A green tourmaline sample from the Tzarevskoye uranium–vanadium deposit, close to the Srednyaya Padma deposit, Lake Onega, Karelia Republic, Russia, has been found to be the second world-occurrence of Cr-rich vanadio-oxy-dravite in addition to the Pereval marble quarry, Sludyanka crystalline complex, Lake Baikal, Russia, type-locality. From the crystal-structure refinement and chemical analysis, the following empirical formula is proposed: X(Na0.96K0.02□0.02)Σ1.00Y(V1.34Al0.68Mg0.93Cu2+0.02Zn0.01Ti0.01)Σ3.00Z(Al3.19Cr1.36V0.03Mg1.42)Σ6.00(TSi6O18)(BBO3)3V(OH)3W[O0.60(OH)0.23F0.17]Σ1.00. Together with the data from the literature, a compositional overview of Al–V–Cr–Fe3+-tourmalines is provided by using Al–V–Cr–Fe3+ diagrams for tourmaline classification. These diagrams further simplify the tourmaline nomenclature as they merge the chemical information over the octahedrally-coordinated sites (Y and Z) by removing the issues of uncertainty associated with cation order–disorder across Y and Z. Results show the direct identification of tourmalines by using the chemical data alone.
The tourmaline-supergroup minerals are chemically complex borosilicates. They are widespread in the Earth's crust, occurring in sedimentary rocks, granites and granitic pegmatites and in low-grade to ultrahigh-pressure metamorphic rocks (e.g. Dutrow and Henry, 2011). In accordance with Henry et al. (2011), the general formula of tourmaline can be written as XY3Z6T6O18(BO3)3V3W, where X = Na+, K+, Ca2+ and □ (□ = vacancy); Y = Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+, Mn2+ and Li+; Z = Al3+, Fe3+, Cr3+, V3+, Mg2+ and Fe2+; T = Si4+, Al3+ and B3+; B = B3+, V = OH1– and O2– and W = OH1–, F1– and O2–. The (non-italicised) letters X, Y, Z, T and B represent groups of cations accommodated at the X, Y, Z, T and B crystallographic sites (identified with italicised letters); the letters V and W represent groups of anions accommodated at the O(3) and O(1) crystallographic sites, respectively. The H atoms occupy the H(3) and H(1) sites, which are related to O(3) and O(1), respectively (e.g. Bosi, 2013; Gatta et al., 2014).
Due to their highly variable chemical composition and refractory behaviour, tourmaline is considered a very useful indicator of geological processes in igneous, hydrothermal and metamorphosed systems (Dutrow and Henry, 2011; van Hinsberg et al.,2011; Ahmadi et al., 2019; Sipahi, 2019) and able to record and preserve the chemical composition of their host rocks.
Vanadium and Cr-bearing hydroxyl- and oxy-tourmaline species have been described widely in the literature (Cossa and Arzruni, 1883; Badalov, 1951; Bassett, 1953; Snetsinger, 1966; Peltola et al., 1968; Jan et al., 1972; Dunn, 1977; Nuber and Schmetzer, 1979; Foit and Rosenberg, 1979; Rumyantseva, 1983; Gorskaya et al., 1984, 1987; Reznitskii et al., 1988; Hammarstrom, 1989; Kazachenko et al., 1993; Reznitskii and Sklyarov, 1996; Ertl et al., 2008; Arif et al., 2010; Lupulescu and Rowe, 2011; Rozhdestvenskaya et al., 2011; Cempírek et al., 2013; Vereshchagin et al., 2014). Currently, they are known from several localities: Sludyanka (Slyudyanka) crystalline complex, Lake Baikal, Russia; Onega region, Central Karelia, Russia; Primorye, Far eastern Russia; Balmat, St. Lawrence County, New York, USA; Silver Knob deposit, Mariposa County, California, USA; Nausahi deposit, Orissa, India; Outokumpu deposit, Finnish North Karelia, Finland; Mingora and Gujar Kili mines, Swat, Pakistan; Alpurai, Pakistan; Shabrovskoe ore deposit, Middle Urals, Russia; Syssertox Dach, Ural Mountains, Russia; Umba Valley, Tanga Province, Tanzania; Kwal District, Kenya; Amstall, Lower Austria, Austria; and Bítovánky, Czech Republic. Also, fluor-rich tourmalines characterised by V and Cr have been reported in the literature with a strong positive relation between F and Cr, but with F contents less than 0.5 atoms per formula unit (Bosi et al., 2017b).
Oxy-tourmalines rich in both V and Cr are unusual minerals and occur almost exclusively in metamorphosed V- and Cr-enriched host rocks such as sulfide-rich black shales, graphite quartzites and calcareous metasediments (Snetsinger, 1966; Kazachenko et al., 1993; Bačik et al., 2011; Cempírek et al., 2013). Most oxy-tourmalines with dominant V and/or Cr (V2O3 or Cr2O3 > 9 wt.%) were found in the Sludyanka crystalline complex, Lake Baikal, Russia (Bosi et al., 2004, 2012, 2013a,b; Reznitskii et al., 2014; Bosi et al., 2014a,b, 2017a,b). Among these is a vanadio-oxy-dravite, ideally NaV3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a rare tourmaline recently described by Bosi et al. (2014a).
The sample studied was found in the Tzarevskoye uranium–vanadium deposit, close to the Srednyaya Padma deposit, Zaonezhye Peninsula, Lake Onega, Karelia Republic, Northern Region, Russia. It is the first occurrence of V-dominant, Cr-rich oxy-tourmaline in Karelia and the second world-occurrence in addition to the Pereval marble quarry (Sludyanka) type-locality. In this work, we describe this tourmaline and provide a compositional overview of Al–V–Cr–Fe3+-tourmalines.
The Srednyaya Padma mine is the largest of the deposits from vanadium, uranium and precious metals of the Onega region and has abnormally high concentrations of gold, palladium, platinum, copper and molybdenum. It is concentrated in the Onega epicratonic trough, which is filled with volcano–sedimentary rocks of Lower Proterozoic age (organic carbon-rich schists, sandstones, dolomites and tuffites prevail) (Boitsov, 1997). The ore mineralisation is located in the albite–mica–carbonate metasomatites upon the Proterozoic aleorolites and schists (Boitsov, 1997). The distribution of these ore-bearing metasomatites is controlled by axial faults and shear zones. In fact the Srednyaya Padma deposit is located in zones of fold-fracture dislocations, which are represented by systems of N–W oriented anticlines with interior portions of the anticlines composed of dolomites and exterior portions composed of schists. The orebodies are situated in steeply-dipping fracture zones in siltstones and in some wedge-shaped zones at the contact with the schungite schists.
The Srednyaya Padma deposit is 3 km long and consists of two orebodies with different amounts of V and U (Boitsov, 1997): the first orebody has a length of 1060 m, thickness 40–50 m, with an average V2O5 and UO2 content of ~3 wt.% and 0.13 wt.%, respectively, whereas the second has a length of 1840 m, vertical size of 100–450 m and an average content of V2O5 and UO2 of ~2.4 wt.% and 0.11 wt.%, respectively.
In accordance with Borozdin et al. (2014), the main minerals of the ore metasomatites are V- and Cr-micas (roscoelite, chromceladonite and Cr-bearing micas of the phengite series), which make-up ~26% of all ores, carbonate marbles (dolomite and calcite), with ~21%, feldspars (albite, which usually prevails over other minerals with a mean content of ~37%), minor V–Cr alkaline pyroxenes (natalyite and Cr-bearing aegirine) and Cr-rich tourmalines.
The tourmaline studied was found in the Tzarevskoye uranium–vanadium deposit, ~14 km from the well-known Srednyaya Padma deposit. The Tzarevskoye deposit is situated in the anticline zone with cores of metamorphosed terrigenous-carbonate rocks in the cores and intensely brecciated, mylonitised and foliated metamorphosed siltstones at the margins of the folds. The tectonic activity was accompanied by hydrothermal–metasomatic and hypogene processes (Boitsov, 1997). The tourmaline sample occurs in micaceous metasomatites, associated with roscoelite, Cr-bearing phengite micas, quartz and dolomite. It forms dark-green to black pyramidal crystals up to 0.1 mm. A similar mineralogical association was observed for the chromium-dravite from the Velikaya Guba gold–copper–uranium occurrence (see below).
Electron-microprobe analyses of the present sample were obtained by a wavelength-dispersive spectrometer (WDS mode) using a CAMECA SX50 instrument at the Istituto di Geologia Ambientale e Geoingegneria (CNR of Rome, Italy), operating at an accelerating potential of 15 kV and a sample current of 15 nA, with a 10 μm beam diameter. Minerals and synthetic compounds were used as standards as follows: wollastonite (Si and Ca), magnetite (Fe), rutile (Ti), corundum (Al), karelianite (V), fluorphlogopite (F), periclase (Mg), jadeite (Na), orthoclase (K), rhodonite (Mn), metallic Cr, Ni, Cu and Zn. Vanadium and Cr concentrations were corrected for interference from the TiKβ and VKβ peaks, respectively. The PAP matrix correction procedure (Pouchou and Pichoir 1991) was applied to reduce the raw data. The results, which are summarised in Table 1, represent mean values of 4 spot analyses. In accordance with Pesquera et al. (2016), the Li2O content was assumed to be insignificant as MgO > 2 wt.% is contained in the sample studied. Calcium, Mn, Fe and Ni were below the detection limits (0.03 wt.%).
Single-crystal structural refinement (SREF)
A pale green crystal fragment (0.037 mm × 0.042 mm × 0.052 mm) of the sample was mounted on an Oxford Gemini R Ultra diffractometer equipped with a Ruby CCD area detector at CrisDi (Interdepartmental Centre for the Research and Development of Crystallography, Turin, Italy) with graphite-monochromatised MoKα radiation from a fine-focus sealed X-ray tube. The sample-to-detector distance was 5.3 cm. A total of 222 exposures (step = 1°, time/step = 48–478 s) with an average redundancy of ~6 was used. Data were integrated and corrected for Lorentz and polarisation background effects, using CrysAlisPro (Agilent Technologies, Version 220.127.116.11, release 27-06-2012 CrysAlis171.36.24). Refinement of the unit-cell parameters was based on 2304 measured reflections. The data were corrected for absorption using the multi-scan method (Scale3 ABSPACK). No violations of R3m symmetry were noted.
Structural refinement was done with the SHELXL-2013 program (Sheldrick, 2013). Starting coordinates were taken from Bosi et al. (2014a). Variable parameters were: scale factor, atomic coordinates, site scattering values and atomic-displacement factors. Attempts to refine the extinction coefficient yielded values within its standard uncertainty, thus it has not been refined. Neutral scattering factors were used for the cations and a fully ionised scattering factor for the oxygen atoms. In detail, the occupancy of the X site was modelled by using the Na scattering factor, the Y site Mg and V scattering factors, and the Z site using Al and Cr scattering factors. The T and B sites were modelled, respectively, with Si and B scattering factors and with a fixed occupancy of 1, because refinement with unconstrained occupancies showed no significant deviations from this value. Three full-matrix refinement cycles with isotropic-displacement parameters for all atoms were followed by anisotropic cycles until convergence was attained. No significant correlations over a value of 0.7 between the parameters were observed at the end of refinement. Table 2 lists crystal data, data-collection information, and refinement details; Table 3 gives the fractional atomic coordinates, site occupancies and displacement parameters; Table 4 gives selected bond distances. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Determination of atomic fractions
In agreement with the SREF results, the B content was assumed to be stoichiometric in the sample studied (B3+ = 3.00 atoms per formula unit, apfu). In fact, both the site-scattering results and the bond lengths of B and T are consistent with the B site fully occupied by B3+ and no amount of B3+ at the T site. The (OH) content can then be calculated by charge balance with the assumption (T + Y + Z) = 15.00 apfu and 31 anions. The atomic fractions were calculated on these assumptions (Table 1). The excellent match between the number of electrons per formula unit (epfu) derived from chemical and structural analysis supports this procedure: 268.90 and 267.95 epfu, respectively.
Determination of site populations and mineral formula
From a classification viewpoint (Henry et al., 2011), this formula corresponds to a Fe3+-rich, V-bearing chromium-dravite (hydroxy-species) belonging to alkali subgroup 1. Compared to the sample studied, significant chemical differences at the octahedrally coordinated sites can be noted between the tourmalines from Karelia: the studied oxy-species (WO = 0.60 apfu) has Mg = 2.05 apfu, Al = 3.86 apfu, V = 1.37 apfu and Cr = 1.38 apfu, whereas the chromium-dravite hydroxy-species (WOH = 0.77 apfu) has Mg = 2.57 apfu, Al = 0.37 apfu, V = 0.22 apfu, Cr = 4.71 apfu and Fe3+ = 1.18 apfu. These differences lead to the following (Y + Z) charge arrangements following Bosi et al. (2019b), Y+Z(R2+2R3+7) for the oxy-species and Y+Z(R2+3R3+6) for the hydroxy-species, which should be reflected in two different compositional diagrams for their classification. Recently, Henry and Dutrow (2018) proposed two ternary diagrams for the Al–V–Cr subsystem and Al–Cr–Fe3+ subsystem of the Al–V–Cr–Fe3+ quaternary system to classify oxy-tourmalines (WO2– > 0.5 apfu). It is worth noting that this diagram includes trivalent cations at both the Y and Z sites to remove issues of uncertainty associated with order–disorder across these sites.
In order to better show the chemical variability of oxy-tourmalines in the Al–V–Cr–Fe3+ quaternary system, we have merged the diagrams Al–Cr–V and Al–Cr–Fe3+ through the edge Al–Cr (Fig. 1). We made these ternaries because no tourmaline rich in both V and Fe3+ has been found so far. With regard to the classification of hydroxy/fluor-tourmalines (OH+F > 0.5 apfu at W), the ternary diagram for the Al–Fe3+–Cr subsystem (Fig. 2) of the Al–V–Cr–Fe3+ quaternary system is used (Henry et al., 2011). This diagram is based on occupancy of the Z site obtained from the tourmaline ordered formula, which also removes issues of uncertainty associated with order–disorder across the Y and Z sites as may occur for example between Fe2+–Al in schorl (Andreozzi et al., 2020). In other words, the use of the diagrams in Figs 1 and 2 is equivalent to classifying tourmalines using only the chemical information of the Y and Z sites.
The plotted data in these diagrams (for a total 109 data sets) are from: Peltola et al. (1968); Foit and Rosenberg (1979); Nuber and Schmetzer (1979); Rumyantseva (1983); Gorskaya et al. (1987, 1989); Cavarretta and Puxeddu (1990); Grice et al. (1993); Grice and Ercit (1993); Ẑàĉek et al.2000; Bosi et al. (2004, 2012, 2013a,b, 2014a,b, 2017a,b); Ertl et al. (2008, 2016); Arif et al. (2010), in which Fe was considered +3 as suggested by the authors; Baksheev et al. (2011); Lupulescu and Rowe (2011); Rozhdestvenskaya et al. (2011); Cempírek et al. (2013); Reznitskii et al. (2014) and Vereshchagin et al. (2014).
The position of Cr-rich vanadio-oxy-dravite from the Tzarevskoye uranium–vanadium deposit close to the chromo-alumino-povondraite boundary is shown in Fig. 1. Moreover, the complete chemical variability of the Al–Cr–V oxy-tourmalines can be compared to the only chemical variability of Fe3+ occurring along the oxy-dravite–bosiite–povondraite series. From a nomenclature viewpoint, the range of the oxy-tourmaline compositions is valid for most of the oxy-tourmalines classified by considering the actual cation distributions over the Y and Z sites as overriding information for the definition of a tourmaline species (Henry et al., 2013). The only exception regards one of the two samples described by Bosi et al. (2012) as oxy-chromium-dravite, which falls in the chromo–alumino–povondraite field. Also note that the V-bearing tourmaline from Silver Knob, California, USA (Foit and Rosenberg, 1979) is classified as V-rich oxy-dravite (Fig. 1).
The position of the chromium-dravite from the Velikaya Guba gold–copper–uranium occurrence (Rumyantseva, 1983) with respect to the other Cr-Fe3+ hydroxy-tourmalines from the literature is shown in Fig. 2. This figure shows the occurrence of a complete chemical variability along the dravite–chromium–dravite series and a partial variability from dravite to the hypothetical end-member NaMg3Fe3+6(Si6O18)(BO3)3(OH)3OH of the samples from Larderello geothermal field, Italy (Cavarretta and Puxeddu, 1990). However, it should be noted that in all the oxy- and hydroxy-tourmalines plotted in Figs 1 and 2 the oxidation state of Fe has always been assumed to be +3 by the various authors, except for the Fe-bearing chromo–alumino–povondraite from the Sludyanka crystalline complex, Russia (Bosi et al., 2013b). The latter was characterised by Mössbauer spectroscopy resulting in Fe2O3 = 2.49 wt.% and FeO = 1.05 wt.%. To date, this is the only experimental information confirming the presence of Fe3+ in Cr-tourmalines (at least the 80% of the Fe3+/ΣFetot).
A classification scheme that disregards details of ion ordering, which typically require techniques that are uncommonly realised in the geosciences community (e.g. crystal structure refinements) is desirable. In this regard, the tourmaline ordered formula would best assist mineralogists and petrologists in identifying tourmaline species. The tourmaline nomenclature can be simplified further by merging the chemical information over the Y and Z sites that results in Al–V–Cr–Fe3+ diagrams.
This study describes the second world-occurrence of the rare vanadio-oxy-dravite from the Tzarevskoye uranium–vanadium deposit, Lake Onega, Karelia Republic, Russia, along with the first world-occurrence of chromium-dravite from the relatively close Velikaya Guba gold–copper–uranium occurrence. These provided an excellent opportunity to use the new Al–V–Cr–Fe3+ diagrams for the tourmaline classification. This approach has also been successfully applied to other oxy- and hydroxy-Al-tourmalines rich in V–Cr–Fe3+ from the literature. Results show the robust classification of tourmalines by using only the chemical data.
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2020.77
We are grateful to M. Serracino who assisted with chemical analyses. Funding by Sapienza University of Rome (Prog. Università 2018 to F. Bosi) is gratefully acknowledged. Comments and suggestions by D. Henry, A. Ertl and the Associate Editor, were much appreciated.