Petrovite, Na₁₀CaCu₂(SO₄)₈, a new fumarolic sulfate from the Great Tolbachik fissure eruption, Kamchatka Peninsula, Russia

Abstract

Petrovite, Na₁₀CaCu₂(SO₄)₈, is a new sulfate mineral discovered on the Second scoria cone of the Great Tolbachik fissure eruption. The mineral occurs as globular aggregates of tabular crystals up to 0.2 mm in maximal dimension, generally with gaseous inclusions. The empirical formula calculated on the basis of O = 32 is Na₆(Na_1.80K_0.20)_Σ2Na(Ca_0.82Na_0.06Mg_0.02)_Σ0.90(Cu_1.84Mg_0.16)_Σ2(Na_0.52□_0.48)_Σ1S_8.12O₃₂. The crystal-chemical formula is CuNa_6−2xCa_x(SO₄)₄, which, for x ≈ 0.5, results in the idealised formula Na₁₀CaCu₂(SO₄)₈. The crystal structure of petrovite was determined using single-crystal X-ray diffraction data; the space group is P2₁/c, a = 12.6346(8), b = 9.0760(6), c = 12.7560(8) Å, β = 108.75(9)°, V = 1385.1(3) ų, Z = 2 and R₁ = 0.051. There are one Cu and six Na sites, one of which is also occupied by the essential amount of Ca. The Cu atom forms five Cu–O bonds in the range 1.980–2.180 Å and two long bonds ≈ 2.9 Å resulting in the formation of the CuO₇ polyhedra, which share corners with SO₄ tetrahedra to form isolated [Cu₂(SO₄)₈]¹²⁻ clusters. The clusters are surrounded by Na sites, which provide their linkage into a three-dimensional framework. The Mohs’ hardness is 4. The mineral is biaxial (+), with α = 1.498(3), β_calc = 1.500, γ = 1.516(3) and 2V = 20(10) (λ = 589 nm). The seven strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.21(27)(110); 6.25(38)(102); 4.47(31)(212); 3.95(21)(30$\bar{2}$); 3.85(17)(121); 3.70(36)(202); and 3.65(34)(22$\bar{1}$). The mineral is named in honour of Prof Dr Tomas Georgievich Petrov (b. 1931) for his contributions to mineralogy and crystallography and, in particular, for the development of technology for the industrial fabrication of jewellery malachite.

Introduction

The fumarole activity of the scoria cones of the Great Tolbachik fissure eruption of 1975–1976 (Fedotov and Markhinin, 1983) and the Tolbachik fissure eruption 2012–2013 (Karpov et al., 2013) is accompanied by intensive exhalation mineralisation, in which sulfate mineralisation plays one of the leading roles. Nowadays, more than 25 new sulfates are described on the Tolbachik volcano, Kamchatka peninsula, Russia, most of which are found on the Second scoria cone of the Great fissure Tolbachik eruption (Pekov et al., 2020). Recently, many new sulfates have been discovered, including Na anhydrous sulfates without additional anions: ivsite, Na₃H(SO₄)₂ (Filatov et al., 2013), bubnovaite, K₂Na₈Ca(SO₄)₆ (Gorelova et al., 2016), puninite, Na₂Cu₃O(SO₄)₃ (Siidra et al., 2017), saranchinaite, Na₂Cu(SO₄)₂ (Siidra et al., 2018), itelmenite, Na₂CuMg₂(SO₄)₄ (Nazarchuk et al., 2018), belomarinaite, KNaSO₄ (Filatov et al., 2019), koryakite, NaKMg₂Al₂(SO₄)₆ (Siidra et al., 2020), natroapthitalite, Na₃K(SO₄)₂ (Shchipalkina et al., 2020), metathénardite, Na₂SO₄ (Pekov et al., 2019) and dobrovolskiite, Na₄Ca(SO₄)₃ (Shablinskii et al., 2020). Reviews on the fumarolic mineralisation of the Tolbachik scoria cones have been provided by Vergasova and Filatov (2012, 2016).

Petrovite has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2018-149b, Filatov et al., 2020). The mineral is named in honour of Prof. Dr. Tomas Georgievich Petrov (b. 1931), a former chairman of the Crystallogenesis Laboratory of the Department of Crystallography, St. Petersburg State University. The scientific activities of Tomas Petrov were devoted to the modelling of crystal growth of minerals (Petrov et al., 1969). He was the first to develop the industrial technology of fabrication of jewellery malachite back in 1977 (Petrov et al., 2013). Tomas Petrov is the author of the two-parameter Alphabet for the Coding of Structural–Chemical Information and RHAT-catalogue of modal mineral compositions of magmatic rocks (Petrov et al., 2012; Petrov, 2014). Type material is stored in the Saint-Petersburg State University Mineralogical Museum, University Emb. 7/9, St. Petersburg 199034, Russia, under catalogue number 1/19696.

Occurrence and association

The holotype material was collected in the fumarole located on the west side of the micrograben of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik fissure eruption that occurred in 1975–1976 on the Tolbachik volcano, Kamchatka, Far-Eastern Region, Russia (55°41'N, 160°14'E, 1200 m asl). The specimen was found in 2000. The temperature of the surface of the fumarole was ~200°C. The mineral is a product of exhalative fumarolic activity. The host rock for the mineral is a basalt scoria. The sample was placed into a glass tube, which was closed and waxed and kept under ambient conditions. When the glass tube was opened, no changes of sample were noted.

The graben at the place of the sample collecting was filled with the pyroclastic material (see Fig. 1). The presence of the gas stream was manifested by relatively high temperatures (up to 200°C), newly formed mineral phases and the development of an oxidation process.

Fig. 1. A view of the fissure of the micrograben on the west side of the Second Cinder cone of Great Tolbachik fissure eruption (photo taken in 1981).

The new mineral in the form of blue cryptocrystalline crusts enveloped fine pyroclastic material. Petrovite occurs in association with tiny black scales of tenorite in close intergrowths with transparent green particles of euchlorine, NaKCu₃O(SO₄)₃ and white particles of dobrovolskiite, Na₄Ca(SO₄)₃. The change of eruptive pyroclastic material was characterised by the wide development of oxidation processes accompanied by the formation of a fine hematite phase. The mineral is relatively stable, which allowed its detailed investigation. The crystallisation of the mineral most likely happened by direct precipitation from volcanic gases.

General appearance and physical properties

The mineral occurs as blue and green globular aggregates of tabular crystals up to 0.2 mm in maximal dimension, generally with gaseous inclusions (Figs 2 and 3). The streak is white and the lustre is vitreous. Fracture is conchoidal. No cleavage or parting was observed. The Mohs’ hardness is 4. The calculated density based on the empirical formula and single-crystal unit-cell parameters is 2.80 g/cm³.

Fig. 2. Scanning electron microscopy image of an individual grain of petrovite. Part of the petrovite sample stored in the Mineralogical Museum of St. Petersburg State University (1/19696).

Fig. 3. Blue cryptocrystalline crusts of petrovite enveloped by fine pyroclastic material. Parts of the petrovite sample stored in the Mineralogical Museum of St. Petersburg State University (1/19696).

The mineral is optically biaxial (+), with α = 1.498(3), β_calc = 1.500, γ = 1.516(3) and 2V = 20(10) (λ = 589 nm). No dispersion or pleochroism was observed. The Gladstone–Dale compatibility index (Mandarino, 1981) based on empirical formula and unit-cell parameters from powder X-ray diffraction data is calculated as (1 – K _P/K _C) = –0.0007 (superior).

Chemical composition

The chemical composition of petrovite was studied using a TESCAN “Vega3” electron microprobe equipped with an Oxford Instruments X-max 50 silicon drift energy-dispersive spectroscopy system, operated at 20 kV and 700 pA, with a beam size of 220 nm. Analytical results are given in Table 1. The data processing was done using Aztec software and an X-MAX-80 mm² detector (Institute of Volcanology and Seismology, Petropavlovsk-Kamchatsky, Russia). The empirical formula calculated on the basis of 32 O atoms per formula unit is Na_9.38Ca_0.82K_0.20Cu_1.84Mg_0.18S_8.12O₃₂. The formula that takes into account the site assignment is Na₃(Na,K)(Cu,Mg)(Na,Ca,Mg)(Na,□)(SO₄)₄. The idealised formula derived from the combination of chemical and structural studies (see below) is Na₁₀Cu₂Ca(SO₄)₈.

Table 1. Chemical composition of petrovite (wt.%).

S.D. – standard deviation

Powder X-ray diffraction

The powder X-ray diffraction data were collected using a Rigaku R-AXIS RAPID II (Gandolfi mode with CoKα) and handled using the domestic software (Britvin et al., 2017). Petrovite is monoclinic, P2₁/c, a = 12.6346(8), b = 9.0760(6), c = 12.7560(8) Å, β = 110.75(9)°, V = 1385.1(3) ų and Z = 2.

The measured and calculated powder-diffraction data are given in Table 2. The seven strongest lines are [d, Å (I, %) (hkl)]: 7.21(27)(110); 6.25(38)(102); 4.47(31)(212); 3.95(21)(30$\bar{2}$); 3.85(17)(121); 3.70(36)(202); and 3.65(34)(22$\bar{1}$).

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

The seven strongest lines are given in bold

Single-crystal X-ray diffraction

The single-crystal X-ray diffraction data were collected using a Bruker Smart APEX II diffractometer equipped with a CCD detector using MoKα radiation. A hemisphere of three-dimensional data was collected using a frame width of 0.5° in ω, with 60 s used to acquire each frame. The data were corrected for Lorentz, polarisation and background effects using the Bruker program APEX. A semi-empirical absorption-correction based on the intensities of equivalent reflections was applied in the SADABS program.

The crystal structure of petrovite was solved by charge flipping and refined on the basis of 2470 unique observed reflections using the JANA2006 program suite (Petříček et al., 2006). The observed isotropic displacement parameter of the Na6 site was too high (0.327 Ų), so the site was split into two mutually exclusive sites with reasonable displacement parameters. Crystallographic data and refinement parameters, atomic coordinates and isotropic displacement parameters, atomic anisotropic displacement parameters and selected interatomic distances are summarised in Tables 3–5. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Table 3. Crystal data, data collection information and structure refinement details for petrovite.

Table 4. Atomic coordinates and displacement parameters (Ų) for petrovite.

Table 5. Selected interatomic distances (Å) for petrovite.

Crystal structure

The crystal structure of petrovite belongs to a new structure type. It contains four symmetrically independent SO₄ tetrahedra with the average <S–O> bond lengths in the range 1.45–1.47 Å, in general agreement with the average value of 1.475 Å given for sulfate minerals (Hawthorne et al., 2000). The short S–O bonds in several tetrahedra can be explained by libration effects, typical for the crystal structures with statistical or dynamical disorder.

There are six Na and one Cu sites with different occupancies. The Cu atom forms five Cu–O bonds in the range 1.980–2.180 Å and two long bonds ≈ 2.9 Å resulting in the formation of the CuO₇ polyhedra with [5+2] coordination of Cu (Fig. 4a). The Cu site is predominantly occupied by Cu (89%) with the admixture of Mg (11%). This type of coordination geometry of Cu²⁺ cations is rather unusual, but was described previously in the crystal structures of saranchinaite, Na₂Cu(SO₄)₂ (Siidra et al., 2018; Kovrugin et al., 2019), the high pressure phase (II) CuGeO₃ (Yoshiasa et al., 2000) and Cu₃(Hbtc)(btc)(bpy)₂ (Nadeem et al., 2010). After saranchinaite, petrovite is the second mineral with heptacoordinated Cu²⁺.

Fig. 4. The crystal structure of petrovite: (a) CuO₇ polyhedron; (b) the [Cu₂(SO₄)₈]^12– cluster; (c) arrangement of the Cu₂(SO₄)₈ clusters in the structure; and (d) three-dimensional framework of the crystal structure.

The CuO₇ polyhedra share corners with SO₄ tetrahedra to form isolated [Cu₂(SO₄)₈]¹²⁻ anionic clusters shown in Fig. 4b, which are the fundamental building blocks for the structure of petrovite. The arrangement of the clusters in shown in Fig. 4c. The four Na sites, Na2, Na3, Na4 and Na5, are fully occupied by Na. In the Na1 site, the essential amount of Ca is present (ca. 47%), which explains its high bond-valence sum (see below). The Na6 site is only partially occupied (53%). The Na1, Na2, Na3, Na5, Na6 and Na6’ atoms are surrounded by six O atoms each, forming distorted octahedra with the average bond lengths equal to 2.29, 2.37, 2.57, 2.35, 2.49 and 2.48 Å, respectively. The Na4 site is coordinated by seven O atoms with the Na4−O bond lengths in the range 2.347–2.993 Å.

The petrovite structure can also be described as a three-dimensional framework, taking into account the essential occupancy of the Na1 site by Ca²⁺ cations. In this description, the [Cu₂(SO₄)₈]¹²⁻ clusters are linked by Na1O₆ octahedra, forming a porous three-dimensional framework (Fig. 4d) with cavities occupied by Na⁺ cations.

The bond-valence calculations were performed using empirical parameters taken from Brown and Altermatt (1985). The results are presented in Table 6. The high bond-valence sums of the S sites and the Na1 site are significantly overbonded, whereas the bond valence sum for Na3 is 0.61 valence units (vu). The deviation of the bond-valence sums from the expected values may be explained by the structural and positional disorder as well as the high ionic mobility of the Na atoms (see below).

Table 6. Bond-valence analysis (vu = valence units) for petrovite.

* The site is occupied by two cations

** The site is partially occupied

Discussion

Taking into account the essential admixture of Ca in the Na1 site, and the low occupancy of the Na6 site, the crystal chemical formula of petrovite can be written as Na_6−2xCa_xCu(SO₄)₄. For petrovite, x ≈ 0.5, which results in the idealised formula Na₁₀CaCu₂(SO₄)₈. However, at least theoretically, the x parameter may vary, resulting in different mineral species. For instance, for x = 0, 1, 1.5 and 2, the idealised formulae are Na₆Cu(SO₄)₄, Na₄CaCu(SO₄)₄, Na₆Ca₃Cu₂(SO₄)₈ and Na₂Ca₂Cu(SO₄)₄, respectively. The x = 2 member of the series, Na₂Ca₂Cu(SO₄)₄, is chemically similar to itelmenite, Na₂Mg₂Cu(SO₄)₄, assuming the Mg-for-Ca replacement. However, the structure types of petrovite and itelmenite are different.

From the chemical point of view, petrovite demonstrates chemical and structural similarities to saranchinaite, Na₂Cu(SO₄)₂. The three-dimensional [Cu(SO₄)₂]²⁻ anionic framework in saranchinaite is formed by corner-sharing SO₄ tetrahedra and CuO_n polyhedra with the Na⁺ cations located in the cavities. In the crystal structure of petrovite, there are [Cu₂(SO₄)₈]¹²⁻ clusters of corner-sharing Cu and S polyhedra.

The chemical formula of petrovite can be obtained from that of saranchinaite through the hypothetical reaction:

$$2{\rm N}{\rm a}_2{\rm Cu}\lpar {{\rm S}{\rm O}_4} \rpar _2\lpar {{\rm saranchinaite}} \rpar + {\rm CaS}{\rm O}_4 + 3{\rm N}{\rm a}_2\lpar {{\rm S}{\rm O}_4} \rpar \to {\rm N}{\rm a}_{10}{\rm CaC}{\rm u}_2\lpar {{\rm S}{\rm O}_4} \rpar_8\lpar {{\rm petrovite}} \rpar.$$

The reaction implies the incorporation of the ionic CaSO₄ and Na₂(SO₄) components into the largely covalent [Cu(SO₄)₂]²⁻ anionic framework with the reduction of the dimensionality of the Cu sulfate polymeric network from three (in saranchinaite) to one (in petrovite). The crystal−chemical behaviour of this kind is known as the dimensional reduction, which has been described previously in oxides and oxysalts (Alekseev et al., 2007; Krivovichev, 2009; Kovrugin et al., 2012).

It is of interest that both petrovite and saranchinaite contain unusual Cu²⁺O₇ polyhedra. According to Siidra et al. (2018), the presence of the Cu²⁺O₇ polyhedra is one of the potential reasons for the crystallisation of saranchinaite in the non-centrosymmetric space group P2₁. Obviously, this kind of reasoning cannot be applied to petrovite, which both contains Cu²⁺O₇ polyhedra and crystallises in the centrosymmetric space group P2₁/c.

Many chemical compounds and materials have been synthesised based on known mineral species (see, e.g. the recent work on synthetic saranchinaite by Siidra et al., 2018). Therefore, the analysis of the functional properties of new minerals is important from the viewpoint of their material science applications. As the Cu²⁺/Cu³⁺ redox potential may provide a high operating voltage (Sun et al., 2015), we have calculated the bond-valence energy landscape (BVEL) of petrovite using the BondStr software (Rodríguez-Carvajal, 2004). As a result, we have found that there are interconnected pathways for the Na⁺ migration in petrovite (Fig. 5), considering a percolation energy of 1.6 eV reported for the mobility of Na⁺ in polyanionic compounds (Boivin et al., 2017; Kovrugin et al., 2019). Thus, the petrovite structure type is promising as a cathode material. The theoretical capacity of petrovite based on the oversimplified formula Na₆Cu(SO₄)₄ (x = 0) is 274.5 mAh/g, but the low Cu content decreases the theoretical capacity to ~46 mAh/g.

Fig. 5. The crystal structure of petrovite with the scheme of Na migration pathways. Migration pathways are calculated in BondStr software (Rodríguez-Carvajal, 2004).