Bakakinite, Ca₂V₂O₇, a new mineral from fumarolic exhalations of the Tolbachik volcano, Kamchatka, Russia

Abstract

The new mineral bakakinite, ideally Ca₂V₂O₇, was found in the high-temperature (not lower than 500°C) exhalations of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with anhydrite, svabite, pliniusite, schäferite, berzeliite, diopside, hematite, powellite, baryte, fluorapatite, calciojohillerite, ludwigite, magnesioferrite, anorthite, titanite and esseneite. Bakakinite forms flattened crystals up to 30 × 5 μm, typically distorted. The mineral is transparent, colourless or pale yellow, with strong vitreous lustre. Electron microprobe analysis gave (wt.%): CaO 37.04, SrO 0.26, SiO₂ 0.16, P₂O₅ 1.48, V₂O₅ 49.47, As₂O₅ 10.85, SO₃ 0.35, total 99.61. The empirical formula calculated on the basis of 7 O apfu is (Ca_1.99Sr_0.01)_Σ2.00(V_1.64As_0.28P_0.06Si_0.01S_0.01)_Σ2.00O₇. The D_calc is 3.463 g cm^–3. Bakakinite is triclinic, P$\bar{1}$, unit-cell parameters are: a = 6.64(2), b = 6.92(2), c = 7.01(2) Å, α = 86.59(7), β = 63.77(7), γ = 83.47(6)°, V = 287.0(5) ų and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 4.647(27)(111, 0$\bar{1}$1), 3.138(76)(002), 3.103(100)(120, 121), 3.027(20)(021), 2.960(81)(200), 2.158(19)(031, 302), 1.791(16)(320), 1.682(16)(114) and 1.584(17)(1$\bar{3}$3, 403). Bakakinite is a natural analogue of synthetic Ca₂V₂O₇. The mineral is named in honour of the outstanding Russian crystallographer and crystal chemist Vladimir Vasilievich Bakakin (born 1933).

Introduction

Divanadate minerals are not numerous in Nature. They are mainly represented by pyrovanadates, the oxysalts with anionic [V₂O₇]^4– groups isolated from each other and composed of two V⁵⁺-centred tetrahedra which share a bridging O atom. Among a dozen such minerals only volborthite, known since the 1830s, is relatively widespread. Typically, pyrovanadates (and natural divanadates in general) are hydrous minerals formed in supergene environments or in late, low-temperature hydrothermal assemblages. Except for volborthite Cu₃V₂O₇(OH)₂⋅2H₂O (Kashaev and Bakakin, 1968; Vladimirova et al., 2021 and references therein) and the related minerals martyite Zn₃V₂O₇(OH)₂⋅2H₂O (Kampf and Steele, 2008) and karpenkoite Co₃V₂O₇(OH)₂⋅2H₂O (Kasatkin et al., 2015), there are fianelite Mn₂(V,As)₂O₇⋅2H₂O (Brugger and Berlepsch, 1996), engelhauptite KCu₃(V₂O₇)(OH)₂Cl (Pekov et al., 2015), mesaite CaMn²⁺₅(V₂O₇)₃⋅12H₂O (Kampf et al., 2017) and donowensite CaFe³⁺₂(V₂O₇)₂⋅3H₂O (Kampf et al., 2022). For pintadoite the formula Ca₂V₂O₇⋅9H₂O is suggested, however, this mineral is poorly studied and has the IMA status Q – questionable (Pasero, 2023). The H-free natural pyrovanadates are represented by chervetite Pb₂V₂O₇ (Bariand et al., 1963; Shannon and Calvo, 1973) and two modifications of Cu₂V₂O₇ – blossite and ziesite (Hughes and Birnie, 1980; Robinson et al., 1987; Hughes and Brown, 1989; Krivovichev et al., 2005); kainotropite Cu₄Fe³⁺O₂(V₂O₇)(VO₄) is the only known mineral containing both pyrovanadate (V₂O₇)^4– and orthovanadate (VO₄)^3– anions (Pekov et al., 2020). Blossite, ziesite and kainotropite are endemics of volcanic fumaroles, as well as the new anhydrous calcium vanadate bakakinite (Cyrillic: бакакинит) described in the present paper. Bakakinite has the ideal, end-member formula Ca₂V₂O₇ and is a divanadate in terms of chemistry, however, from the crystal chemical viewpoint, it is not a pyrovanadate. This mineral contains more complex vanadate anionic groups – isolated tetramers [V₄O₁₄]^8– built from 4-fold (tetrahedra) and 5-fold V⁵⁺-centred polyhedra.

The new mineral is named in honour of the outstanding Russian crystallographer and crystal chemist Professor Vladimir Vasilievich Bakakin (born 1933) who works in Nikolaev Institute of Inorganic Chemistry of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk. Prof. Bakakin made a significant contribution to the field of structural mineralogy. In particular, Kashaev and Bakakin (1968) first determined the crystal structure of volborthite and showed that this mineral is a pyrovanadate and not an ortho-vanadate as it was assumed before.

Both the new mineral and its name (symbol Bkkn) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2022-046, Pekov et al., 2022a). The holotype specimen is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow with the catalogue number 98012.

Occurrence and general appearance

The specimens with the new mineral were collected by us in July 2021 from the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka peninsula, Far-Eastern Region, Russia, 55°41’N, 160°14’E, 1200 m elevation. This active fumarole that contains very rich and diverse high-temperature sublimate mineralisation has been described by Pekov et al. (2018) and Shchipalkina et al. (2020).

Bakakinite was found in several open pockets at the deepest (depths of 3–4 m under the day surface) and hottest zone of Arsenatnaya. The temperatures measured using a chromel–alumel thermocouple in these pockets during sampling varied from 430 to 490°C. We believe that bakakinite crystallised at temperatures not lower than 500°C. It can be deposited directly from hot gas as a volcanic sublimate, however, in our opinion, it seems more probably that the mineral was formed as a result of the interaction between fumarolic gas and basalt scoria. The latter could be a source of calcium which has very low volatility in such post-volcanic systems (Symonds and Reed, 1993).

Bakakinite occurs, sporadically in significant amounts, as a constituent of polymineralic exhalation incrustations together with anhydrite, svabite, pliniusite, schäferite, berzeliite, diopside, hematite and powellite. Minor amounts of baryte, fluorapatite, calciojohillerite, ludwigite, magnesioferrite, anorthite, titanite and esseneite also occur in this mineral assemblage. Cavernous polymineralic aggregates containing up to 25 vol.% bakakinite form thin (usually not thicker than 0.02 mm) crusts up to several cm² in area on anhydrite crystal crusts that cover basalt scoria altered by fumarolic gas to aggregates mainly consisting of diopside and hematite.

Bakakinite forms flattened crystals typically not larger than 10 μm, rarely up to 30 μm across and up to 5 μm thick. Some crystals are well-formed and complicated in shape (Fig. 1a), however, commonly bakakinite crystals are crude and distorted. The crystals, even with a well-developed outer shape, have skeletal (Fig. 1b) and/or blocky characters. Epitactic bakakinite overgrowths on svabite were observed (Fig. 1b). Clusters of bakakinite crystals are up to 0.1 mm across; in such clusters, bakakinite is intimately intergrown with other minerals, usually with anhydrite and svabite–pliniusite series members.

Figure 1. Morphology of crystals and aggregates of bakakinite and its relations with associated minerals: (a) flattened crystals of bakakinite (1) on pliniusite (2) with crystals of powellite (3) and diopside (4); (b) epitactic overgrowths of bakakinite skeletal crystals (1) on svabite (5) in association with anhydrite crusts (6) and schäferite crystals (7). Scanning electron microscopy image – back-scattered electron mode.

Physical properties and optical data

Bakakinite is transparent, colourless or pale yellow, with a white streak and strong vitreous lustre. It is brittle, cleavage or parting was not observed. The fracture is uneven (observed under the scanning electron microscope). The density value calculated using the averaged empirical formula is 3.463 g cm^–3.

The new mineral is transparent and optically anisotropic, however, its optical studies were carried out in reflected light due to high refractive indices. The mean refractive index calculated on the basis of the Gladstone–Dale equation is 1.93.

Under the microscope in reflected light, bakakinite is grey; pleochroism was not observed. Bireflectance is weak, ΔR = 1.2% (589 nm). Anisotropy is very weak; internal reflections were not observed. The reflectance values measured in air using the SiC standard (Zeiss, No. 545) are given in Table 1.

Table 1. The reflectance data of bakakinite.

The values for wavelengths (λ) recommended by the IMA Commission on Ore Mineralogy are marked in boldtype.

Chemical composition

The composition of bakakinite was studied by electron microprobe 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 standards used are listed in Table 2. Contents of other elements with atomic numbers >6 were below detection limits.

Table 2. Chemical composition of bakakinite (in wt.%).

S.D. = standard deviation

The chemical composition of bakakinite in wt.% is given in Table 2. The empirical formula calculated on the basis of 7 O atoms per formula unit is (Ca_1.99Sr_0.01)_Σ2.00(V_1.64As_0.28P_0.06Si_0.01S_0.01)_Σ2.00O₇. The simplified formula is Ca₂(V,As)₂O₇. The ideal, end-member formula of bakakinite is Ca₂V₂O₇ which requires CaO 38.14, V₂O₅ 61.86, total 100 wt.%.

X-ray crystallography

Attempts to obtain single-crystal X-ray diffraction (XRD) data for bakakinite were unsuccessful due to the small size and low quality (skeletal and/or blocky character) of crystals.

Powder XRD data (Table 3) 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 exposure of 15 min. 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).

Table 3. Powder X-ray diffraction data (d in Å) of bakakinite and synthetic Ca₂V₂O₇.

* The powder X-ray diffraction pattern also contains lines with d = 3.492 [a], 2.904 [s], 2.842 [a,s], 2.822 [s] and 1.875 [a,s] Å which are overlapped reflections of bakakinite and anhydrite [a], svabite [s] or both these minerals [a,s]. The strongest reflections are marked in boldtype.

Despite the absence of single-crystal XRD data, it is clear that bakakinite is a natural analogue of a well-known synthetic calcium divanadate, Ca₂V₂O₇, of which the crystal structure was reported by Trunov et al. (1983) and Tong et al. (2011). Intimate intergrowths of bakakinite with other minerals hampered the Rietveld refinement of the crystal structure, however, its structural identity with synthetic Ca₂V₂O₇ found from powder XRD data is clear: see Discussion and Tables 3 and 4. Bakakinite is triclinic, with space group P $\bar{1}$. The unit-cell parameters are reported in Table 4.

Table 4. Crystal data of bakakinite and synthetic Ca₂V₂O₇ and Ca₂As₂O₇.

Raman spectroscopy

The Raman spectrum of bakakinite (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 ~6 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 mm^–1 and spectral resolution was 6 cm^–1. The diameter of the focal spot on the sample was ~10 μm. The back-scattered Raman signal was collected with a 60× objective; signal acquisition time for a single scan of the spectral range was 1000 ms and the signal was averaged over 30 scans.

Figure 2. The Raman spectrum of bakakinite.

The assignment of bands in the Raman spectrum of bakakinite can be performed based on the data reported by Griffith and Wickins (1966), Nakamoto (1986), Hardcastle and Wachs (1991), Russu (2008) and Chong et al. (2019) and taking into account the presence of a complex anionic group [V₄O₁₄]^4– in Ca₂V₂O₇ (Trunov et al., 1983; Tong et al., 2011), a synthetic end-member analogue of the mineral.

The Raman spectrum of synthetic Ca₂V₂O₇ (= Ca₄[V₄O₁₄]: for the structure data see below) reported by Russu (2008) is similar to the spectrum of bakakinite in its general pattern. However, unlike this synthetic vanadate, bakakinite contains admixed arsenic which partially substitutes vanadium. We cannot clearly identify the bands corresponding to As⁵⁺–O vibrations due to overlap with bands of V⁵⁺–O vibrations (Nakamoto, 1986), however, it is necessary to take into account the effect of the As admixture on the Raman spectrum. This probably results in the broadening of some bands in comparison with the spectrum of synthetic Ca₂V₂O₇ (Russu, 2008), but also other features. The position of an intense sharp band corresponding to ^IVM–O–^VM (superscript Roman numerals mean coordination numbers) stretching vibrations, which is at 944 cm^–1 in a pure vanadate synthetic compound (M = V⁵⁺) and at 928 cm^–1 in bakakinite (M = V⁵⁺ > As⁵⁺). A band at 928 cm^–1 is related to ν(^VV⁵⁺–O–^IVAs⁵⁺) and ν(^VV⁵⁺–O–^IVV⁵⁺), wide bands with maxima at 872, 845 cm^–1 and at 789 and 775 cm^–1 are assigned to symmetric (ν₁) and asymmetric (ν₃) vibrations, respectively, in (VO₄)^3–, [V₂O₈]^6– and (AsO₄)^3– groups. A strong band at 695 cm^–1 and a weak band at 527 cm^–1 correspond to vibrations of the (V–O–V) and (V–O–As), and the low-frequency shoulder of a band at 695 cm^–1 may be connected with vibrations in [V₂O₈]^6–. The group of bands in the region of 480–300 cm^–1 corresponds to overlapping split bending modes δ₂ and δ₄ of (VO₄)^3– and (AsO₄)^3– tetrahedra and bending modes in [V₂O₈]^6– groups. The bands with frequencies below 300 cm^–1 are assigned to translational modes of calcium cations (Ca–O) and lattice modes.

The Raman spectrum of bakakinite was obtained from a small (20 μm) flattened grain which overgrows pliniusite Ca₅(VO₄)₃F. The strongest band in the Raman spectra of pliniusite occurs at 868–873 cm^–1 and the next in intensity band is situated at 350–356 cm^–1 (Pekov et al., 2022b). In the above-described bakakinite spectrum, we observe a strong broad band at 872 cm^–1 and a weak low-frequency shoulder of a broad band with maximum at 386 cm^–1. We cannot exclude that the broadening of these bands may be a result of, in addition to the effect of admixed As⁵⁺, the overlap of spectral bands of bakakinite (prevailing) and pliniusite (admixed).

Discussion

The ideal formula of bakakinite is Ca₂V₂O₇. However, all electron-microprobe analyses demonstrate some arsenic admixture. Synthetic Ca₂V₂O₇ (Trunov et al., 1983; Tong et al., 2011) and Ca₂As₂O₇ (Pertlik, 1980) are not isotypic. They significantly differ from one another in symmetry, unit-cell metrics (Table 4) and crystal structure (Fig. 3). In both structures the layers of Ca cations are connected via anionic units built by V⁵⁺- or As⁵⁺-centred polyhedra. Noteworthy, in the papers on synthetic Ca₂V₂O₇, the descriptions of Ca-centred polyhedra are slightly different: Trunov et al. (1983) characterised them as nine- and eight-fold polyhedra whereas Tong et al. (2011) described both crystallographically non-equivalent Ca polyhedra as nine-fold ones. The analysis of interatomic Ca–O distances shows that strongly elongated Ca–O distances (more than 2.9 Å) were included in the coordination spheres of Ca in both papers. For clarity and better comparison with Ca₂As₂O₇, we omit strongly elongated Ca–O distances in the description below and in drawings given in Fig. 3. Following this approach, Ca cations in Ca₂V₂O₇ have distorted octahedral and seven-fold oxygen coordination whereas in Ca₂As₂O₇ calcium cations are located only in distorted octahedra. In both structures edge-sharing Ca-centred polyhedra form layers, but these layers in Ca₂V₂O₇ and Ca₂As₂O₇ strongly differ topologically (Figs 3c,d). The anionic units in these compounds are also quite different in arrangement and coordination of V and As atoms. In Ca₂As₂O₇, As⁵⁺ has only tetrahedral coordination whereas V⁵⁺ cations in Ca₂V₂O₇ centre tetrahedra and five-fold polyhedra (distorted trigonal bipyramids). In the structure of Ca₂V₂O₇ we see a linear anionic cluster, tetramer [V₄O₁₄]^8–, which is built of two edge-connected distorted trigonal bipyramids VO₅ (core of the cluster) and two VO₄ tetrahedra connected with these trigonal bipyramids via common vertices (‘wings’ of the cluster) (Fig. 3e). For this reason, Tong et al. (2011) wrote the formula of this vanadate as Ca₄V₄O₁₄ rather Ca₂V₂O₇. In the structure of Ca₂As₂O₇, arsenate tetrahedra form pyrogroups [As₂O₇]^4–, isolated from each other (Fig. 3f), i.e. it is a pyroarsenate.

Figure 3. The crystal structures of synthetic analogue of bakakinite, Ca₂V₂O₇ (left column: drawn after Trunov et al. (1983); the unit cell is outlined) and Ca₂As₂O₇ (right column: drawn after Pertlik (1980); the unit cell is outlined): general view (a, b), the layers of Ca-centred polyhedra (c, d) and the arrangement of anionic [V₄O₁₄]^8– tetramer units (e) and [As₂O₇]^4– pyrogroups (f).

The presence of pentacoordinated V⁵⁺ in Ca₂V₂O₇ prevents its isotypism with the formula analogues, numerous natural and synthetic compounds Me ₂T ₂O₇ (T = P, As and Si) in which T has only tetrahedral coordination. At the same time, our electron-microprobe analyses show that bakakinite contains a distinct As admixture (Table 2) and, thus, we suggest that Ca₂V₂O₇ and Ca₂As₂O₇ can form a solid-solution series, at least, in the V-dominant region. The structure determinations were performed only for synthetic end-members (Pertlik, 1980; Trunov et al., 1983; Tong et al., 2011) and, thus, we do not know which chemical composition in this hypothetic series corresponds to the point of transition from the Ca₂V₂O₇ structure type (Fig. 3a) to the Ca₂As₂O₇ type (Fig. 3b). Unfortunately, we also know nothing on the distribution of V and admixed As between structure positions in bakakinite. If vanadium and arsenic occupy the sites with different coordination, then the increase of As⁵⁺ content could result in the formation of the hypothetical V–As-ordered bakakinite-type compound with the ideal formula Ca₂VAsO₇.

In the powder XRD data, bakakinite and synthetic Ca₂V₂O₇, being very close to one another (Table 3), differ strongly from Ca₂As₂O₇. In particular, the characteristic, low-angle region (d > 2.5 Å) of the powder XRD pattern of Ca₂As₂O₇ contains strong lines with d = 3.37, 3.34 and 2.77 Å and intensities I = 53, 100 (the strongest reflection of Ca₂As₂O₇) and 35%, respectively (Pertlik, 1980), which are absent in the powder XRD diagrams of bakakinite and synthetic Ca₂V₂O₇ (Table 3).

Of note, the crystal structure, unit-cell metrics and powder XRD pattern of another natural divanadate with large cations, chervetite Pb₂V₂O₇ (Shannon and Calvo, 1973) are quite different from those of both Ca₂V₂O₇ and Ca₂As₂O₇. All known polymorphs of Cu₂V₂O₇ including the minerals blossite and ziesite (Krivovichev et al., 2005), also possess quite different structures.