Three new iron-phosphate minerals from the El Ali iron meteorite, Somalia: Elaliite Fe82+ Fe³⁺(PO₄)O₈, elkinstantonite Fe₄(PO₄)₂O, and olsenite KFe₄(PO₄)₃

Occurrence

Elaliite, elkinstantonite, and olsenite occur along with wüstite, troilite, sarcopside, and Ca-bearing graftonite within inclusions in the iron-nickel metal (kamacite, 9.4 wt% Ni) that makes up the bulk of El Ali (Figs. 1–3). Most inclusions are ~100 μm across, although larger, millimeter-scale inclusions have the greatest mineralogical diversity. In larger inclusions, elaliite typically occurs as ~10 × 50 μm euhedral, elongate laths at the interfaces of troilite and/or wüstite and graftonite-rich areas (Fig. 1). Graftonite-rich areas also contain <5 μm grains of Si-bearing sarcopside, elaliite, and troilite (Fig. 1). In smaller (<200 μm) inclusions, elaliite occurs with troilite set in a matrix of sarcopside or graftonite (Fig. 2); this is also the typical setting for olsenite (Fig. 3). Elkinstantonite has thus far only been found in a smaller inclusion, as subhedral grains associated with troilite and wüstite symplectites, troilite, and elaliite (Fig. 2). Minor terrestrial alteration extends along fractures and grain boundaries, which is consistent with the specimen being taken from the exterior of the meteorite (Fig. 1).

Figure 1

Backscattered electron image of a large inclusion in the El Ali meteorite (including void and hematite interpreted to be from terrestrial alteration). Inset shows elongate crystals of elaliite (medium gray) in a sarcopside/graftonite-rich cavity (dark gray), surrounded by troilite with a symplectic intergrowth of wüstite (light gray, and medium-light gray, respectively), and low-Ni iron (white).

Figure 2

Backscattered electron image of elkinstantonite (dark gray) in low-Ni iron (white) with elaliite (medium gray) and troilite (light gray). Slight differences in grayscale within elkinstantonite are likely caused by minor compositional variations (observed by FE-SEM-EDS).

Figure 3

Backscattered electron image of olsenite (dark gray) with graftonite (medium gray), elaliite (light gray), troilite (very light gray), and low-Ni iron (white).

Appearance, physical and optical properties

Elaliite occurs as elongate laths mainly from 1 × 5 μm to 7 × 20 μm in size (Fig. 1) and is dark brown in color. Elkinstantonite occurs as equant grains, ~10 to 20 μm in size (Fig. 2), and is transparent with a light brown color. Olsenite occurs as irregular crystals, 1 to 5 μm in size (Fig. 3); its color was not observed. As a result of their small grain sizes, optical properties and several physical properties of these minerals were not observed (including: streak, luster, Mohs hardness, cleavage, parting, tenacity, fracture, and measured density). Micro-hardness and reflectance measurements were not undertaken. These three minerals did not show visible cathodoluminescence.

Chemical compositions

The chemical compositions (mean, standard deviation, range) of elaliite, elkinstantonite, and olsenite measured by electron probe microanalysis are presented in Table 1. These analyses show the empirical formulas for elaliite, elkinstantonite, and olsenite (respectively) to be: ( Fe7.9432+Fe1.0203+ Cr_0.010Ni_0.006Ca_0.004Mn_0.004)_Σ8.987(P_0.932Si_0.077S_0.005)_Σ1.014O₁₂, ( Fe3.9472+ Mn_0.016Ni_0.003Ca_0.001Cr_0.001)_Σ3.968(P_1.986Si_0.014S_0.013)_Σ2.013O₉, and (K_0.820Na_0.135Ca_0.004)_Σ0.959(Fe_3.829Mn_0.050)_Σ3.879(P_2.972S_0.058Si_0.017)_Σ3.047O₁₂, respectively. The ideal formulas of elaliite, elkinstantonite, and olsenite are Fe82+ Fe³⁺(PO₄)O₈, Fe₄(PO₄)₂O, and KFe₄(PO₄)₃, respectively, which require (each sum 100.00 wt%): elaliite = P₂O₅ 9.78, FeO 79.22, and Fe₂O₃ 11.00 wt%; elkinstantonite = P₂O₅ 33.06, and FeO 66.94 wt%; and olsenite = K₂O 8.60, FeO 52.50, P₂O₅ 38.90 wt%.

Crystallography and structures

Electron backscatter diffraction was used to confirm the crystal structures of the three new minerals. Chemistry-limited searches of the Inorganic Crystal Structure Database (Hellenbrandt 2004) yielded excellent matches to synthetic materials: elaliite = Fe₉(PO₄)O₈, Venturini et al. (1984), mean angular deviation 0.32 to 0.37° (Fig. 4); elkinstantonite = Fe₄(PO₄)₂O, Bouchdoug et al. (1982), mean angular deviation 0.22 to 0.36° (Fig. 5); and olsenite = KFe₄(PO₄)₃, Matvienko et al. (1981), mean angular deviation 0.44 to 0.48° (Fig. 6). From the unit-cell parameters determined by EBSD (Table 2), the densities were calculated (from the empirical formulas and EBSD unit-cell volumes), along with the mean refractive indices given by the Gladstone-Dale relationship (Mandarino 1976).

Figure 4

EBSD pattern of elaliite (left), indexed (right) in space group Cmmm based on the structure of Venturini et al. (1984). (Color online.)

Figure 5

EBSD pattern of elkinstantonite (left), indexed (right) in space group P2₁/c based on the structure of Bouchdoug et al. (1982). (Color online.)

Figure 6

EBSD pattern of olsenite (left), indexed (right) in space group Pnnm based on the structure of Matvienko et al. (1981). (Color online.)

The crystal structure of synthetic Fe₉(PO₄)O₈, equivalent to elaliite, was reported by Venturini et al. (1984) to have site-occupancy disorder on the tetrahedral position between P and Fe²⁺, and two of the oxygen positions are positionally disordered; it is probable that an unresolved superstructure is present. Elaliite is isotypic with Mg₇Ga₂GeO₁₂ and (Mg,Ga)₈(Ga,Ge)₂O₁₂ (Barbier and Hyde 1987; Barbier and Frampton 1992). This structure-type has been described (Barbier and Hyde 1987) as an intergrowth between the wüstite (halite structure) and β-Ga₂O₃ structures (Fig. 7).

Figure 7

Clinographic projection of the crystal structure of elaliite in the Cmmm setting, after Venturini et al. (1984). The Fe²⁺-centered octahedra are shown in blue, the Fe³⁺-centered octahedra in orange with dashes, and the tetrahedra in red with crosses. (Color online.)

The crystal structure of synthetic Fe₄(PO₄)₂O, equivalent to elkinstantonite, was reported by Bouchdoug et al. (1982). This structure consists of a dense framework of edge- and corner-sharing Fe²⁺-centered octahedra and five-coordinated Fe²⁺-centered polyhedra, which is further connected by phosphate tetrahedra that share edges and corners with the various Fe²⁺-centered polyhedra (Fig. 8). The analogous copper oxyphosphate and copper oxyarsenate compounds, Cu₄(PO₄)₂O and Cu₄(AsO₄)₂O, both occur in triclinic and orthorhombic polymorphs that differ structurally from elkinstantonite (Anderson et al. 1978; Brunel-Laügt et al. 1978; Schwunck et al. 1998; Pekov et al. 2014; Volkova 2023). The copper oxyarsenates have been described as the minerals ericlaxmanite (triclinic) and kozyrevskite (Pekov et al. 2014).

Figure 8

Clinographic projection of the crystal structure of elkinstantonite, after Bouchdoug et al. (1982). The Fe²⁺-centered octahedra are shown in blue, the five-coordinated Fe²⁺-centered polyhedra in orange with dashes, and the phosphate tetrahedra in red with crosses. (Color online.)

The crystal structure of synthetic KFe₄(PO₄)₃, equivalent to olsenite, was reported by Matvienko et al. (1981). This structure-type has been described (Sugiyama and Kimiyama 2009) as a framework that consists of buckled slabs of Fe²⁺-centered octahedra and phosphate tetrahedra that are further linked through five-coordinated Fe²⁺-centered polyhedra and phosphate tetrahedra, with channels that run parallel to [100] (Fig. 9). Several isotypic compounds have been reported, e.g., KCo₃Fe(PO₄)₃ (Assaaoudi et al. 2006); several of these are reviewed in Yakubovich et al. (2018). Olsenite is not structurally related to the minerals of the alluaudite supergroup or the fillowite group, despite similarities in their stoichiometries (Hatert 2019; Hatert et al. 2021; Tait et al. 2021). Rather, according to Britvin et al. (2020), the structure-type of olsenite is a derivative of the α-CrPO₄ type (Attfield et al. 1988).

Figure 9

Clinographic projection of the crystal structure of olsenite in the Pnnm setting, after Matvienko et al. (1981). The Fe²⁺-centered octahedra are shown in blue, the five-coordinated Fe²⁺-centered polyhedra in orange with dashes, the phosphate tetrahedra in red with crosses, and the potassium atoms in gray. (Color online.)

Raman spectroscopy

The Raman spectra of elaliite, elkinstantonite, and olsenite acquired at both 532 and 785 nm are in good agreement, confirming that the observed bands are indeed Raman signals and not artifacts or fluorescent signals. Each of the spectra shows P-O stretching modes between 930 and 1020 cm^–1, out-of-plane bending modes between 450 and 650 cm^–1, in-plane bending modes (for elkinstantonite) around 400 cm^–1, and lattice modes below 250 cm^–1 (compare Frost et al. 2002; Litasov and Podgornykh 2017; Pieczka et al. 2018) (Figs. 10–12). The Raman spectrum of elaliite has a single P-O symmetric stretching mode at 937 cm^–1, a broad hump centered around 488 cm^–1, and various lattice modes whose positions show variation among different grains (Fig. 10). Elaliite was found to be very sensitive to laser damage, consistent with potential oxidation, as is observed in Fe-oxide and sulfide minerals (de Faria et al. 1997; Fries and Steele 2018). The Raman spectrum of elkinstantonite has a triplet in the P-O symmetric stretching region at 930, 942, and 976 cm^–1, and a weak band at 1016 cm^–1, with other weak bands between 450 and 650 cm^–1, and lattice modes whose positions show variation among different grains (Fig. 11). The Raman spectrum of olsenite has a single symmetric stretching mode at 968 cm^–1 and a weak band around 648 cm^–1; no lattice modes were resolved for the Raman spectrum of olsenite (Fig. 12). The Raman spectrum of each mineral is superimposed on a fluorescent background that differs for each mineral. The low relative peak intensities of the Raman bands, especially in the spectrum of elaliite, arise from the deliberate use of low laser power during acquisition to mitigate sample damage.

Figure 10

Raman spectrum of elaliite acquired with 532 nm wavelength radiation.

Figure 11

Raman spectrum of elkinstantonite acquired with 532 nm wavelength radiation.

Figure 12

Raman spectrum of olsenite acquired with 532 nm wavelength radiation.

Relationship to other species

There are at least five other iron-bearing oxyphosphate mineral species reported—beershevaite [CaFe₃(PO₄)₃O], crocobelonite [CaFe₂(PO₄)₂O], grattarolaite [Fe₃(PO₄)O₃], moabite [NiFe(PO₄)O], and stanĕkite [FeMn(PO₄)O]—all of which have Fe³⁺ as the dominant oxidation state of iron, and most of which occur in pyrometamorphic rocks (Cipriani et al. 1997; Britvin et al. 2021a, 2021b, 2023); stanĕkite occurs in a pegmatite (Keller et al. 1997). Elaliite and elkinstantonite are the only natural oxyphosphates that contain Fe²⁺ as the dominant oxidation state of iron, and which occur in iron meteorites. All of the iron oxyphosphate minerals (except perhaps stanĕkite), therefore, appear to have formed at relatively high temperatures, regardless of their differing parageneses. In the Dana classification, elaliite and elkinstantonite both belong to the “Anhydrous Phosphates with miscellaneous formulas,” and they are most closely related (compositionally) to grattarolaite (Dana number is 38.05.12.01). In the Nickel-Strunz classification, grattarolaite similarly belongs to “Phosphates without additional anions and without H₂O and with only medium-sized cations and RO₄ >2:1” and its Nickel-Strunz number is 08.BE.10 (webmineral.com; accessed June 2022).

The formula of olsenite, KFe₄(PO₄)₃ is analogous to that of the alluaudite group of monoclinic phosphates and arsenates, whose general formula can be simplified for comparative purposes to A₂M₃(TO₄)₃ (Tait et al. 2021), where A = Na, Ca, Cu, or □; M = Na, Mg, Ca, Mn, Fe²⁺, Fe³⁺, Cu, Zn, or Cd; and T = P or As. The fillowite group of rhombohedral phosphates also has compositional similarities, with the simplified formula Na₃CaC₁₁(PO₄)₉ (Hatert et al. 2021), in which C = Mg, Mn, or Fe²⁺ for chladniite, fillowite, and johnsomervilleite, respectively. The mineral galileiite, NaFe₄(PO₄)₃, which is the sodium analog of olsenite, has been commonly attributed to the fillowite group because the majority of galileiite’s X-ray diffraction lines (measured with a Gandolfi camera) could be indexed to a fillowite-type unit cell; the crystal structure of natural galileiite has not yet been determined. A synthetic hypersodic analog of galileiite, Na_xFe₄(PO₄)₃ with (1.1 ≤ x ≤ 1.2) has been reported with a monoclinic structure that is isotypic with synthetic NaCo₄(PO₄)₃ (Baies et al. 2006; Zhang et al. 2018). Olsen and Steele (1997) concluded that the fillowite-type unit cell interpreted from the XRD results for galileiite is not entirely satisfactory because the d-spacing of the 024 reflection (rhombohedral indexing), which has 40% relative intensity, shows a large difference from the calculated value: d_obs 5.83 vs. d_calc 5.51 Å. It may be notable that synthetic Na_xFe₄(PO₄)₃ (space group P2₁/n, a 6.369, b 9.950, c 15.666 Å, β 91.927°) has its calculated 101 reflection, with relative intensity 100%, at 5.829 Å (Zhang et al. 2018). The diffraction lines of galileiite (from 5.83 to 1.48 Å) found in the Grant IIB meteorite and reported by Olsen and Steele (1997) in their Table 2 can be re-indexed to yield: a = 6.38, b = 9.92, c = 15.57 Å, β = 92.0°, using the programs PowderCell and UnitCell (Kraus and Nolze 1996; Holland and Redfern 1997), based on the structure of Zhang et al. (2018). The monoclinic structure of Na_xFe₄(PO₄)₃ and NaCo₄(PO₄)₃ is related to the orthorhombic structure of olsenite and its isotypes (Yakubovich et al. 2018), which in turn is derivative from the α-CrPO₄ type (Attfield et al. 1988).

In the original description of galileiite, Olsen and Steele (1997) mention that “[an] occurrence of the potassium analog of galileiite, KFe₄(PO₄)₃ has been located in another IIIA iron meteorite.” This is presumably the same material from the Sand-town IIIA iron meteorite whose analysis is reported in Table 2 of Olsen et al. (1999) with a sum of 99.60 wt%, and which yields the empirical formula: (K_0.596Na_0.390Ca_0.017)_Σ1.003(Fe_3.350Mn_0.615Cr_0.026)_Σ3.991P_2.994O₁₂. This K-rich phase has the simplified formula: (K_0.6Na_0.4)(Fe_3.4Mn_0.6)(PO₄)₃, and occurs in direct contact with graftonite and johnsomervilleite in the Sandtown IIIA meteorite (Olsen et al. 1999); it probably represents another occurrence of olsenite.

Olsenite and galileiite belong, in the Dana classification, to the Anhydrous Phosphates (A⁺B²⁺)₅(PO₄)₃, which also includes the recently described triclinic mineral xenophyllite, Na₄Fe₇(PO₄)₆, found in the troilite nodules of the Augustinovka IIIAB iron meteorite (Britvin et al. 2020). The Dana numbers of galileiite and xenophyllite are 38.02.05.04 and 38.02.05.05, respectively. Although its crystal structure has not been determined, xenophyllite is probably related structurally to the monoclinic synthetic compounds RbNa₃Fe₇(PO₄)₆ and KNa₃Fe₇(PO₄)₆; xenophyllite has an I-centered subcell that is similar to the primitive unit cell of olsenite (Queen et al. 2007; Britvin et al. 2020). In the Nickel-Strunz classification, galileiite and xenophyllite belong to “Phosphates without additional anions and without H₂O and with medium-sized and large cations” and their Nickel-Strunz numbers are both 08.AC.50 (webmineral.com; accessed July 2022).

Raman spectroscopy

The Raman spectra of elaliite, elkinstantonite, and olsenite were obtained from grains with sizes exceeding 2 μm, ensuring the acquired spectra were devoid of additions from surrounding phases. There is also no indication of Raman modes from the surrounding phases wüstite, troilite, sarcopside, or graftonite in the Raman spectra of the new phosphate minerals. The Raman spectra of each novel mineral exhibit distinctive Raman bands, confirming their unique compositions and crystal structures. The P-O stretching mode between 930 and 1020 cm^–1 is consistently the strongest and well-defined feature in each spectrum, indicative of a high degree of crystallinity.

The low relative peak intensity of the major modes, high fluorescence background, and lower frequency of the P-O symmetric stretching mode at 937 cm^–1 in the Raman spectrum of elaliite are likely a result of the elevated Fe-content of the mineral. A broad hump in the bending region at around 488 cm^–1 may indicate the presence of disorder, mentioned earlier, in the crystal structure of elaliite.

The presence of a triplet in the Raman spectrum of elkinstantonite, instead of a single band observed in the spectra of elaliite and olsenite, is due to either the non-equivalence of the three phosphate groups in the structure, or overtones and/or combination frequencies (Šoptrajatov and Petrov 1967).

Olsenite is the potassium analog of galileiite, and the Raman spectrum of galileiite has a single P-O symmetric stretching band at around 980 cm^–1, and weak bands at around 595, 556, 417, and 1125 cm^–1 (Xie and Chen 2020). Comparing their Raman spectra, both olsenite and galileiite have different stretching and bending modes but both have a similar single band in the P-O symmetric stretch region; for olsenite this mode occurs at lower frequency, probably because of the larger radius of potassium.

Petrogenesis

Phosphate-rich inclusions often occur with troilite in meteorites, e.g., Augustinovka IIIAB (Britvin et al. 2020), Chelyabinsk LL5 chondrite (Sharygin et al. 2016), Darinskoe IIC (Sharygin 2022), Elga IIIE (Litasov and Podgornykh 2017), Graves Nunataks (GRA) 95209 lodranite (Grew et al. 2010), Krymka LL3.1 chondrite (Semenenko and Perron 2005), Sahara 03505 sulfide-rich iron (D’Orazio et al. 2009), and the Yanzhuang H6 chondrite (Xie and Chen 2020). Most occurrences in non-iron meteorites (e.g., chondrites, lodranites) may be attributable to shock metamorphism (e.g., Grew et al. 2010). In iron meteorites, iron-rich phosphates are predominantly sarcopside and graftonite; chromite is the most common O-bearing associated mineral (e.g., Olsen et al. 1999; Litasov and Podgornykh 2017). Phosphate minerals vary in composition and identity among iron meteorites, but are commonly Na±Ca±Mg-dominant (e.g., Olsen et al. 1999; Litasov and Podgornykh 2017; Britvin et al. 2020). In the Morasko IAB iron, the main phosphates are buchwaldite (NaCaPO₄), brianite [Na₂CaMg(PO₄)₂], moraskoite [Na₂Mg(PO₄)F], and czochralskiite [Na₄Ca₃Mg(PO₄)₄]; these are found in association with Ca-phosphates such as merrillite and fluorapatite (Karwowski et al. 2016; Rubin and Ma 2021). The mineralogy of meteoritic phosphates is reviewed by Olsen et al. (1999) for IIIAB iron meteorites and more broadly by Rubin and Ma (2017, 2021). The observation that phosphate minerals in IIIAB meteorite are carriers of the incompatible lithophile elements Fe, Mn, Na, Ca, Mg, and K (Olsen et al. 1999), appears to be the general case for most inclusions in iron meteorites.

The common paragenesis of the iron phosphates found in the El Ali meteorite—including elaliite, elkinstantonite, olsenite, and graftonite/sarcopside—is in association with wüstite and troilite within inclusions of two general size populations: <200 μm across (“small”) and >200 μm across (“large”) (Figs. 1–3). The mineral assemblages vary between inclusion types, with small inclusions typically containing troilite + wüstite + elaliite ± elkinstantonite ± sarcopside/graftonite. Large inclusions consist of troilite + wüstite on their margins—wherein wüstite is present as blebs within troilite, and graftonite-rich areas in their cores (Fig. 1). Elaliite in this setting is present at the boundary of these two areas and as small (<10 μm) grains within the graftonite-rich areas, along with small (<10 μm) grains of sarcopside and troilite (Fig. 1, inset). We interpret the texture of the large inclusions as reflecting the solidification of a sulfide-rich melt, with wüstite blebs representing unmixing of an FeO-component from troilite, followed by the solidification of a more O-rich (and S-poor) melt, represented by the graftonite-rich areas. Within this context, elaliite crystallizes at the boundary between these melts and in association with graftonite in the inclusion cores, where the activity of oxygen (the oxygen fugacity) is sufficiently high to form this mixed-valence phosphate. Terrestrial alteration is present in the form of voids lined with hematite (Fig. 1); however, these crosscut the iron phosphate mineral assemblage and, therefore, were not involved in their formation.

In small inclusions, the paragenesis is similar; however, it appears that the melt that solidified in each small inclusion varied in bulk composition, as shown by differences in their mineral assemblages (Figs. 2–3). It is notable that elkinstantonite and olsenite have thus far been found in only one small inclusion each. Elaliite occurs as ~10 μm grains between troilite ± wüstite and elkinstantonite (Fig. 2) and as ~1–2 μm grains included within olsenite or as sub-micrometer inclusions within graftonite/sarcopside (Fig. 3), suggestive of differences in crystallization sequence in different inclusions. We conclude that the mineral assemblage in each inclusion—and therefore the occurrence of elaliite, elkinstantonite, and olsenite—depends on the relative proportions of Fe, S, P, K, Ca, and O trapped within a given inclusion, and the crystallization sequence. In all cases, the system is constrained to low oxygen fugacity by the host iron meteorite.