Methods of investigation

The morphology and chemical composition of yamhamelachite, phosphides and associated minerals were studied using Phenom XL and Quanta 250 EDS-equipped scanning electron microscopes (SEM) (Institute of Earth Sciences, University of Silesia, Poland). The mineral chemical composition was measured with a Cameca SX100 electron microprobe analyser (EMPA, Micro-Area Analysis Laboratory, Polish Geological Institute–National Research Institute, Warsaw, Poland): WDS mode, acceleration voltage = 15 kV, beam current = 20 nA (phosphates, sulfides) or 40 nA (phosphides), and beam diameter ∼1 μm. The following standards and lines were used: albite = NaKα; apatite = CaKα; PKα; celestine = SrLα; chalcopyrite = CuKα; diopside = MgKα; orthoclase = KKα and AlKα; pentlandite = FeKα, NiKα and SKα; rutile = TiKα; V metal = VKα; Cr₂O₃ = CrKα; and wollastonite = SiKα.

Raman spectra of yamhamelachite were recorded on a WITec alpha 300R Confocal Raman Microscope (Department of Earth Science, University of Silesia, Poland) equipped with an air-cooled solid state laser (532 nm) and a CCD camera. 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 a monochromator with an 1800 mm^–1 grating. The laser power at the sample position was ∼20 mW. Fifteen scans with an integration time of 3 s and a resolution of 1.5 cm^–1 were collected and averaged.

Single-crystal X-ray diffraction (SCXRD) studies of yamhamelachite and barringerite were performed using a SuperNova diffractometer with a mirror monochromator [CuKα, λ = 1.54184 Å (yamhamelachite); MoKα, λ = 0.71073 Å (barringerite)] and an Atlas CCD detector (formerly Agilent Technologies, currently Rigaku Oxford Diffraction) at the Institute of Physics, University of Silesia, Poland.

Occurrence and general appearance

High-temperature pyrometamorphic rocks of the Hatrurim Complex and their alteration products are distributed widely along the Dead Sea rift in the territories of Israel, Palestine and Jordan. The most typical rocks are spurrite marble, larnite pseudoconglomerate and gehlenite hornfels (Bentor, Reference Bentor1960; Gross, Reference Gross1977; Vapnik et al., Reference Vapnik, Sharygin, Sokol and Shagam2007; Novikov et al., Reference Novikov, Vapnik and Safonova2013). The highest temperature rocks of the Complex are paralavas of various types, most of which comprise oxidised mineral associations (Galuskina et al., Reference Galuskina, Galuskin, Pakhomova, Widmer, Armbruster, Krüger, Grew, Vapnik, Dzierażanowski and Murashko2017). The rarest type encompasses diopside-bearing and gehlenite-bearing reduced paralavas, which are associated with the presence of phosphides (Britvin et al., Reference Britvin, Murashko, Vapnik, Polekhovsky and Krivovichev2015; Galuskin et al., Reference Galuskin, Galuskina, Vapnik and Zieliński2023a, Reference Galuskin, Kusz, Galuskina, Książek, Vapnik and Zieliński2023b, Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2024a).

Yamhamelachite was discovered in phosphide-bearing breccia found in 2019 in the Hatrurim Basin on the artificial outcrop formed as a result of the construction of the Arad-Dead Sea road. The geological description of this unique and highly inhomogeneous breccia with cement composed of gehlenite–flamite (±rankinite, pseudowollastonite) paralava can be found in a number of our papers alongside descriptions of minerals previously known only from meteorites, such as osbornite, allabogdanite, dmitryivanovite, grokhovskyite, caswellsilverite and rubinite (Galuskin et al., Reference Galuskin, Galuskina, Kamenetsky, Vapnik, Kusz and Zieliński2022, Reference Galuskin, Galuskina, Vapnik and Zieliński2023a, Reference Galuskin, Kusz, Galuskina, Książek, Vapnik and Zieliński2023b, Reference Galuskin, Galuskina, Vapnik, Kusz, Marciniak-Maliszewska and Zieliński2024a).

Yamhamelachite is a rare mineral in the breccia and forms thin zones of 2–3 µm on zonal aggregates of phosphides with the following zonation: barringerite → schreibersite → eutectic: schreibersite + native iron (Fig. 1). Phosphides are concentrated at the boundary of the paralava and thermally altered sedimentary xenoliths (Fig. 2a). Inclusions of Cr–V-bearing pyrrhotite are often noted at the rim of these aggregates (Galuskin et al., Reference Galuskin, Galuskina, Kamenetsky, Vapnik, Kusz and Zieliński2022). The occurrence of late-generation barringerite, replacing schreibersite, and its association with minerals of the merrillite subgroup and fluorapatite is a characteristic feature of phosphide aggregates with yamhamelachite (Fig. 1b).

Figure 1. (a) Zonal aggregate of phosphides showing a barringerite–schreibersite-eutectic (schreibersite–native iron) sequence. The fragment magnified in part (b) is shown in the frame. (b) Thin zones of yamhamelachite up to 5 μm thick on zonal aggregates of phosphides containing two generations of barringerite. Bgr – barringerite, Bgr-I – barringerite, generation I; Bgr-II – barringerite, generation II; Gh – gehlenite; Fap – fluorapatite; Fe – native iron; Hem – hematite; Mrl – merrillite; Pwo – pseudowollastonite; Scb – schreibersite; and Ymm – yamhamelachite.

Figure 2. (a) Polished slab of phosphide-bearing breccia. I – paralava, II – altered fragment of sedimentary rock. The frames labelled b and a’ indicate the fragments enlarged in part (b) and Fig. 3a, respectively. (b) Hematite aggregate formed after pyrrhotite, in which the largest yamhamelachite deposit was found. The fragment magnified in (c) is shown in the frame. (c, d) Optical image of yamhamelachite, reflected light, c – PP (polarised light), d – XP (cross polarised light). Key: Bgr – barringerite; Fe – native iron; Hem – hematite; Hgr – hydrogrossular; Prv – perovskite; Pwo – pseudowollastonite; Tch – tacharanite; and Ymm – yamhamelachite.

In one case, a zone of yamhamelachite up to 30 µm thick was found in the hematite aggregate formed after pyrrhotite (Fig. 2), which was the source of grains for single-crystal structural and optical studies. Next to this yamhamelachite is a zoned aggregate: barringerite I → eutectic: schreibersite + barringerite → barringerite II → yamhamelachite → chromite + magnetite → ferromerrillite (Fig. 3). The eutectic zone contains daubréelite inclusions, and partially oxidised pyrrhotite inclusions were noted in barringerite II (Fig. 3b). As ‘barringerite I’ and ‘barringerite II’ could have different structures—hexagonal (barringerite) or orthorhombic (allabogdanite)—we carried out structural studies using SCXRD, which confirmed that both generations of Fe₂P minerals correspond to barringerite.

Figure 3. (a) Zonal aggregate of phosphides with edges of Fe–Cr spinels and ferromerrillite. The fragment magnified in (b) is outlined. (b) In the zonal aggregate of phosphides, the following changes can be observed: barringerite of generation I → eutectic: schreibersite–barringerite with daubréelite inclusions → barringerite of generation II replaced by schreibersite with inclusions of Cr-V-bearing pyrrhotite. Key: Bgr-I – barringerite, generation I; Bgr-II – barringerite, generation II; Dbr – daubréelite; Gh – gehlenite; Hem – hematite; Fmel – ferromerrillite; Hgr – hydrogrossular; Mag – magnetite; Chr – chromite; Pyh – pyrrhotite; Pwo – pseudowollastonite; Scb – schreibersite; Tch – tacharanite; and Ymm – yamhamelachite.

It should be added that pyrrhotite was widespread in the rock, which often had lamellae of Cr and V-enriched pyrrhotite and/or daubréelite (Galuskin et al., Reference Galuskin, Galuskina, Vapnik and Zieliński2023a). In samples containing yamhamelachite, pyrrhotite was almost completely replaced by hematite, forming characteristic framework pseudomorphs reminiscent of the lamellar structure of pyrrhotite (Figs 1b, 2c).

Yamhamelachite forms dark green granular aggregates in which the grain size does not exceed 25–30 mm (Fig. 2d). The mineral is transparent with a glassy lustre. Yamhamelachite is a brittle mineral with a conchoidal fracture indicating a lack of cleavage. Mohs hardness = 4. The density calculated on the basis of the empirical formula and the structural data is 3.035 g·cm^–3. Selected yamhamelachite grains are usually very small (∼10 μm) and intergrow with Fe-oxides. Therefore, extraction of a pure yamhamelachite grain suitable for optical characterisation was a challenging task. Only minimal [α = 1.640(3)] and maximal [γ = 1.662(3)] refractive indexes could be measured, but the superior Gladstone–Dale compatibility (1–(K_P/K_C) = –0.0076) shows that the measurements are accurate.

Chemical composition

The empirical formula of yamhamelachite calculated on the basis of average microprobe analyses is (K_0.89Ca_0.01□_0.10)_Σ1.00Cr³⁺_0.50V³⁺_0.33Al_0.15Fe³⁺_0.04Ti⁴⁺_0.03)_Σ1.05P_1.98O₇ (Table 1, an. 1) and contains the main end-members: KCr³⁺P₂O₇ – 50%; KV³⁺P₂O₇ – 33%; and KAlP₂O₇ – 15%. The maximum chromium content in yamhamelachite is 16.43 wt.% Cr₂O₃, which corresponds to 0.57 Cr per formula unit (Table 1, an. 2). In rare cases, V > Cr (Table 1, an. 3), and the empirical formula of the mineral becomes (K_0.87□_0.13)_Σ1.00(V³⁺_0.53Cr³⁺_0.40Al_0.12Fe³⁺_0.03Ti⁴⁺_0.04)_Σ1.12P_1.95O₇. This can be simplified to the formula KVP₂O₇, indicating that it may be another new mineral. The EMPA points of the pyrophosphate analysis are shown in the classification diagram KCr³⁺P₂O₇ – KAlP₂O₇ – KV³⁺P₂O₇ (Fig. 4).

Figure 4. Points of analysis of K-pyrophosphates in the V–Cr–Al ternary diagram.

Table 1. Chemical composition of yamhamelachite (1,2), V-analogue of yamhamelachite (3), ferromerrillite (4) and ferroalluaudite (5)

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

Ferromerrillite, (Na_0.76K_0.14Sr_0.01)_Σ0.91(Fe²⁺_0.97Mg_0.11Cr³⁺_0.09V³⁺_0.02Al_0.03)_Σ1.22Ca_8.91(P_6.90Si_0.05)_Σ6.95O₂₈ (Fig. 3a; Table 1, an. 3), known only in meteorites (Britvin et al., Reference Britvin, Krivovichev and Armbruster2016), and a phase close in composition to ferroalluaudite (Na_0.70Ca_0.41K_0.15)_Σ1.26(Fe²⁺_0.98Fe³⁺_0.88Cr³⁺_0.53Al_0.18V³⁺_0.05Mg_0.04Ti⁴⁺_0.01)_Σ2.67(P_3.05Si_0.01)O₁₂ (Table 1, an. 4), associate with yamhamelachite. Yamhamelachite usually occurs between zones of barringerite II and an inhomogeneous zone of Fe–Cr spinels where chromite grains are embedded in magnetite (Fig. 3, Table 2).

Table 2. Chemical composition (wt.%) of chromite (1) and magnetite (2)

S.D. – standard deviation

Zonal phosphide aggregates are represented by early homogeneous barringerite I (P $\bar 6$2m, a = 5.8649(4) and c = 3.4625(4) Å) with minor impurities of Cr and V (Table 3, an. 1), which is replaced by rather rare schreibersite–barringerite eutectics in this breccia (Fig. 3b). In eutectic formations, Cr and V tend to accumulate in barringerite (Table 3, an. 2, 3). In contrast, in schreibersite with few barringerite and cohenite inclusions (Fig. 3b), Cr and V are concentrated in small daubréelite inclusions with the composition Fe(Cr³⁺_1.4V³⁺_0.6)_Σ2S₄ (EDS data). Barringerite II (P $\bar 6$2m, a = 5.8465(4) and c = 3.48176(19) Å) which replaces schreibersite, is characterised by low Cr and V content. The effects of the replacement of early phosphides by later ones with higher P content is observed in other fragments of the studied sample, in which yamhamelachite can be found. Specifically, there is a replacement of schreibersite by late barringerite in eutectic schreibersite–barringerite (Fig. 5a,b; Table 4) and a replacement of barringerite by murashkoite with pyrrhotite inclusions (Fig. 5c,d; Table 5).

Figure 5. (a) Eutectic of schreibersite–barringerite with schreibersite rim. (b) Grain similar to that shown in (a), in which some of the schreibersite in the eutectic is replaced by later barringerite. (c) Barringerite grain with reaction rim of murashkoite. Fragment magnified in (d) is shown in the frame. (d) Sulfide inclusions are observed in porous murashkoite. Key: Bgr-I – barringerite, generation I; Bgr-II – barringerite, generation II; Dbr – daubréelite, Gh – gehlenite; Hem – hematite; Hgr – hydrogrossular; Muh – murashkoite; Pyh – pyrrhotite; Pwo – pseudowollastonite; and Scb – schreibersite.

Table 3. Phosphide chemical compositions from aggregate with yamhamelachite, wt.%: 1 – barringerite I; 2,3 – eutectic: schreibersite (2) – barringerite (3); 4 – barringerite II

S.D. – standard deviation

Table 4. Composition of barringerite–schreibersite grains shown in Fig. 5b: barringerite (2) – schreibersite intergrowth and schreibersite rim (1), secondary barringerite after schreibersite (3)

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

Table 5. Chemical composition of grain shown in Fig. 5b: barringerite (1) with reaction rim of porous murashkoite (2) with pyrrhotite inclusions (3)

S.D. – standard deviation

Raman spectroscopy and structure of yamhamelachite

The Raman spectrum of yamhamelachite is similar to that of its synthetic analogue (Elouafi et al., Reference Elouafi, Ouahbi, Ezairi, Lmai, Tizliouine and Lassri2023). The following bands are observed in the yamhamelachite spectrum (Fig. 6, cm^–1): ν_as(PO₂) – 1260, 1222; ν_s(PO₂) – 1190, 1134, 1102; ν_as(POP) – 1059, 1025, 917; ν_s(POP) – 775; δ(PO₂) – 608, 588, 564; δ(POP) – 478, 441, 423, 365; T(Cr,V) – 234; T(K) – 194; T(P₂O₇) + L – 152, 117. Data from previous studies of Raman spectra of various pyrophosphates (Cornilsen and Condrate, Reference Cornilsen and Condrate1977; Stranford et al., Reference Stranford, Condrate and Cornilsen1981; Cornilsen, Reference Cornilsen1984; Szczygieł et al., Reference Szczygieł, Macalik, Radomińska, Znamierowska, Mączka, Godlewska and Hanuza2007; Capitilli et al., Reference Capitelli, Dridi, Arbib, Valentini and Mattei2007; El Arni et al., Reference El Arni, Hadouchi, Assani, Saadi, El Marssi, Lahmar and El Ammari2023) informed the band assignments.

Figure 6. Raman spectrum of yamhamelachite.

Single-crystal XRD data were collected for a small yamhamelachite crystal fragment (30×10×8 μm) using a SuperNova diffractometer. The predominance of Cr over V was confirmed by semi-quantitative analysis of selected grains using EDS/SEM. The structure of yamhamelachite was refined using the SHELX-2019/2 program (Sheldrick, Reference Sheldrick2015). Its crystal structure was refined from the atomic coordinates of synthetic KCrP₂O₇ (Gentil et al., Reference Gentil, Andreica, Lujan, Rivera, Kubel and Schmid1997). Experimental details and refinement data are summarised in Tables 6–9. The XRD data (Table S1) and crystallographic information file are available as Supplementary material (see below). The structure of yamhamelachite (P2₁/c, a = 7.3574(3) Å, b = 9.9336(4) Å, c = 8.1540(4) Å, β = 106.712(5)°, V = 570.77(5) ų and Z = 4) is shown in Fig. 7a–c. It can be described as layered, with single layers formed by Сr³⁺(V³⁺)-octahedra connected by (Р₂О₇)^4– groups (Fig. 7d), and as consisting of columns of octahedra and pyrophosphate groups (Fig. 7e). In the (P₂O₇)^4– group, the two corner-linked PO₄ tetrahedra are slightly distorted as might be expected, where the linking (P1,P2–O4) distances are both ∼1.61 Å while the remaining three P–O distances are ∼1.49–1.52 Å in each case. The (P₂O₇)^4– group is significantly bent with a dihedral angle for the (P1–O4–P2) bond of 124.09(11), which is due to the (P₂O₇)^4– group being able to act as a bidentate ligand, forming a 6-membered chelate ring with Cr³⁺ via bonds to O1 and O6 (both ∼1.52 Å), while acting as a monodentate ligand for the remaining four O atom positions of the (CrO₆)^9– coordination octahedron (Fig. 7d, Table 9). Interestingly, the (CrO₆)^9– octahedron is only slightly distorted even in the presence of significant V³⁺ and Al³⁺ replacing Cr³⁺ in the central metal site. The resulting structure, which can be described as a 3D covalent lattice formed by the corner bonding of (CrO₆)^9– octahedra and (P₂O₇)^4– pyrophosphate groups, results in open channels parallel to [001], which can then accommodate the K cations in large 10-coordinate sites, if K–O distances of up to 3.22 Å are considered viable contacts (Fig. 7c, Table 9).

Figure 7. The structure of yamhamelachite: (a) projection on (100); (b) projection on (010); (c) projection on (001); (d) projection on (001) – note only one structural layer is shown and the chelate ring is shown; (e) columns in yamhamelachite formed by pyrophosphate groups (P₂O₇)^4– and octahedra [(Cr,V)O₆]^9–. Key: green octahedra – [(Cr,V)O₆]^9–; dark-blue tetrahedra – (P1O₄); light-blue tetrahedra – (P2O₄); pink balls – K. Drawn using CrystalMaker 2.7® software.

Table 6. Crystal data and structure refinement details for yamhamelachite

* Weighting scheme: w = 1/[σ²(F _o²) + (0.0427P)² + 0.1098P], where P = (F _o² + 2F _c²)/3

Table 7. Atomic coordinates, equivalent-isotropic displacement parameters (Ų) and site occupancy for yamhamelachite

Table 8. Anisotropic displacement parameters (Ų)

Table 9. Selected bond lengths (Å), angles (°) and BVS* calculations for yamhamelachite

* Calculated using ECoN21 (Ilinca, Reference Ilinca2022)

The structural formula of yamhamelachite K_0.953(Cr_0.77Al_0.23)P₂O₇ is close to the empirical formula (K_0.89Ca_0.01□_0.10)_Σ1.00Cr³⁺_0.50V³⁺_0.33Al_0.15Fe³⁺_0.04Ti⁴⁺_0.03)_Σ1.05P_1.98O₇ (Tables 1, 6). The resulting K- and Cr-site occupancies give site-scattering values of 18.11 and 21.47 electrons per formula unit (structural formula) and 17.11 and 23.24 epfu (empirical formula), respectively. It is likely that the grain used for structural studies contained more Al than the calculated average Al content (Table 1, an.1).