Halilsarpite was identified in a small number of hand specimens collected in the Oumlil mine, Bou Azzer district, southern Morocco (30^∘${\mathrm{31}}^{\prime }{\mathrm{22}}^{\prime \prime }$ N, 6^∘${\mathrm{47}}^{\prime }{\mathrm{6}}^{\prime \prime }$ W), in 2012. The Bou Azzer mining district is one of the world's main Co producers from Co–Ni–Fe arsenide ores. The arsenide ore minerals include löllingite (FeAs₂), safflorite ((Co,Ni,Fe)As₂), skutterudite (CoAs₃), and arsenopyrite (FeAsS), associated with Neoproterozoic peridotite and quartz diorite intrusions in more than 60 orebodies of which the Oumlil deposit is one (Ahmed et al., 2009). Intense weathering of the arsenides has resulted in the formation of a diverse series of secondary ferric arsenite and arsenate minerals including arseniosiderite, karibibite, pharmacosiderite, scorodite, and walentaite (Favreau and Dietrich, 2006).

Halilsarpite spherules occur on resinous brown smolyaninovite (Fig. 1) and brown karibibite. Electron microprobe analysis of the smolyaninovite gave the formula Ca_1.1Co_0.9${\mathrm{Fe}}_{\mathrm{2.7}}^{\mathrm{3}+}$(AsO₄)₄⋅11H₂O. Other associated minerals are Al-rich scorodite, karibibite, and pharmacosiderite, all of which are older than halilsarpite. Safflorite and skutterudite occur as inclusions in the hand specimens. The safflorite is heavily altered and may be the source for As and Fe in the formation of halilsarpite. No primary source of Mo was located in the hand specimens, but both molybdenite and powellite, CaMoO₄, occur at Bou Azzer (Favreau and Dietrich, 2006). Yellow crystals and spherules of walentaite or walentaite-like minerals with variable chemistry have been found on several occasions at the Oumlil mine and nearby localities (Georges Favreau, personal communication, 2019), but only the present material has been identified as halilsarpite.

Figure 1Yellow spherules of halilsarpite on orange-brown smolyaninovite from the Oumlil mine, Morocco. FOV 3.15 mm. Photo by Ole-Thorsein Ljøstad.

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Halilsarpite forms lemon-yellow spherules, about 0.1 mm in diameter (Fig. 2). Scanning electron microscope (SEM) inspection of the spherules showed that they are composed of layers of platelets, some tens of micrometres in diameter but only 1–2 µm thick (Fig, 3). The plane of the platelets is {100} and this was the only discernible form. The platelets have a vitreous lustre and are brittle with perfect {100} cleavage. The calculated density is 2.89 g cm⁻³, based on the empirical formula and PXRD cell.

Figure 2Spherules of halilsarpite, FOV 0.7 mm. Photo by Ole-Thorstein Ljøstad.

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Figure 3Backscattered electron image of halilsarpite spherules.

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Optically, halilsarpite is biaxial (–), with the indices of refraction α=1.646(calc), β=1.765(5), and γ=1.815(5), measured in white light. The 2V (meas.) = 62(1)^∘ from extinction data using EXCALIBR (Gunter et al., 2004). Dispersion is weak with r<v. The optical orientation could only be partially determined as X=a. The pleochroism involves shades of yellow, with $X<Y\approx Z$. Because X is perpendicular to the very thin plates, α could not be measured; consequently, it has been calculated from β, γ, and 2V. The tiny plates exhibited uniform extinction, although they generally consist of multiple slightly offset plates. The Gladstone–Dale compatibility, 1-(K_P∕K_C) (Mandarino, 2007), is −0.042 (GOOD) based upon the empirical formula and density calculated using the powder XRD unit cell and the indices of refraction.

A Raman spectrum on crystal aggregates was obtained using a Renishaw inVia Raman microscope with 514.5 nm argon green laser excitation and a Leica short-working-distance objective. A 90 µm slit was employed. The power at the sample was approximately 0.5 mW and the accumulation time was 30 s.

Figure 4Raman spectrum for halilsarpite.

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The Raman spectrum in the range from 20 to 4000 cm⁻¹ is shown in Fig. 4. In the O–H stretching region, a broad band with peaks at 3270 and 3420 cm⁻¹ corresponds to H-bonded H₂O while a weak sharp peak at 3590 cm⁻¹ is most likely due to hydroxyl groups. A broad H–O–H bending mode vibration is centred at 1640 cm⁻¹. The halilsarpite crystal structure comprises layers of FeO₆ octahedra and AsO₄ tetrahedra that have the same topology as jarosite-group minerals, specifically segnitite, PbFe₃(AsO₄)(AsO₃OH)(OH)₆. Therefore, the As⁵⁺–O and Fe³⁺–O Raman spectra peaks below 1000 cm⁻¹ were interpreted with the aid of publications of Raman spectra by Sasaki et al. (1998) on jarosite minerals and Frost et al. (2005) on segnitite. The interpretation is complicated by the presence of peaks due to Mo⁶⁺–O and As³⁺–O stretching vibrations, which overlap with As⁵⁺–O peaks. For Mo⁶⁺–O and As³⁺–O peaks, the publication by Cejka et al. (2010) on the Raman spectrum of vajdakite, [(Mo⁶⁺O₂)₂(H₂O)₂${\mathrm{As}}_{\mathrm{2}}^{\mathrm{3}+}$O₅] ⋅ H₂O, was helpful. On the basis of these studies, the peak at 925 cm⁻¹ is an Mo⁶⁺–O stretching vibration, while that of As⁵⁺–O is at 860 cm⁻¹ and As³⁺–O is at 800 cm⁻¹. Other stretching vibrations due to these species are unresolved in the envelope from 700 to 1000 cm⁻¹. The peaks centred around 590 cm⁻¹ correspond to both Mo⁶⁺–O and As³⁺–O bending vibrations (Cejka et al., 2010). The band centred at 450 cm⁻¹ corresponds to the bending region for AsO₄ (Frost et al., 2005) but also has a contribution from Fe³⁺–O vibrations. Other Fe³⁺–O vibrations are at 340 and 210 cm⁻¹ (Sasaki et al., 1998).

Halilsarpite crystal aggregates were mounted in a polished section and analysed using wavelength-dispersive spectrometry on a JEOL JXA-8500F Hyperprobe operated at an accelerating voltage of 15 kV and a beam current of 5 nA. The beam was defocused to 5 µm. Standards were scorodite (As), wollastonite (Ca), albite (Na), adularia (K), AlPO₄ (P), hematite (Fe), MgAl₂O₄ (Mg), and CaMoO₄ (Mo). Analytical results (average of 16 analyses) are given in Table 1. The water content is from the crystal structure (to give 6 H₂O pfu). The As₂O₃∕As₂O₅ were apportioned to give 2 (As⁵⁺ + P) pfu, consistent with the crystal structure. The slightly low analysis total is most likely due to the thinness of the platelets. For the calculation of the empirical formula, the oxide mass fraction (wt%) values in Table 1 were scaled to give Σoxides + H₂O = 100.

Table 1Analytical data (wt%) for halilsarpite.

^a Total Fe as Fe₂O₃, based on the crystal structure. ^b H₂O calculated based on 6 H₂O pfu, from the crystal structure.

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The atomic proportions, normalized to 21 anions, are Na_0.11K_0.01Ca_0.69Mg_0.62${\mathrm{Fe}}_{\mathrm{2.55}}^{\mathrm{3}+}$Mo_0.56${\mathrm{As}}_{\mathrm{2.05}}^{\mathrm{3}+}$${\mathrm{As}}_{\mathrm{1.97}}^{\mathrm{5}+}$P_0.03O₂₁ H_12.1.

Combining the analyses with crystal-structure refinement results gives the empirical formula [(Mg_0.42 Ca_0.19 ${\mathrm{Fe}}_{\mathrm{0.11}}^{\mathrm{3}+}$ □_0.28)_Σ1.00 (H₂O)₆] [((Ca_0.5 Mg_0.2 Na_0.11 K_0.01□_0.13)_Σ0.95 (As³⁺)_2.05)_∑3.00 (${\mathrm{Fe}}_{\mathrm{2.44}}^{\mathrm{3}+}$${\mathrm{Mo}}_{\mathrm{0.56}}^{\mathrm{6}+}$)_Σ3.00 ((${\mathrm{As}}_{\mathrm{0.985}}^{\mathrm{5}+}$ ${\mathrm{P}}_{\mathrm{0.015}}^{\mathrm{5}+}$)_Σ1.00 O₄)₂ O_6.9(OH)_0.1].

The simplified formula for halilsarpite is [(Mg, Ca, Fe³⁺, □) (H₂O)₆] [(Ca, Mg, Na, □) ${\mathrm{As}}_{\mathrm{2}}^{\mathrm{3}+}$(Fe³⁺, Mo⁶⁺)₃ (AsO₄)₂O₇].

The ideal formula is [Mg(H₂O)₆][${\mathrm{CaAs}}_{\mathrm{2}}^{\mathrm{3}+}$ (${\mathrm{Fe}}_{\mathrm{2.67}}^{\mathrm{3}+}$${\mathrm{Mo}}_{\mathrm{0.33}}^{\mathrm{6}+}$) (AsO₄)₂O₇], which requires Fe₂O₃ 23.89, CaO 6.28, MgO 4.51, MoO₃ 5.32, As₂O₃ 22.16, As₂O₅ 25.74, H₂O 12.10, and total 100.00 wt.%.

A 0.1 mm spherule of halilsarpite was used to collect PXRD data with a Rigaku Synergy-S diffractometer equipped with a hybrid photon counting area detector (HyPix6000HE), housed at the Natural History Museum (NHM) in Oslo. Data were collected in the two-theta range 4 to 82^∘ using monochromatized CuKα radiation. A Gandolfi-like motion on the φ and ω axes was used to randomize the sample. Rietveld refinements were made using FullProf (Rodriguez-Carvajal, 1990). Refined orthorhombic unit cell parameters are a=26.516(2) Å, b=7.4036(6) Å, c=10.4192(12) Å, and V=2045.5(3) ų. The indexed PXRD pattern for halilsarpite is given in Table 2.

Table 2Powder X-ray data for halilsarpite (d in Å).

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Single-crystal X-ray studies were carried out with a platelet separated from a rosette of Oumlil mine crystals and mounted on a cryoloop with oil for room temperature data collections using a Rigaku Synergy-S diffractometer with sealed micro-sources (Mo and Cu) and a HyPix-6000HE detector. The data collection, integration, and face absorption corrections were all carried out in Rigaku's CrysAlis Pro software. A 49 h data collection was made at room temperature using MoKα radiation and a detector distance of 34 mm. The data quality was also checked using CuKα radiation at a longer detector distance of 45 mm. This showed that the reflections were multiply split due to contributions from sub-parallel lamellae. In the Mo dataset, the splitting was minimal, and the reflections were successfully integrated from the whole crystal, giving R_int=0.050 for equivalent reflections. Other details of the data collection are given in Table 3.

Table 3Data collection and refinement conditions for halilsarpite.

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Crystal-structure refinement

The atom coordinates for walentaite in space group Imma (Grey et al., 2019a) were used to start the refinement, with tetrahedral As⁵⁺ replacing P⁵⁺ at the T site. Based on previous experience with refinement of the walentaite and natrowalentaite structures, the metal atoms per formula unit (apfu) from the EMP analyses were distributed at the different metal atom sites in the following way: Mo was ordered, together with Fe at M2, Fe was ordered at M1, and any remaining Fe was located at the interlayer site A1. Ca was distributed between the interlayer A1^′ site and the surface-attached B site. Mg was distributed uniformly over the three sites A1, A1^′, and B. The Ca and Fe (in A1) contents were fixed, and the Mg content was refined at the three sites. The Fe and Mo coordinates at the M2 site were refined independently and the sum of their occupancies was restrained to full occupancy of the site. The occupancies of split, partially occupied Ow sites in the interlayer region and the split, partially occupied As2 and As3 sites were refined. With anisotropic displacement parameters for the heteropolyhedral-layer atoms and isotropic displacement parameters for the partially occupied interlayer and surface-attached atoms, a refinement using JANA2006 (Petříček et al., 2014) converged to wR_obs=0.046 for 1402 unique reflections. Other details of the refinement are given in Table 3. The refined coordinates and equivalent isotropic displacement parameters are reported in Table 4, anisotropic displacement parameters in Table 5, and polyhedron bond distances are given in Table 6. Note that although the 0.50(2) Mg apfu obtained from the site occupancy refinements (Table 4) agrees reasonably with the EMP-derived 0.62 Mg apfu, the site occupancies given in Table 4 for the A1, A1^′, and B sites should be considered with caution because all three sites are occupied by multiple cations as well as vacancies.

Table 4Refined coordinates, equivalent isotropic displacement parameters, and site occupation factors (sof) for halilsarpite.

* The sof multiplied by 4 gives the number of atoms per formula unit.

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Table 5Anisotropic displacement parameters (Ų) for halilsarpite.

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Table 6Polyhedral distances in halilsarpite (Å).

* Disordered atoms nA and nB cannot both be present in the same polyhedron.

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Crystallographic data for halilsarpite are available in the Supplement.

The supplement related to this article is available online at: https://doi.org/10.5194/ejm-32-89-2020-supplement.

TH conducted preliminary analyses and identified the new mineral as related to walentaite. IEG analysed the experimental data and wrote the paper. HF and FDB carried out the single-crystal and powder XRD experiments. ARK made the optical measurements. CMM performed the EMP analyses. WGM assisted in the crystal structure refinement. OTL provided the specimens and photographed them for Figs. 1 and 2. FS obtained the Raman spectrum.

The authors declare that they have no conflict of interest.

This paper was edited by Cristian Biagioni and reviewed by Ferdinando Bosi and one anonymous referee.