New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. XX. Evseevite, Na₂Mg(AsO₄)F, the first natural arsenate with an antiperovskite structure

Abstract

The new mineral evseevite was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Evseevite is represented by two chemical varieties. The variety close to the end-member Na₂Mg(AsO₄)F (holotype) is associated with sanidine, hematite, tenorite, aegirine, cassiterite, sylvite, halite, johillerite, badalovite, calciojohillerite, hatertite, arsmirandite, yurmarinite, axelite, polyarsite, aphthitalite, potassic-magnesio-fluoro-arfvedsonite, litidionite, ferrisanidine and tridymite. The P- and S-enriched variety (cotype) is associated with hematite, fluorophlogopite, svabite, fluorapatite, tilasite, calciojohillerite, forsterite, cassiterite, belomarinaite and aphthitalite. Evseevite occurs as prismatic, acicular or hair-like crystals up to 0.7 mm long combined in clusters up to 0.5 mm, brushes or crusts up to 2 × 2 mm. It is transparent, colourless or pale pinkish, with vitreous lustre. D_calc is 3.377 g cm^–3 for the holotype and 3.226 g cm^–3 for the cotype. Evseevite is optically uniaxial (+), α = 1.545(2), β = 1.546(2), γ = 1.549(2) and 2V_meas = 40(10)°. The empirical formulae calculated based on O+F = 5 apfu are (Na_1.99Ca_0.03K_0.01)_Σ2.03(Mg_0.98Fe³⁺_0.01Zn_0.01Cu_0.01)_Σ1.01[(As_0.98Si_0.01)_Σ1.01O₄](F_0.97O_0.03) for the holotype and Na_2.02(Mg_1.00Fe³⁺_0.03)_Σ1.03[(As_0.69P_0.25S_0.07)_Σ1.01O₄](F_0.78O_0.22) for the cotype. Evseevite is orthorhombic, Pbcn, a = 5.3224(1), b = 14.1255(3), c = 12.0047(3) Å, V = 902.53(4) ų and Z = 8. Strong reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 4.001(100)(121), 3.479(56)(023), 3.041(45)(042), 2.657(44)(200), 2.642(68)(142) and 2.613(36)(104). The crystal structure was solved from single-crystal XRD data and refined on powder data by the Rietveld method, R_wp = 0.0068, R_p = 0.0047 and R_obs = 0.0435. Evseevite is isostructural to moraskoite Na₂Mg(PO₄)F. The structure of evseevite can be described in terms of anion-centred polyhedra. F-centred octahedra [FNa₄Mg₂]⁷⁺ share faces to form chains [FNa₂Mg]³⁺ and AsO₄ tetrahedra are located between the chains. Evseevite belongs to a small set of minerals with antiperovskite structures and is the first natural arsenate with antiperovskite units. The mineral is named in honour of the Russian mineralogist Aleksandr Andreevich Evseev (born 1949).

Introduction

This paper continues the series of articles in which we characterise new arsenates from the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975–1976, Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia. Twenty two new mineral species have been described in the previous papers of the series: yurmarinite Na₇(Fe³⁺,Mg,Cu)₄(AsO₄)₆ (Pekov et al., 2014a), two polymorphs of Cu₄O(AsO₄)₂, ericlaxmanite and kozyrevskite (Pekov et al., 2014b), popovite Cu₅O₂(AsO₄)₂ (Pekov et al., 2015b), structurally related shchurovskyite K₂CaCu₆O₂(AsO₄)₄ and dmisokolovite K₃Cu₅AlO₂(AsO₄)₄ (Pekov et al., 2015a), katiarsite KTiO(AsO₄) (Pekov et al., 2016b), melanarsite K₃Cu₇Fe³⁺O₄(AsO₄)₄ (Pekov et al., 2016a), pharmazincite KZnAsO₄ (Pekov et al., 2017), arsenowagnerite Mg₂(AsO₄)F (Pekov et al., 2018b), arsenatrotitanite NaTiO(AsO₄) (Pekov et al., 2019d), the two isostructural minerals edtollite K₂NaCu₅Fe³⁺O₂(AsO₄)₄ and alumoedtollite K₂NaCu₅AlO₂(AsO₄)₄ (Pekov et al., 2019e), anatolyite Na₆(Ca,Na)(Mg,Fe³⁺)₃Al(AsO₄)₆ (Pekov et al., 2019b), zubkovaite Ca₃Cu₃(AsO₄)₄ (Pekov et al., 2019a), pansnerite K₃Na₃Fe³⁺₆(AsO₄)₈ (Pekov et al., 2020c), badalovite NaNaMg(MgFe³⁺)(AsO₄)₃ (Pekov et al., 2020b), calciojohillerite NaCaMgMg₂(AsO₄)₃ (Pekov et al., 2021a), yurgensonite K₂SnTiO₂(AsO₄)₂ (Pekov et al., 2021b), paraberzeliite NaCaCaMg₂(AsO₄)₃ (Pekov et al., 2022a), khrenovite Na₃Fe³⁺₂(AsO₄)₃ (Pekov et al., 2022b), and axelite Na₁₄Cu₇(AsO₄)₈F₂Cl₂ (Pekov et al., 2023).

In this article we describe the new mineral evseevite (Cyrillic: евсеевит), ideally Na₂Mg(AsO₄)F. It is named in honour of the Russian mineralogist Aleksandr Andreevich Evseev (born 1949) who works in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow. He is a specialist in the history of mineralogy and the application of geographical information in mineralogy.

Both the new mineral and its name (symbol Evs) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2019-064, Pekov et al., 2019c). The holotype specimen is deposited in the systematic collection of the Fersman Mineralogical Museum with the catalogue number 96701.

Occurrence and general appearance

The Arsenatnaya fumarole, discovered by us in 2012, is located at the summit of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, a monogenetic volcano formed in 1975 (Fedotov and Markhinin, 1983). Arsenatnaya is one of the hottest Tolbachik fumaroles today: the temperatures measured by us using a chromel–alumel thermocouple in the period 2012–2022, reached 500°C in its deep levels. It is a locality outstanding in mineral diversity and originality: more than 200 mineral species have been reliably identified here, including 67 new minerals. The mineralogy and zonation of sublimate incrustations of Arsenatnaya were described recently by Pekov et al. (2018a) and Shchipalkina et al. (2020); the general mineralogical features of oxidising-type fumaroles of Tolbachik were overviewed by Vergasova and Filatov (2016) and Pekov et al. (2020a).

The specimens with the new mineral were first collected by us in July 2018 from hot pockets situated 1.5–2 m below the day surface, within zone IV (Shchipalkina et al., 2020), or polymineralic zone. Evseevite is represented by two chemical varieties. The variety close to the end-member Na₂Mg(AsO₄)F gave the specimen considered as the holotype (sample¹ #6328): a suite of complex studies was carried out on samples of this composition, including the crystal structure determination. The variety enriched with admixed P and S, which partially substitute As, is considered as the cotype (#6260) and was subjected to scanning electron microscopy (SEM), electron microprobe analysis (EMPA) and powder X-ray diffraction (XRD). Additional specimens of the variety chemically close to the holotype but morphologically different were collected from other pockets in the same area in July 2022 (#7529). The temperature measured in the pockets with evseevite during sampling varied from 380 to 450°C.

These chemical varieties of evseevite occur in different mineral assemblages. The holotype (and later collected sample #7529) is associated with sanidine, hematite, tenorite, aegirine, cassiterite, sylvite, halite, johillerite, badalovite, calciojohillerite, hatertite, arsmirandite, yurmarinite, axelite, polyarsite, aphthitalite, potassic-magnesio-fluoro-arfvedsonite, litidionite, ferrisanidine and tridymite. The minerals associated with the cotype are hematite, fluorophlogopite, svabite, fluorapatite, tilasite, calciojohillerite, forsterite, cassiterite, belomarinaite and aphthitalite.

Evseevite occurs in cavities as prismatic, typically long-prismatic to acicular or hair-like crystals up to 0.1 mm, rarely up to 0.7 mm long and up to 0.03 mm thick. They are elongated along [100] and usually combined in parallel, near-parallel, sheaf-, bush- or brush-like aggregates. Clusters (up to 0.5 mm across) of randomly oriented crystals are also common. Acicular crystals form interrupted brushes and hair-like crystals compose pilous crusts (Fig. 1) up to 2 × 2 mm in area. Some crystals are skeletal and case-like.

Figure 1. Morphology of evseevite: (a–с: holotype, #6328) – aggregates of prismatic to acicular crystals overgrowing badalovite (b – with large hematite crystal); (d–e: cotype, #6260) – long-prismatic to acicular and hair-like crystals on hematite and fluorophlogopite; f – sample #7529, pilous crust completely covering calciojohillerite crystals (with bright white tenorite crystals). SEM images, SE (a, b, d, e) and BSE (c, f) modes.

We suggest that evseevite was deposited directly from the gas phase as a volcanic sublimate or, more probably, formed as a result of the interaction between fumarolic gas and basalt scoria at the temperatures not lower than 450°C. Basalt could be a source of Mg which has low volatility in such fumarolic systems (Symonds and Reed, 1993).

Physical properties and optical data

Evseevite is transparent, colourless or pale pinkish, with a white streak and vitreous lustre. The mineral is brittle, cleavage or parting was not observed. The fracture is uneven. Density calculated using the empirical formula and unit-cell volume found from powder XRD data is 3.377 g cm^–3 for the holotype and 3.226 g cm^–3 for the P- and S-enriched cotype.

The optical data were obtained for the holotype specimen. It is optically biaxial (+), α = 1.545(2), β = 1.546(2), γ = 1.549(2), 2V_meas = 40(10)° and 2V_calc = 60° (589 nm). Dispersion of optical axes was not observed. Orientation: X = a. Extinction is straight and elongation is negative. In transmitted plane-polarised light the mineral is colourless and non-pleochroic.

Raman spectroscopy

The Raman spectrum of the holotype evseevite (Fig. 2) was obtained on a randomly oriented crystal using an EnSpectr R532 instrument with a green laser (532 nm) at room temperature. The output power of the laser beam was ~10 mW. The spectrum was processed using the EnSpectr expert mode program in the range from 4000 to 100 cm^–1 with the use of a holographic diffraction grating with 1800 lines per cm^–1 and a resolution of 6 cm^–1. The diameter of the focal spot on the sample was ~10 μm. The back-scattered Raman signal was collected with a 40× objective; signal acquisition time for a single scan of the spectral range was 1500 ms and the signal was averaged over 20 scans.

Figure 2. The Raman spectrum of evseevite (the holotype).

The bands in the Raman spectrum of evseevite are assigned according to Nakamoto (1986). Bands in the region between 950 and 770 cm^–1 correspond to As⁵⁺–O stretching vibrations of AsO₄^3– anions. The presence of two strong bands in this region (with maxima at 883 and 808 cm^–1) is caused by significant distortion of AsO₄ tetrahedron (see below). Bands with frequencies lower than 520 cm^–1 correspond to bending vibrations of AsO₄ tetrahedra, Mg–O stretching vibrations and lattice modes. The absence of bands with frequencies higher than 950 cm^–1 indicates the absence of groups with O–H, C–H, C–O, N–H, N–O and B–O bonds in evseevite.

Chemical composition

The chemical composition of evseevite was studied by EMPA using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter.

The chemical data in wt.% are given in Table 1. Contents of other elements with atomic numbers higher than carbon were below detection limits.

Table 1. Chemical composition (in wt.%) of evseevite.

S.D. = standard deviation; ‘–’ indicates the content is below the detection limit; *averaged for four spot analyses for the holotype and for seven analyses for the cotype; **the trivalent state of admixed iron is assumed because of the strongly oxidising conditions of mineral formation in the Arsenatnaya fumarole: all iron minerals known from here contain only Fe³⁺ (Pekov et al., 2018a).

The empirical formulae calculated on the basis of O+F = 5 atoms per formula unit are (Na_1.99Ca_0.03K_0.01)_Σ2.03(Mg_0.98Fe³⁺_0.01Zn_0.01Cu_0.01)_Σ1.01[(As_0.98Si_0.01)_Σ1.01O₄](F_0.97O_0.03) for the holotype and Na_2.02(Mg_1.00Fe³⁺_0.03)_Σ1.03[(As_0.69P_0.25S_0.07)_Σ1.01O₄](F_0.78O_0.22) for the cotype. The idealised formula is Na₂Mg(AsO₄)F which requires Na₂O 27.16, MgO 17.66, As₂O₅ 50.36, F 8.33, –O=F –3.51, total 100 wt.%.

The Gladstone–Dale compatibility index (Mandarino, 1981) for the holotype evseevite, 1 – (K _p/K _c) = 0.028, excellent.

X-ray crystallography and crystal structure determination details

Single-crystal XRD studies of the holotype sample of evseevite were carried out at room temperature using an Xcalibur S diffractometer equipped with a CCD detector (MoKα-radiation). Powder XRD data for both holotype and cotype samples were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with cylindrical image plate detector (radius 127.4 mm) using Debye-Scherrer geometry, CoKα radiation (rotating anode with VariMAX microfocus optics), 40 kV, 15 mA and 15 min exposure. Angular resolution of the detector is 0.045°2θ (pixel size 0.1 mm). The data were integrated using the software package Osc2Tab (Britvin et al., 2017). Powder XRD data (in Å for CoKα) for the holotype are given in Table 2. The strongest reflections of powder XRD patterns and unit-cell parameters calculated from powder data for both holotype and cotype are presented in Table 3.

Table 2. Powder X-ray diffraction data (d in Å) of evseevite (the holotype).

* For the calculated pattern, only reflections with intensities ≥1 are given. The strongest reflections are marked in boldtype.

Table 3. Comparative data of evseevite (holotype and cotype) and moraskoite.

* A powder X-ray diffraction study of moraskoite was not carried out, only the calculated data were reported.

** It is likely that the mean refractive index for moraskoite was determined wrongly. Our calculation of the Gladstone–Dale compatibility index for moraskoite using n _mean = 1.550 gave 1 – (K _p/K _c) = –0.079, fair. The estimated value of n _mean for moraskoite calculated using the Gladstone–Dale equation (Mandarino, 1981) is ca. 1.51 [1.510 for 1 – (K _p/K _c) = 0.000].

The single-crystal XRD dataset used for the structure model determination was obtained on the same diffractometer Xcalibur S CCD. Data reduction was performed using CrysAlisPro, version 1.171.37.34 (Agilent Technologies, 2014). The data were corrected for Lorentz factor and polarisation effects. The crystal structure was solved by direct methods and refined in the space group Pbcn using the SHELX software package (Sheldrick, 2015). The low quality of the crystal and consequently of the experimental data precluded obtaining an excellent agreement between observed and calculated F values but resulted in an acceptable agreement with R _hkl = 0.1106 for 1116 reflections with I>2σ(I). Reasonable values of interatomic distances and displacement parameters of atoms, as well as good agreement between the measured and calculated powder XRD patterns (Table 2; Fig. 3) showed that the obtained structure model is correct. Further refinement of the structure of holotype evseevite was performed by the Rietveld method using this model.

Figure 3. Measured and calculated powder X-ray diffraction patterns of evseevite (the holotype). The solid line corresponds to calculated data, the crosses correspond to the measured pattern, vertical bars mark all possible Bragg reflections. The difference between the measured and calculated patterns is shown by curve at the bottom.

Data treatment and the Rietveld structure analysis were carried out using the JANA2006 program package (Petříček et al., 2006). The profiles were modelled using a pseudo-Voigt function. The structure was refined in isotropic approximation of atomic thermal displacements, the values of U _iso for all anions were restricted to be equal. The interatomic distances for As-centred tetrahedron and Mg-centred octahedron were softly restrained nearby the values obtained for the single-crystal model. Final agreement factors are: R _wp = 0.0068, R _p = 0.0047, R _obs = 0.0435. The observed and calculated powder XRD diagrams demonstrate very good agreement (Fig. 3; Table 2).

Data collection information and structure refinement details for both single-crystal and powder XRD studies are presented in Table 4, coordinates and thermal displacement parameters of atoms are given in Table 5, selected interatomic distances in Table 6, and bond valence calculations in Table 7. The data presented in Tables 5–7 are obtained in the result of the Rietveld refinement. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Crystal data, data collection information and structure refinement details for evseevite (the holotype).

Table 5. Coordinates and isotropic displacement parameters (U _iso, in Ų) of atoms for evseevite (the holotype).

Table 6. Selected interatomic distances (Å) in the structure of evseevite (the holotype).

Table 7. Bond valence calculations for evseevite (the holotype).

Bond-valence parameters for As–O, Mg–O and Na–O are taken from Gagné and Hawthorne (2015), and for Mg-F and Na-F from Brese and O'Keeffe (1991).

Discussion

Evseevite, ideally Na₂Mg(AsO₄)F, is the isostructural arsenate analogue of moraskoite Na₂Mg(PO₄)F (Karwowski et al., 2015) and synthetic Na₂Mg(PO₄)F (Swafford and Holt, 2002). The structure of evseevite (Fig. 4a) is based upon the heteropolyhedral (010) layers formed by [100] chains of Mg-centred octahedra connected via AsO₄ tetrahedra (Fig. 4b). A repeat unit of the chain is dimer [Mg₂O₆F₂]^10– consisting of two MgO₄F₂ octahedra sharing a common face of O(2)–F(1)–O(2). Adjacent dimers are linked via F(2) vertices. Na cations occupy two crystallographically non-equivalent sites. Na(1) cations centre Na(1)O₄F₂ octahedra [in moraskoite this site centres seven-fold polyhedron with one elongated Na(1)–O(1) distance of 2.93 Å; in evseevite this distance is > 3.0 Å and, thus, O(1) is excluded from the coordination sphere of the Na(1) cation]. Na(2) cations occupy seven-fold polyhedra Na(2)O₅F₂. The Na(1) sites are located between the heteropolyhedral layers whereas the Na(2) sites are situated at both sides of the heteropolyhedral layer.

Figure 4. The crystal structure of evseevite (a) projected along the a axis and (b) the heteropolyhedral layer in it. The unit cell is outlined.

The crystal structure of evseevite, as well as of moraskoite, can be described in terms of anion-centred polyhedra. Both F(1) and F(2) sites are octahedrally coordinated by two Mg and four Na cations each. F-centred octahedra [FNa₄Mg₂]⁷⁺ share faces to form [100] chains [FNa₂Mg]³⁺; AsO₄ tetrahedra are located between the chains (Fig. 5). This approach was used for the description of nacaphite Na₂Ca(PO₄)F (Krivovichev et al., 2007), the monoclinic (P2₁/c) mineral which is structurally related to moraskoite and evseevite and has the same stoichiometry. The relation of the crystal structures and unit-cell metrics of nacaphite and moraskoite were reported in detail by Karwowski et al. (2015).

Figure 5. F-centred octahedra FNa₄Mg₂ and AsO₄ tetrahedra in the structure of evseevite (two projections). The unit cell is outlined.

Evseevite, moraskoite and nacaphite belong to a group of fairly few minerals with antiperovskite structure, i.e. they have perovskite-type structures but with anions replaced by cations and vice versa. The group of natural antiperovskites is known to include sulfates, phosphates and silicates (Krivovichev, 2008; Karwowski et al., 2015), and now with the addition of evseevite has the first arsenate mineral with antiperovskite units: its structure is based on F-centred octahedra [FNa₄Mg₂]⁷⁺ which form chains similar to those in hexagonal 2H-perovskites. We have not found any information on synthetic arsenate antiperovskites in literature and databases, however, the X-centred octahedra [XNa₆]⁵⁺ (X = F, OH) are present in the structures of synthetic cubic arsenates Na₇F(AsO₄)₂⋅19H₂O and Na₇(OH)(AsO₄)₂⋅19H₂O (Baur and Tillmanns, 1974) isostructural to the mineral natrophosphate, ideally Na₇F(PO₄)₂⋅19H₂O. Avdontceva et al. (2021) suggested that the presence of the [FNa₆]⁵⁺ units makes it possible to consider natrophosphate-type compounds as a precursor for the formation of antiperovskite structure motifs based upon anion-centred octahedra.