Introduction
This paper continues the series of descriptions of new arsenate mineral species found in the Arsenatnaya fumarole situated at the apical part of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975–1976 (NB GTFE), Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41'N 160°14'E, 1200 m a.s.l.). Twenty new minerals have been characterised in the previous papers of the series: yurmarinite Na7(Fe3+,Mg,Cu)4(AsO4)6 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Lykova, Vigasina, Sidorov and Pushcharovsky2014a), two polymorphs of Cu4O(AsO4)2, ericlaxmanite and kozyrevskite (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Vigasina, Sidorov and Pushcharovsky2014b), popovite Cu5O2(AsO4)2 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Vigasina, Sidorov and Pushcharovsky2015a), structurally related shchurovskyite K2CaCu6O2(AsO4)4 and dmisokolovite K3Cu5AlO2(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Belakovskiy, Yapaskurt, Vigasina, Sidorov and Pushcharovsky2015b), katiarsite KTiO(AsO4) (Pekov et al., Reference Pekov, Yapaskurt, Britvin, Zubkova, Vigasina and Sidorov2016a), melanarsite K3Cu7Fe3+O4(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Polekhovsky, Vigasina, Belakovskiy, Britvin, Sidorov and Pushcharovsky2016b), pharmazincite KZnAsO4 (Pekov et al., Reference Pekov, Yapaskurt, Belakovskiy, Vigasina, Zubkova and Sidorov2017), arsenowagnerite Mg2(AsO4)F (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Yapaskurt, Chukanov, Belakovskiy, Sidorov and Pushcharovsky2018c), arsenatrotitanite NaTiO(AsO4) (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Belakovskiy, Vigasina, Yapaskurt, Sidorov, Britvin and Pushcharovsky2019a), the two isostructural minerals edtollite K2NaCu5Fe3+O2(AsO4)4 and alumoedtollite K2NaCu5AlO2(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Ksenofontov, Pautov, Sidorov, Britvin, Vigasina and Pushcharovsky2019b), anatolyite Na6(Ca,Na)(Mg,Fe3+)3Al(AsO4)6 (Pekov et al., Reference Pekov, Lykova, Yapaskurt, Belakovskiy, Turchkova, Britvin, Sidorov and Scheidl2019c), zubkovaite Ca3Cu3(AsO4)4 (Pekov et al., Reference Pekov, Lykova, Agakhanov, Belakovskiy, Vigasina, Britvin, Turchkova, Sidorov and Scheidl2019d), pansnerite K3Na3Fe3+6(AsO4)8 (Pekov et al., Reference Pekov, Zubkova, Koshlyakova, Agakhanov, Belakovskiy, Vigasina, Yapaskurt, Britvin, Turchkova, Sidorov and Pushcharovsky2020a), badalovite NaNaMg(MgFe3+)(AsO4)3 (Pekov et al., Reference Pekov, Koshlyakova, Agakhanov, Zubkova, Belakovskiy, Vigasina, Turchkova, Sidorov and Pushcharovsky2020b), calciojohillerite NaCaMgMg2(AsO4)3 (Pekov et al., Reference Pekov, Koshlyakova, Agakhanov, Zubkova, Belakovskiy, Vigasina, Turchkova, Sidorov and Pushcharovsky2021a), yurgensonite K2SnTiO2(AsO4)2 (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Yapaskurt, Belakovskiy, Vigasina, Britvin, Turchkova, Sidorov and Pushcharovsky2021b) and paraberzeliite NaCaCaMg2(AsO4)3 (Pekov et al., Reference Pekov, Koshlyakova, Belakovskiy, Vigasina, Zubkova, Agakhanov, Britvin, Sidorov and Pushcharovsky2022).
Here we characterise khrenovite, a mineral with the end-member formula Na3Fe3+2(AsO4)3. It belongs to the alluaudite group within the alluaudite supergroup and is the member of this supergroup richest in sodium (Hatert, Reference Hatert2019). In Pekov et al. (Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018b) it was mentioned as “unnamed Na3Fe3+2(AsO4)3”.
Khrenovite (Cyrillic: хреновит) is named in honour of the Russian volcanologist and geologist Anatoly Petrovich Khrenov (1946–2016) who worked in the Institute of Volcanology and Seismology of the Far Eastern Branch of the Russian Academy of Sciences, Petropavlovsk-Kamchatsky, and, from1993–2016, at the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM) of the Russian Academy of Sciences, Moscow. Dr. Khrenov has made significant contribution to studies of the Kamchatka volcanoes including Tolbachik.
Both the new mineral and its name (symbol Khr) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2017–105, Pekov et al. Reference Pekov, Koshlyakova, Belakovskiy, Vigasina, Zubkova, Agakhanov, Britvin, Sidorov and Pushcharovsky2018a). The holotype material is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow with the catalogue number 96190.
Occurrence and general appearance
The active Arsenatnaya fumarole and its mineralogical features, including zonation in the distribution of mineral associations, were characterised by Pekov et al. (Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Lykova, Vigasina, Sidorov and Pushcharovsky2014a, Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018b) and Shchipalkina et al. (Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020).
Khrenovite is one of the rarest minerals in the Arsenatnaya fumarole. It was found in several specimens collected by us in July 2016 from the upper part of the zone enriched by alluaudite-group arsenates (zone Va: Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020). It is situated at the depth of ~1.8 m from the surface. The temperature measured by us using a chromel–alumel thermocouple in this area was 430°C. We believe that khrenovite was formed at temperatures not lower than 430–450°C. It probably crystallised directly from the fumarolic gas phase as a volcanic sublimate.
The minerals associated with khrenovite are aphthitalite-group sulfates (aphthitalite, belomarinaite and metathénardite), badalovite, calciojohillerite, nickenichite, johillerite, tilasite, svabite, achyrophanite, ozerovaite, pansnerite, arsenatrotitanite, anhydrite, sanidine (As-bearing variety), hematite, cassiterite, rutile and pseudobrookite.
Khrenovite occurs as coarse prismatic crystals (Fig. 1) up to 0.2 × 0.3 × 0.8 mm and their clusters up to 1 mm across. They contain numerous hematite inclusions and are located typically within aggregates of aphthitalite-group sulfates overgrowing the surface of basalt scoria altered by fumarolic gas.
Red-brown coarse prismatic crystals of khrenovite with colourless aphthitalite and iron-black hematite. Field of view width: 1.5 mm; specimen #96190. Photo: I.V. Pekov & A.V. Kasatkin.
Physical properties and optical data
Khrenovite is a transparent honey-coloured, red–, orange– or yellow–brown mineral. Its streak is yellowish and the lustre is vitreous. Khrenovite is brittle; cleavage or parting was not observed, and the fracture is uneven. The Mohs hardness is ca. 3½. The density calculated using the empirical formula and unit-cell volume obtained from single-crystal X-ray diffraction data is 4.257 g cm–3.
In plane-polarised transmitted light, khrenovite is weakly pleochroic, with the following absorption scheme: X (pale yellow-brownish) > Y ≈ Z (nearly colourless). It is optically biaxial (+), α = 1.825(7), β = 1.834(7), γ = 1.845(7) (589 nm), 2Vmeas. = 80(10)° and 2Vcalc. = 85°. Dispersion of optical axes is strong, r > v. The optical orientation is presumably Y = b (by analogy with other alluaudite-group arsenates).
Raman spectroscopy
The Raman spectrum of khrenovite (Fig. 2) was obtained on a randomly orientated crystal using an EnSpectr R532 instrument (Dept. of Mineralogy, Moscow State University) with a green laser (532 nm) at room temperature. The output power of the laser beam was ~16 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 ~16 μm. The back-scattered Raman signal was collected with a 40× objective; signal acquisition time for a single scan of the spectral range was 1000 ms and the signal was averaged over 50 scans.
The Raman spectrum of khrenovite.
The Raman spectrum of khrenovite was interpreted according to Nakamoto (Reference Nakamoto1986). The bands with maxima at 961, 859 and 809 cm–1 correspond to As5+–O stretching vibrations of AsO43– anions and the band with maximum at 475 cm–1 and shoulder at 537 cm–1 is assigned to Fe3+–O stretching vibrations. Bands with frequencies lower than 450 cm–1 correspond to bending vibrations of AsO4 tetrahedra, Mn–O and Cu–O stretching vibrations and lattice modes. The absence of bands with frequencies higher than 900 cm–1 indicates the absence of groups with O–H, C–H, C–O, N–H, N–O and B–O bonds.
Chemical composition
Chemical studies of khrenovite were performed 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 10 μm beam diameter.
The chemical data are given in Table 1. Contents of other elements with atomic numbers >6 were below detection limits. The empirical formula calculated on the basis of 12 O atoms per formula unit (apfu) is (Na2.26K0.16Ca0.02Mn0.35Cu0.33Zn0.15Al0.02Fe3+1.62)Σ4.91(As2.98Si0.01V0.01)Σ3.00O12. The idealised, end-member formula, in accordance with the actual nomenclature of the alluaudite group (Hatert, Reference Hatert2019), is Na3Fe3+2(AsO4)3.
Chemical composition (wt.%) of khrenovite.
*Averaged for seven spot analyses; S.D. – standard deviation
X-ray crystallography and crystal structure determination
The powder X-ray diffraction (XRD) data for khrenovite (Supplementary Table S1) were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with cylindrical image plate detector (r = 127.4 mm) using Debye-Scherrer geometry, CoKα radiation (rotating anode with VariMAX microfocus optics), 40 kV, 15 mA and 12 min exposure. The 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). The unit-cell parameters refined from the powder data are: a = 12.248(6), b = 12.818(4), c = 6.657(4) Å, β = 112.97(4)° and V = 962(1) Å3. These values slightly differ from ones obtained from the single crystal data, probably due to some chemical variations from crystal to crystal.
Single-crystal XRD studies of khrenovite were carried out using an Xcalibur S diffractometer equipped with a CCD detector. A full sphere of three-dimensional data was collected. Intensity data were corrected for Lorentz and polarisation effects. The crystal structure of the new mineral was refined using the calciojohillerite structure (Pekov et al., Reference Pekov, Koshlyakova, Agakhanov, Zubkova, Belakovskiy, Vigasina, Turchkova, Sidorov and Pushcharovsky2021a) as the starting model with the SHELX software package (Sheldrick, Reference Sheldrick2015) to R = 0.0287 on the basis of 1906 independent reflections with I > 2σ(I). Crystal data, data collection information and structure refinement details for khrenovite are given in Table 2, coordinates, equivalent displacement parameters of atoms and bond-valence sums in Table 3, and selected interatomic distances in Table 4. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Crystal data, data collection information and structure refinement details for khrenovite.
Coordinates, equivalent displacement parameters (U eq, Å2) and bond-valence sums (BVS) for atoms in the structure of khrenovite.*
*Bond-valence parameters were taken from Gagne and Hawthorne (Reference Gagné and Hawthorne2015). Bond-valence sums were calculated taking into account cation distribution.
Selected interatomic distances (Å) in the structure of khrenovite.
Discussion
Khrenovite possesses the alluaudite-type structure (Fig. 3) based upon a three-dimensional framework composed of zig-zag chains of edge-sharing M(1)O6 and M(2)O6 octahedra connected with T(1)O4 and T(2)O4 tetrahedra. The M-octahedral chains consist of [M(2)2O10] dimers of distorted M(2)O6 octahedra connected via distorted M(1)O6 octahedra isolated from one another (Fig. 3a). T(1)O4 tetrahedra share all vertices with the M-centred octahedra to form the (010) heteropolyhedral layers (Fig. 3b) whereas each T(2)O4 tetrahedron shares three vertices with the MO6 octahedra of one layer and the fourth vertex with the octahedron of the adjacent layer, thus linking the layers to a three-dimensional framework. Two large-cation positions A(1) and A(2)’ are situated in channels running through the framework parallel to [001] (Fig. 3c). The M and A cation sites are labelled according to the scheme proposed by Hatert et al. (Reference Hatert, Keller, Lissner, Antenucci and Fransolet2000) and revisited by Krivovichev et al. (Reference Krivovichev, Vergasova, Filatov, Rybin, Britvin and Ananiev2013).
The crystal structure of khrenovite: (a) chain of [M(2)2O10] dimers connected via isolated M(1)O6 octahedra; (b) heteropolyhedral layer; (c) general view of the crystal structure projected along the c axis, with the unit cell outlined.
The first channel can be described as a chain of A(1)O6 cubes sharing common faces, and the second as a chain of A(2)’O8 polyhedra connected via common edges. Average A(1)–O and A(2)’–O distances are 2.411 and 2.744 Å, respectively. Both A(1) and A(2)’ sites are Na-dominant and, based on sizes of polyhedra, we assume admixtures of Mn in the A(1) site and K in the partially vacant A(2)’ site. The A(1) site occupancy was refined using scattering curves of Na vs Mn, and A(2)’ using the scattering curve of Na (observed scattering of 9.24 electrons per site). There were no maxima in the difference-Fourier map in the A(1)’ site, indicating the absence of Cu2+ in the channels.
In khrenovite the larger M(1)O6 octahedron has an average M–O distance of 2.263 Å, and the smaller M(2)O6 octahedron has an M–O distance of 2.042 Å. Based on the polyhedra sizes and data on hatertite (Krivovichev et al., Reference Krivovichev, Vergasova, Filatov, Rybin, Britvin and Ananiev2013), we assign Na, Mn and Zn to the M(1) site, and Fe3+ and Cu2+ to the M(2) site. The occupancy of the M(1) site was refined using scattering curves of Na, Mn and Zn with the sum of their occupancy factors fixed at 1.00, for the M(2) site the Fe-scattering curve was used (the e ref for this site is 26.08). Admixed Cu cations were added to M(2) similarly to cation distribution in hatertite and for consistency with chemical data.
Both T(1)O4 and T(2)O4 tetrahedra are As5+-centred, insignificant admixtures of V and Si were not considered during refinement.
The crystal chemical formula of the structurally studied khrenovite crystal can be written, taking into account electron microprobe data and ignoring minor admixtures of Al, Ca, Mg, Si, V, P and S, as: A (1)(Na0.93Mn0.07)A (2)’(Na0.71K0.06□0.23)M (1)(Na0.54Mn0.34Zn0.12)M (2)(Fe3+1.70Cu0.30)(TAsO4)3, which is close to the averaged composition obtained by electron microprobe (Table 1). Thus, the structural formula of the mineral, simplified to the species-defining constituents, is A (1)NaA (2)’Na M (1)NaM (2)Fe3+2(TAsO4)3 that gives the end-member formula Na3Fe3+2(AsO4)3.
Khrenovite is the first mineral of the alluaudite supergroup with Na prevailing in the M(1) position and the third, after alluaudite and yazganite, with trivalent cations prevailing in the M(2) site; in all other known natural alluaudite-type arsenates and phosphates, divalent cations dominate in M(1) and di- or trivalent cations in M(2) (Hatert, Reference Hatert2019). In addition, a synthetic alluaudite-type arsenate Na3In3+2(As3O4)3 is known (Lii and Ye, Reference Lii and Ye1997; Khorari et al., Reference Khorari, Rulmont and Tarte1997). It is the compound most similar to khrenovite in terms of crystal chemistry. A synthetic compound Na3Fe3+2(AsO4)3 with the alluaudite-type structure is unknown, though two other modifications of this arsenate have been synthesised, namely trigonal phase (R $\bar{3}$
c, a = 13.698 and c = 18.59 Å, d'Yvoire et al., Reference d'Yvoire, Bretey and Collin1988) with the structure very similar to that of yurmarinite Na7(Fe3+,Mg,Cu)4(AsO4)6 discovered in the same Arsenatnaya fumarole (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Lykova, Vigasina, Sidorov and Pushcharovsky2014a), and a cubic phase with the garnet structure (Ia $\bar{3}$
d, a = 12.25 Å, Ouerfelli et al., Reference Ouerfelli, Guesmi, Mazza, Zid and Driss2008). It is not excluded that significant admixtures of divalent cations in the M and/or A sites stabilise the alluaudite-like structure of khrenovite.
Acknowledgements
We thank Fernando Cámara and an anonymous referee for their valuable comments. This study was supported by the Russian Science Foundation, grant no. 19-17-00050. The technical support by the SPbSU X-Ray Diffraction Resource Center in the powder XRD study is acknowledged.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2022.64
Competing interests
The authors declare none.
