Moabite, NiFe³⁺(PO₄)O, a new natural oxyphosphate

Moabite, NiFe³⁺(PO₄)O, is a new natural oxyphosphate discovered in pyrometamorphic rocks of the Daba-Siwaqa complex, a subdivision of the Hatrurim Formation in central Jordan. The mineral is named for the Kingdom of Moab, an ancient state that existed on the territory of the modern Jordan. Moabite is an accessory phase in the phosphide–phosphate assemblages, where it associates with diopside; anorthite; crocobelonite, CaFe${}_{\mathrm{2}}^{\mathrm{3}+}$(PO₄)₂O; yakubovichite, CaNi₂Fe³⁺(PO₄)₃; hematite; negevite, NiP₂; murashkoite, FeP; transjordanite, Ni₂P; halamishite, Ni₅P₄; native iron (α-Fe); and an alluaudite-group phosphate whose composition is exactly midway between the two endmembers NaNaCa(Fe³⁺Mg)(PO₄)₃ and □NaCa(Fe³⁺Fe³⁺)(PO₄)₃. The mineral forms isometric to short prismatic crystals and euhedral grains up to 30 µm across. Macroscopically, it has a deep-brown colour. In the polished sections in transmitted light, the mineral is translucent red-brown. It has a Mohs hardness rating of 4. Cleavage was not observed. The density, 4.324 g cm⁻³, was calculated based on the empirical formula and unit-cell parameters obtained from single-crystal refinement. The chemical composition was as follows (electron microprobe, wt %): NiO 29.75, CuO 1.73, MgO 0.45, Fe₂O₃ 36.04, Al₂O₃ 0.19, Cr₂O₃ 0.18, V₂O₅ 0.47, P₂O₅ 31.22, total 100.03. The empirical formula calculated on the basis of 5 oxygen atoms per formula unit (apfu) is (Ni_0.90Cu_0.05Mg_0.03)_Σ0.98(Fe³⁺ _1.01Al_0.01Cr_0.01)_Σ1.03(P_0.99V⁵⁺ _0.01)_Σ1.00O₅, corresponding to the ideal NiFe³⁺(PO₄)O. Moabite is orthorhombic; the space group is Pnma (no. 62); and a=7.2161(16), b=6.4064(15), c=7.4706(19) Å, V=345.4(1) ų and Z=4. The strongest lines of X-ray powder diffraction pattern are as follows [d in Å (I) (hkl)]: 5.20(63)(101), 3,321(37)(102), 3.251(83)(201), 2.7262(100)(121), 2.5946(37)(202), 2.3542(25)(103) and 2.3044(24)(122). The crystal structure has been solved and refined to R₁=0.033 for 389 unique observed reflections. Moabite is the first mineral that crystallizes in the α-Fe₂PO₅ (α-Fe₂OPO₄) structure type. It has a direct synthetic analogue, and it is isotypic to antiferromagnetic transition metal oxyphosphates of the general formula $A^{\mathrm{2}+}{B}^{\mathrm{3}+}$OPO₄, where $A^{\mathrm{2}+}=$ Fe, Ni, Co and Cu and $B^{\mathrm{3}+}=$ Fe, V and In.

1 Introduction

The Hatrurim Formation, a large complex of pyrometamorphic rocks in the Middle East (Gross, 1977), is distinguished by the unprecedented diversity of phosphorus-bearing minerals. They include a palette of silicophosphates and carbonate phosphates (Galuskin et al., 2015, 2016, 2018a, b, 2019, 2021; Sokol et al., 2015), anhydrous orthophosphates (Britvin et al., 2021c, 2023b; Galuskin et al., 2023a, 2024; Juroszek et al., 2023; Krzątała et al., 2023), phosphides (Britvin et al., 2015, 2020a, b, c, 2021b, 2022a; Murashko et al., 2022; Galuskin et al., 2023b), condensed phosphates (Britvin et al., 2021d), and oxyphosphates (Galuskin et al., 2018b, 2021; Britvin et al., 2023a). The majority of these minerals are endemic, reflecting the unique and still controversially interpreted processes that triggered the emergence of the formation (Gross, 1977; Burg et al., 1992; Fleurance et al., 2013; Sokol et al., 2007, 2010, 2012; Novikov et al., 2013; Britvin et al., 2021a, 2022b, c, d). In this paper we focus on a specific subset of phosphate minerals, pure oxyphosphates, which are characterized by (1) an anhydrous composition, (2) the presence of a so-called “additional” oxygen atom in their chemical formulas and (3) the absence of other anions. These oxyphosphates are now comprised by six terrestrial species and two minerals of meteoritic origin (Table 1). We herein present a description of moabite, NiFe³⁺(PO₄)O, a new representative of natural oxyphosphates. The mineral is named for the Kingdom of Moab, an ancient state that emerged in the late 10th century to 11th century BCE on the territory of the modern Hashemite Kingdom of Jordan (e.g. Finkelstein and Lipschits, 2011). The holotype specimen of moabite is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5627/1.

Figure 1 Location of the Hatrurim Formation outcrops across the territory of the southern Levant. The largest massif is a Daba-Siwaqa complex in Jordan. Adapted from Britvin et al. (2021d) (CC-BY license).

Figure 2 (a) Moabite crystal in an anorthite–diopside matrix. (b) Thin section of the paralava containing moabite (Mab), unnamed alluaudite-group phosphate (UN) and hematite (Hem) in diopside matrix (Di). Transmitted light, parallel polars.

Table 1 Oxyphosphate minerals with no other additional anions.

^* Origin: terrestrial (T) or meteoritic (M).

2 Samples and methods

Electron microprobe analysis was conducted on the polished carbon-coated thin sections, using a WAVE 500 wavelength-dispersive spectrometer (Oxford Instruments) attached to a Hitachi S-3400 N scanning electron microscope. The spectrometer was operated at an acceleration voltage of 20 kV, beam current of 15 nA and beam diameter of 2 µm. The following standards and Kα lines were used: diopside (Mg), trevorite (Ni), Cu metal (Cu), hematite (Fe), gehlenite (Al), V metal (V), Cr metal (Cr) and chlorapatite (P). The same polished sections were used for subsequent spectroscopic studies after the removal of carbon film. Optical reflectance data were acquired by means of a Leica DM4500P microscope and TIDAS E MSP spectrophotometer calibrated against the Si standard.

The Raman spectrum was obtained by means of a LabRAM HR800 (HORIBA Jobin Yvon) spectrometer equipped with an Olympus microscope, 50× confocal objective and He–Ne laser (λ= 632.8 nm). X-ray single-crystal data were collected using a Bruker Kappa APEX DUO Diffractometer (CCD detector, charge-coupled device), equipped with a microfocus tube (MoKα radiation). The crystal structure was solved and refined using the SHELX-2018 set of programs (Sheldrick, 2015) embedded into the Olex2 graphical user interface (Dolomanov et al., 2009). Data collection and structure refinement details are summarized in the section on crystal structure. Full crystallographic information can be retrieved from the crystallographic information file (CIF) attached to the Supplement. X-ray powder diffraction data were obtained by means of a Rigaku RAXIS Rapid II diffractometer (semi-cylindrical imaging plate, Debye–Scherrer geometry, r= 127.4 mm), under the following conditions: CoKα radiation (rotating anode with microfocus tube), 40 kV, 15 mA and 60 min exposure. Conversion of the image plate to the profile and processing were carried out using the ocs2xrd program (Britvin et al., 2017) and Stoe WinXPOW v2.03 (Stoe & Cie GmbH, Darmstadt, Germany).

Table 2 Reflectance values of moabite measured in air (%).

3 Occurrence

The geological setting of the Hatrurim Formation is described in numerous reports and reviews (Gross, 1977; Burg et al., 1992; Fleurance et al., 2013; Novikov et al., 2013; Abzalov et al., 2015; Sokol et al., 2019; Vereshchagin et al., 2024b). In brief, the formation, also known as “the Mottled Zone”, is comprised of dozens of rock outcrops exposed on both the Jordanian and Israeli sides of the Dead Sea, with some patches traced far north along the Jordan River (Fig. 1). The spread of the Mottled Zone outcrops coincides with the area affected by the tectonics of the Dead Sea transform fault system (Ben-Avraham et al., 2008). The age dating of the pyrometamorphic event(s) gives controversial results, from 16 Ma to even ∼ 250 ka (e.g. Sokol et al., 2019). Thermal metamorphic processes, initiated and maintained by one or more as yet poorly defined heat sources, resulted in temperatures up to 1400 °C at a near-atmospheric pressure (e.g. Vapnik et al., 2007). These P–T (pressure–temperature) conditions led to calcination and even fusion of sedimentary strata – chalks, marls and limestones – to form a variety of metamorphic lithologies, from marbles to the so-called “paralavas” (Vapnik et al., 2007). The 300 km² Daba-Siwaqa complex in central Jordan is a largest representative of the Hatrurim Formation (Abzalov et al., 2015; Vereshchagin et al., 2024b). The numerous abandoned quarries formerly operated for building stone expose the whole palette of fresh pyrometamorphic rocks. Moabite-bearing assemblages were found in a small quarry (31°21^′52^′′ N, 36°10^′55^′′ E), within the blocks of veined anorthite–diopside paralava, whose assemblages were previously described in detail (Britvin et al., 2023a; Vereshchagin et al., 2024b). Moabite is an accessory phase in these assemblages, where it associates with diopside; anorthite; native iron; hematite; and a series of recently discovered phosphides and phosphates – negevite, NiP₂; murashkoite, FeP; transjordanite, Ni₂P; halamishite, Ni₅P₄; crocobelonite, CaFe${}_{\mathrm{2}}^{\mathrm{3}+}$(PO₄)₂O; yakubovichite, CaNi₂Fe³⁺(PO₄)₃; and an alluaudite-group phosphate whose composition is exactly midway between the two endmembers NaNaCa(Fe³⁺Mg)(PO₄)₃ and □NaCa(Fe³⁺Fe³⁺)(PO₄)₃. The relationships between the minerals indicate that moabite is the earliest phosphate but formed after hematite, iron and phosphide minerals.

4 Appearance and physical and optical properties

Moabite forms isometric to short prismatic crystals (Fig. 2a) and anhedral grains (Fig. 2b) up to 30 µm in size disseminated in a fine-grained matrix of anorthite–diopside paralava. The mineral often intergrows with other phosphates (Fig. 1b). Macroscopically, moabite has very dark-brown, almost-black, colour with vitreous lustre. In the thin sections in transmitted light, the mineral is semi-transparent with a red-brown colour. Due to the very high predicted value of the mean refractive index, 1.99, determination of refractive indices in transmitted light was considered impractical. In reflected light, the mineral has a grey colour with red-brown internal reflections. It is non-pleochroic, with weak anisotropy and bireflectance. Reflectance values are presented in Table 2.

Table 3 Chemical composition of moabite (wt %).

5 Raman spectroscopy

The Raman spectrum of moabite (Fig. 3) contains the following bands (cm⁻¹): 115, 130, 160, 179, 198, 255, 292sh, 307, 361 and 388 (internal vibrations of [MO₆] octahedra and lattice modes); 410, 423, 449 and 469 [ν₂ (symmetric bending vibrations of (PO₄) tetrahedra)]; 545, 590, 628 and 650sh [ν₄ (asymmetric bending vibrations of (PO₄) tetrahedra)]; 843, 871 and 918 [ν₁ (symmetric stretching modes of P–O bonds)]; and 988, 1019, 1090 and 1157 [ν₃ (asymmetric stretching vibrations of P–O)]. The band assignments were made based on the data previously reported for α-Fe₂PO₅ (Wu et al., 2018) and CoIn(PO₄)O (El Arni et al., 2023). The absence of bands in the region of stretching vibrations of O–H bonds (3800–2800 cm⁻¹) and bending modes of H₂O at 1600–1650 cm⁻¹ provides evidence of the absence of water in the chemical composition of moabite.

Figure 3 Raman spectrum of moabite.

Figure 4 Zigzag chains of octahedra [NiO₆] (blue) and [FeO₆] (brown) in the crystal structure of moabite. The octahedra within the chains are connected via alternation of common edges and faces. The chains are propagated along the b axis and linked by [PO₄] tetrahedra (not shown for clarity).

6 Chemical composition

Moabite crystals and grains have rather uniform chemical composition, with no zoning. Among the non-essential elements substituting for Ni and Fe, one can mention Cu, which was not previously detected in phosphates of the Hatrurim Formation. The Raman spectrum, the results of the structural study and bond-valence calculations are consistent with the absence of water in the composition of the mineral. Consequently, the average results of microprobe analyses (Table 3) can be calculated on the basis of 5 oxygen atoms per formula unit (apfu), yielding the empirical formula (Ni_0.90Cu_0.05Mg_0.03)_Σ0.98(Fe${}_{\mathrm{1.01}}^{\mathrm{3}+}$Al_0.01Cr_0.01)_Σ1.03 (P_0.99V${}_{\mathrm{0.01}}^{\mathrm{5}+}$)_Σ1.00O₅, which corresponds to the ideal NiFe³⁺(PO₄)O.

7 X-ray powder diffraction and crystal structure

Indexing of X-ray powder diffraction data for moabite (Table 4) and refinement of the unit-cell parameters (orthorhombic system, space group Pnma) give the following results: a= 7.2145(3), b= 6.4031(3), c= 7.4657(3) Å and V= 344.88(2) ų. These parameters are consistent with the results obtained by the single-crystal study, which are provided in Table 5 along with the basic parameters of single-crystal data collection and structure refinement. Fractional atomic coordinates and displacement parameters for moabite are presented in Tables 6 and 7.

Table 4 X-ray powder diffraction data for moabite.

The measured intensities of the seven strongest lines are highlighted in bold. The theoretical pattern was calculated on the basis of atomic coordinates obtained from structure refinement and unit-cell parameters refined from powder data. Calculated lines with an intensity of less than 1 have been omitted.

Table 5 Crystal parameters, data collection and structure refinement details for moabite.

Moabite has a synthetic analogue, NiFe³⁺(PO₄)O (El Khayati et al., 2001); both crystallize in the α-Fe₂PO₅ structure type (Modaressi et al., 1983; Chemseddine and El Hajbi, 1999). The mineral and synthetic compounds exhibit almost identical bond-valence sums (Table 8), providing evidence for the proper assignment of the metal oxidation states in moabite. Moabite is the first mineral that adopts the α-Fe₂PO₅ structure type (Table 9). Synthetic orthorhombic α-Fe₂PO₅ ≡ Fe²⁺Fe³⁺(PO₄)O, the structural prototype and Fe²⁺ analogue of moabite, is a high-temperature polymorph of Fe₂PO₅. Its low-temperature modification, β-Fe₂PO₅, adopts monoclinic (distorted-tetragonal) symmetry (Ech-Chahed et al., 1988; Ijjaali et al., 1990; Elkaïm et al., 1996). The crystal structure of the mineral consists of zigzag chains of alternating octahedra of [NiO₆] and [FeO₆] connected via the common faces, where [FeO₆] octahedra are located at the bends of the chains (Fig. 4). The oxide chains, propagated along the b axis, are linked both through the external corners of the [FeO₆] octahedra and via the corners of the [PO₄] tetrahedra.

Moabite is isotypic to transition metal oxyphosphates of the general formula $A^{\mathrm{2}+}{B}^{\mathrm{3}+}$(PO₄)O [$\equiv A^{\mathrm{2}+}{B}^{\mathrm{3}+}$OPO₄], where $A^{\mathrm{2}+}=$ Fe, Ni, Co and Cu and $B^{\mathrm{3}+}=$ Fe, V and In, which are extensively studied due to their magnetic and electrochemical properties (Ech-Chahed et al., 1988; El Khayati et al., 2001; Aziam et al., 2018, 2020; El Arni et al., 2023). There are two known oxyphosphate minerals with the stoichiometry M₂PO₅: staněkite, Mn²⁺Fe³⁺(PO₄)O (Keller et al., 1997), and joosteite, Mn²⁺Mn³⁺(PO₄)O (Keller et al., 2007a) (Table 9). These minerals have crystal structures different from moabite and synthetic M₂PO₅ oxyphosphates.

Table 6 Fractional atomic coordinates and isotropic displacement parameters (U_iso, Ų) for moabite.

Note: Crystallographic tables were created using the PublCIF software (Westrip, 2010). Site multiplicities and Wyckoff symbols are given in parentheses.

Table 7 Anisotropic displacement parameters (Ų) for moabite.

Table 8 Selected bond length (Å) and bond-valence sum (BVS; v.u., valence unit) values for cationic sites of moabite and synthetic NiFe(PO₄)O^*.

^* Bond-valence sums for moabite were calculated using microprobe-determined site populations normalized to 1, and bond-valence parameters were derived by Gagné and Hawthorne (2015). Bond-valence sums for synthetic NiFe(PO₄)O were taken from El Khayati et al. (2001).

Table 9 Crystallographic data for moabite and some related phosphates.

^* References are as follows. (1) This work. (2) El Khayati et al. (2001). (3) Modaressi et al. (1981). (4) Elkaïm et al. (1996). (5) Keller et al. (2006). (6) Keller et al. (2007b).

8 Discussion and conclusions

Moabite, which was discovered in pyrometamorphic assemblages, is a product of high-temperature pyrolitic oxidation of natural iron–nickel phosphides, as shown for crocobelonite, CaFe${}_{\mathrm{2}}^{\mathrm{3}+}$(PO₄)₂O (Britvin et al., 2023a). Phosphide assemblages are the typical feature of the Hatrurim Formation; they are known from several localities across Israel, Palestine and Jordan (Britvin et al., 2015, 2020a; Galuskin et al., 2023a). It is noteworthy that rare phosphide findings were also reported from other localities of pyrometamorphic origin (Savina et al., 2020). In addition, natural phosphides occur in terrestrial basaltic iron assemblages (Klöck et al., 1986; Ulff-Møller, 1990; Vereshchagin et al., 2024a); they were found in coal-burnt dumps (Nishanbaev et al., 2002), mantle lithologies (Yang et al., 2005), fulgurites (Pasek and Block, 2009; Plyashkevich et al., 2016) and seabed sediments (Borodaev et al., 1982). Not all of these occurrences are suitable for postformational oxidation of phosphides, yet pyrometamorphism appears to present the most variable redox conditions. Phosphides, such as murashkoite, FeP; transjordanite–barringerite series, Ni₂P-Fe₂P; zuktamrurite, FeP₂; negevite, NiP₂; schreibersite, (Fe,Ni)₃P; and others (Britvin et al., 2020c; Galuskin et al., 2023a), are frequently associated with native iron, forming a fruitful basement for high-temperature oxidative transformations. In the case of oxidation alone, pure Fe–Ni phosphates are being formed, according to e.g. the following scheme: 2(FeNi)P (barringerite–transjordanite) + 5O₂→ 2FeNi(PO₄)O (moabite).

In general, however, side reactions with the surrounding Ca-bearing silicates do occur, resulting in formation of more common Ca–Fe-bearing phosphates and oxyphosphates (Britvin et al., 2023a). The oxidation state of Ni in phosphates reaches Ni²⁺, whereas Fe state may vary between Fe²⁺ and Fe³⁺, depending on the local redox environment (Britvin et al., 2021d, 2023a).

Moabite, a Fe–Ni oxyphosphate, is a potential candidate for the occurrence in iron and stony-iron meteorites, where it could be formed by the oxidation of Fe–Ni metal–phosphide assemblages during high-temperature ablation processes. The recent discovery of ablation-formed iron oxyphosphates in the El Ali iron meteorite (Herd et al., 2024) shows that ablation-formed phosphates deserve further investigation, with the possibility of the recognition of new mineral phases.