendency to increase towards the edge of the crystal.

Figure 1. (a) Sulfide nodules in diopside–anorthite–tridymite paralava. I - porous rock and II - massive rock. The fragment outlined (holotype specimen 6005/1) is enlarged in Fig. 1b. (b) Reflected light image of the differentiated sulfide nodule with a gas bubble (upper part) and composed of troilite and pentlandite zones. Relatively large inclusions of tetrataenite and nickelphosphide are present in the troilite zone. Tro = troilite; Ttae = tetrataenite; Mol = molybdenite; Pn = pentlandite and Nic = nickelphosphide [Mineral symbols here and below according to Warr (Reference Warr2021)].

Ferrodimolybdenite forms separate tabular crystals in a sulfide matrix, with dimensions ranging from 3 µm to 20 µm in the cross-section (Figs 2c, 3, 4). Ferrodimolybdenite crystals are typically found in the troilite zone, with a thin pentlandite film commonly observed between ferrodimolybdenite and troilite (Fig. 3b–d). Ferrodimolybdenite is grey in colour with a dark grey streak. The mineral is opaque with a metallic lustre. Its Mohs hardness is ∼3. Ferrodimolybdenite crystals show cleavage in three directions: perfect on {001}, good on {100} and poor on {010}. Its tenacity is sectile, and its fracture is smooth. Its density, 5.445 g⋅cm^–3, was calculated from the empirical formula and unit cell volume refined from the single-crystal XRD data. The mineral is slightly magnetic. In reflected light, it is grey to light grey with a bluish tinge. It is strongly anisotropic (Fig. 4). No internal reflections have been observed. The reflectivity varies between 34.2% and 40.0% (Table 1).

Figure 2. (a) Back-scattered electron (BSE) images of a sulfide nodule in which ferrodimolybdenite was discovered. The areas outlined are magnified in: Fig. 2b (‘B’) and Fig. 3a (‘A’). (b) BSE image of nickelphosphide and tetrataenite inclusions in troilite. The frame (‘C’) outlines the area magnified in Fig. 2c. (c) Reflected light image of the ferrodimolybdenite crystal has a inductive growth surface with troilite (arrow, simultaneous growth) and idiomorphism with respect to pentlandite and tetrataenite. Fdmol = ferrodimolybdenite; Krw = karwowskiite; Mol = molybdenite; Nic = nickelphosphide; Pn = pentlandite; Tro = troilite; and Ttae = tetrataenite.

Figure 3. BSE images. (a) Fragment of sulfide nodule (from Fig. 2a) with ferrodimolybdenite crystals. The frames outline the areas magnified in Fig. 3b–d, respectively. (b) Relatively large ferrodimolybdenite crystal used for the single-crystal XRD study. (c, d) Fine lamellar crystals of ferrodimolybdenite (cross-section is ∼ parallel to Z), usually with a thin film of pentlandite. Tro = troilite; Ttae = tetrataenite; Mol = molybdenite; Pn = pentlandite; Nic = nickelphosphide; Fdmol = ferrodimolybdenite; and Pwl = powellite.

Figure 4. Ferrodimolybdenite (centre right) in reflected light is (a) grey to light-grey with (b) a bluish tinge; the mineral exhibits strong bireflectance. This crystal is also shown, magnified, in Fig. 2c. Tro = troilite; Ttae = tetrataenite; Pn = pentlandite; and Fdmol = ferrodimolybdenite.

Table 1. Reflectivity of ferrodimolybdenite*

* The values required by the Commission on Ore Mineralogy (COM) are given in bold.

The chemical composition of ferrodimolybdenite is close to stoichiometric (Fe²⁺_0.99Cu²⁺_0.07Ni²⁺_0.04)_Σ1.10Mo³⁺_1.94(S^2–_3.98P^3–_0.02)_Σ4.00 (Table 2). The simplified formula of ferrodimolybdenite is (Fe²⁺,Cu²⁺,Ni²⁺)Mo₂S₄, leading to the ideal formula FeMo₂S₄. Interestingly, a 3 μm grain of the Cu-analogue of ferrodimolybdenite, with the composition (Cu_0.78Fe_0.22)Mo₂S_∼4 was found in the inclusion in the tetrataenite.

Table 2. Chemical composition (wt.%) of ferrodimolybdenite

n = number of spot analyses; S.D. = standard deviation.

Raman spectroscopy

In the Raman spectrum of ferrodimolybdenite at laser beam polarisation (∼ ⊥ Z, ∼ parallel to molybdenite layers), a single weak band at 243 cm^–1 related to the stretching Mo–S vibration is observed (Fig. 5a). When laser beam power is increased, a dark spot appears on the surface of ferrodimolybdenite in which the Raman spectrum corresponds to that of molybdenite (Fig. 5b,c).

Figure 5. (a) Raman spectra of ferrodimolybdenite; crystal orientation during the measurements is shown. (b) Raman spectrum of thermally altered ferrodimolybdenite. (c) Raman spectrum of molybdenite from the sulfide nodule.

Crystal structure

Single-crystal XRD and refinement showed the cation and anion sites are fully occupied. Details of data collection and structure refinement are given in Table 3, final atomic coordinates are summarized in Table 4, and anisotropic displacement parameters are given in Table 5. Selected bond lengths and bond valence sum (BVS) calculations are given in Table 6. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 3. Parameters for X-ray data collection and crystal-structure refinement for ferrodimolybdenite

Table 4. Atom coordinates (x,y,z), equivalent isotropic displacement parameters (U _eq, Ų) for ferrodimolybdenite

Table 5. Anisotropic displacement parameters (Ų) for ferrodimolybdenite

Table 6. Selected bond distances (Å) and BVS (bond-valence sum) for ferrodimolybdenite

BVS calculated using ECoN21 (Ilinca, Reference Ilinca2022) and the bond-valence parameters of Brown and Altermatt (Reference Brown and Altermatt1985).

Ferrodimolybdenite (С2/c; a = 11.8249(8) Å, b = 6.5534(3) Å, c = 13.0052(10) Å, β = 114.474(9)° and V = 917.27(12) ų), like its synthetic analogue (C1c1; a = 11.8148(2) Å, b = 6.5499(1) Å, c = 13.014(2) Å, β = 114.455(1)° and V = 916.75 ų) (Guillevic et al., Reference Guillevic, Le and Grandjean1974; Vaqueiro et al., Reference Vaqueiro, Kosidowski and Powell2002), crystallises with monoclinic symmetry and has a structure formed by layers of octahedra ∞(MoS₂)^– with columns of Fe²⁺-octahedra between them (Fig. 6a). Ferrodimolybdenite has a centrosymmetric structure, two types of Mo-octahedra and one type of Fe-octahedra (Fig. 6a,b). In the synthetic analogue there is no inversion centre and a reduction in symmetry is illustrated by the presence of four types of Mo-octahedra and two types of Fe-octahedra. The ferrodimolybdenite structure can be considered a molybdenite structure, where molybdenite layers are divided by parallel columns of Fe-octahedra. In natural molybdenite the ∞(MoS₂)⁰ layers consist of trigonal prisms (MoS₆)^8–, not octahedra. Distorted octahedra have only been found in synthetic molybdenite–1Т Mo (Samy et al., Reference Samy, Zeng, Birowosuto and El Moutaouakil2021). Mo-bearing sulfides with the general formula AMo₂S₄ (A = V, Cr, Fe and Co) are isostructural with Cr₃S₄ (space group C2/m). In this space group, the symmetry only allows distortions involving the formation of zigzag cation chains (Vaqueiro et al., Reference Vaqueiro, Kosidowski and Powell2002). A reduction in symmetry from С2/m (archetype of the Cr₃S₄ structure) to С1c1 for artificial FeMo₂S₄ can be related both to its synthesis conditions (Canadell et al., Reference Canadell, LeBeuze, El Khalifa, Chevrel and Myung-Hwan Whangbo1988) and to the deformation of its crystals during pulverisation for powder diffraction.

Figure 6. (a) The structure of ferrodimolybdenite (C2/c) is built up by octahedral layers (MoS₂)^1–, between which Fe²⁺ cations are located. Projection on (010). (b) Octahedral layers in ferrodimolybdenite are formed by two types of octahedra (MoS₆)^9–, between which there are columns of Fe-octahedra (FeO₆)^10– (only three columns are shown). Drawn using CrystalMaker 2.7® software.

Taking into account the systematic absences, it was found that two space groups are possible for ferrodimolybdenite, namely C1c1 (no. 9) and C2/c (no.15). Statistical analysis of the intensities of the reflections using the GRAL programme implemented in the CrysAlis^Pro package (Rigaku Oxford Diffraction, 2015) suggests that the space group should be centrosymmetric, i.e. the mean | E² – 1| value is equal to 1.202 (expected 0.968 centrosymmetric and 0.736 non-centrosymmetric), suggesting that the space group should be centrosymmetric (Howells et al., Reference Howells, Phillips and Rogers1950; Dolomanov et al., Reference Dolomanov, Bourhis, Gildea, Howard and Puschmann2009). The refinement of the crystal structures showed that in the space group C1c1, there were 127 necessary parameters and R ₁ = 0.0332, whereas in the space group C2/c, there were 64 necessary parameters and R ₁ = 0.0297. We believe that the studied crystal of ferrodimolybdenite, which formed at high temperature from sulfide melt at low pressure, has С2/c symmetry.