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, Reference Fedotov and Markhinin1983). 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. (Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a) and Shchipalkina et al. (Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020); the general mineralogical features of oxidising-type fumaroles of Tolbachik were overviewed by Vergasova and Filatov (Reference Vergasova and Filatov2016) and Pekov et al. (Reference Pekov, Agakhanov, Zubkova, Koshlyakova, Shchipalkina, Sandalov, Yapaskurt, Turchkova and Sidorov2020a).

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., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020), 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 (sampleFootnote ¹ #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, Reference Symonds and Reed1993).

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 (Reference Nakamoto1986). 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., Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a).

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, Reference Mandarino1981) 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., Reference Britvin, Dolivo-Dobrovolsky and Krzhizhanovskaya2017). 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, Reference Mandarino1981) 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, Reference Sheldrick2015). 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., Reference Petříček, Dušek and Palatinus2006). 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 (Reference Gagné and Hawthorne2015), and for Mg-F and Na-F from Brese and O'Keeffe (Reference Brese and O'Keeffe1991).