Introduction
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, Reference Dutrow and Henry2011). In accordance with Henry et al. (Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011), 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 [9]X, [6]Y, [6]Z, [4]T and [3]B crystallographic sites (identified with italicised letters); the letters V and W represent groups of anions accommodated at the [3]O(3) and [3]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, Reference Bosi2013; Gatta et al., Reference Gatta, Bosi, McIntyre and Skogby2014).
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, Reference Dutrow and Henry2011; van Hinsberg et al., Reference van Hinsberg, Henry and Dutrow2011; Ahmadi et al., Reference Ahmadi, Tahmasbi, Khalaji and Zal2019; Sipahi, Reference Sipahi2019) 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, Reference Cossa and Arzruni1883; Badalov, Reference Badalov1951; Bassett, Reference Bassett1953; Snetsinger, Reference Snetsinger1966; Peltola et al., Reference Peltola, Vuorelainen and Hakli1968; Jan et al., Reference Jan, Kempe and Symes1972; Dunn, Reference Dunn1977; Nuber and Schmetzer, Reference Nuber and Schmetzer1979; Foit and Rosenberg, Reference Foit and Rosenberg1979; Rumyantseva, Reference Rumyantseva1983; Gorskaya et al., Reference Gorskaya, Frank-Kamenetskaya, Rozhdestvenskaya and Frank-Kamenetskii1984, Reference Gorskaya, Frank-Kamenetskaya, Rozhdestvenskaya, Frank-Kamenetskii, Rumyantseva, Kozlov, Urusov and Pushcharovsky1987; Reznitskii et al., Reference Reznitskii, Sklyarov, Ushapovskaya and Belichenko1988; Hammarstrom, Reference Hammarstrom, Kazmi and Snee1989; Kazachenko et al., Reference Kazachenko, Butsik, Sapin, Kitaev, Barinov and Narnov1993; Reznitskii and Sklyarov, Reference Reznitskii and Sklyarov1996; Ertl et al., Reference Ertl, Rossman, Hughes, Ma and Brandstätter2008; Arif et al., Reference Arif, Henry and Moon2010; Lupulescu and Rowe, Reference Lupulescu and Rowe2011; Rozhdestvenskaya et al., Reference Rozhdestvenskaya, Vereshchagin, Frank-Kamenetskaya, Zolotarev and Pekov2011; Cempírek et al., Reference Cempírek, Houzar, Novák, Groat, Selway and Šrein2013; Vereshchagin et al., Reference Vereshchagin, Rozhdestvenskaya, Frank-Kamenetskaya and Zolotarev2014). 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., Reference Bosi, Reznitskii, Hålenius and Skogby2017b).
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, Reference Snetsinger1966; Kazachenko et al., Reference Kazachenko, Butsik, Sapin, Kitaev, Barinov and Narnov1993; Bačik et al., Reference Bačík, Méres and Uher2011; Cempírek et al., Reference Cempírek, Houzar, Novák, Groat, Selway and Šrein2013). 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., Reference Bosi, Lucchesi and Reznitskii2004, Reference Bosi, Reznitskii and Skogby2012, Reference Bosi, Reznitskii and Sklyarov2013a,Reference Bosi, Skogby, Hålenius and Reznitskiib; Reznitskii et al., Reference Reznitskii, Clark, Hawthorne, Grice, Skogby, Hålenius and Bosi2014; Bosi et al., Reference Bosi, Skogby, Reznitskii and Hålenius2014a,Reference Bosi, Reznitskii, Skogby and Håleniusb, Reference Bosi, Cámara, Ciriotti, Hålenius, Reznitskii and Stagno2017a,Reference Bosi, Reznitskii, Hålenius and Skogbyb). Among these is a vanadio-oxy-dravite, ideally NaV3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a rare tourmaline recently described by Bosi et al. (Reference Bosi, Skogby, Reznitskii and Hålenius2014a).
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.
Geological setting
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, Reference Boitsov1997). The ore mineralisation is located in the albite–mica–carbonate metasomatites upon the Proterozoic aleorolites and schists (Boitsov, Reference Boitsov1997). 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, Reference Boitsov1997): 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. (Reference Borozdin, Polekhovskii, Bushmin, Glebovitskii, Belyatskii and Savva2014), 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, Reference Boitsov1997). 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).
Experimental Methods
Electron-microprobe analysis
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 Reference Pouchou, Pichoir, Heinrich and Newbury1991) 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. (Reference Pesquera, Gil-Crespo, Torres-Ruiz, Torres-Ruiz and Roda-Robles2016), 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.%).
Table 1. Chemical composition for Cr-rich vanadio-oxy-dravite from the Tzarevskoye deposit, Russia.
Notes: Errors for oxides are standard deviations (in parentheses) of 4 spot analyses; apfu = atoms per formula unit.
* Calculated by stoichiometry.
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 1.171.36.20, 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, Reference Sheldrick2013). Starting coordinates were taken from Bosi et al. (Reference Bosi, Skogby, Reznitskii and Hålenius2014a). 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).
Table 2. Single-crystal X-ray diffraction data: details for Cr-rich vanadio-oxy-dravite from the Tzarevskoye deposit, Russia.
Notes: R int = merging residual value; R 1 = discrepancy index, calculated from F-data; wR 2 = weighted discrepancy index, calculated from F 2-data; GoF = goodness of fit; Δρmax, Δρmin = maximum and minimum residual electron density.
Table 3. Displacement parameters (Å2), fractional atom coordinates and site occupancy for Cr-rich vanadio-oxy-dravite from the Tzarevskoye deposit, Russia.
* Equivalent (U eq) and isotropic (U iso) displacement parameters; H-atom was constrained to have a U iso 1.2 times the U eq value of the O(3) oxygen.
Table 4. Selected bond distances (Å) for Cr-rich vanadio-oxy-dravite from the Tzarevskoye deposit, Russia.
Notes: Standard uncertainty in parentheses. Symmetry codes: a = (y – x, y, z); b = (y – x, –x, z); c = (x, x – y, z); d = (y – x + ⅓, –x + ⅔, z + ⅔); e = (–y + ⅔, x – y + ⅓, z + ⅓); and f = (–y, x – y, z). Transformations relate coordinates to those of Table 2.
* Positioned in adjacent unit cell.
Results
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
The anion site populations in the sample studied follow the general preference suggested for tourmaline (e.g. Henry et al., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011): the O(3) site (V position in the general formula) is occupied by (OH), while the O(1) site (W position in the general formula) can be occupied by O2–, (OH) and F–. The T site is fully occupied by Si. The cation distribution at the Y and Z sites can be optimised according the procedure of Bosi et al. (Reference Bosi, Reznitskii, Hålenius and Skogby2017b) and the ionic radii of Bosi (Reference Bosi2018). In detail, the site distribution of Mg, Al, V3+ and Cr3+ was obtained by minimising the residuals between calculated and observed structural data (such as mean bond distance, site scattering expressed in terms of mean atomic number) by using a least-square approach. The minor amounts of Ti, Cu2+ and Zn were assumed fixed at the Y site. The resulting empirical crystal-chemical formula is
$$\eqalign{& ^{\rm X} ({\rm N}{\rm a}_{0.96}\,{\rm K}_{0.02}\,{\squ}_{0.02})_{\Sigma 1.00}{}^{\rm Y} \left( {{\rm V}_{1.34}{\rm A}{\rm l}_{0.68}{\rm M}{\rm g}_{0.93}{\rm C}{\rm u}^{2 + }_{0.02} {\rm Z}{\rm n}_{0.01}{\rm T}{\rm i}_{0.01}} \right)_{\Sigma 3.00} \cr & ^{\rm Z} \left( {{\rm A}{\rm l}_{3.19}{\rm C}{\rm r}_{1.36}{\rm V}_{0.03}{\rm M}{\rm g}_{1.42}} \right)_{\Sigma 6.00}\left( {^{\rm T} {\rm S}{\rm i}_6{\rm O}_{18}} \right)\left( {^{\rm B} {\rm B}{\rm O}_3} \right)_3{}^{\rm V} \left( {{\rm OH}} \right)_3 \cr & ^{\rm W} \left[ {{\rm O}_{0.60}{\left( {{\rm OH}} \right)}_{0.23}{\rm F}_{0.17}} \right]_{\Sigma 1.00}} $$The observed mean atomic number and mean bond length values and those calculated from the optimised site-populations are in excellent agreement (Table 5). This cation distribution is consistent with the studies of Bosi et al. (Reference Bosi, Reznitskii, Hålenius and Skogby2017b) and Bosi (Reference Bosi2018), which showed that the preference of Al3+, V3+ and Cr3+ for the Y and Z sites is controlled mainly by the cation size according to the sequence: YV3+ > YCr3+ > YAl3+ and ZAl3+ > ZCr3+ > ZV3+. Because <Y–O> is always greater than <Z–O> in tourmaline, the Y site will in fact tend to incorporate relatively large cations, whereas the Z site will tend to incorporate relatively small cations. This trend is documented by the preference of V3+ over Cr3+ to dominate the Y site in the vanadio-oxy-chromium-dravite compositions. Compared to Al3+, V3+ prefers the Y site (and Al3+ the Z site) as observed in the vanadio-oxy-dravite samples.
Table 5. Optimised cation site populations (apfu), mean atomic numbers and mean bond lengths (Å) for Cr-rich vanadio-oxy-dravite from the Tzarevskoye deposit, Russia.
Note: apfu = atoms per formula unit.
* Calculated from empirical ionic radii (in Å) of Bosi (Reference Bosi2018): Al = 0.547, Fe3+ = 0.675, Fe2+ = 0.776, Mn2+ = 0.809, Zn = 0.740, Li = 0.751 and Ti = 0.605; the mean Y and Z anion radii are functions of constituent-anion radius (1.360 and 1.357, respectively).
** Fixed in the final stages of refinement
The optimised empirical formula can be recast in its ordered form for classification purposes (Henry et al., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011) by ordering all trivalent cations at the Z site up to 6.00 apfu, according to the site preference Al > Cr > V > Fe3+ (Bosi, Reference Bosi2018), and assigning any excess to Y along with the other cations:
$$\eqalign{& ^{\rm X} \left( {{\rm N}{\rm a}_{0.96}{\rm K}_{0.02}{\squ }_{0.02}} \right)_{\Sigma 1.00}^{\quad \,\,\,\, \rm Y} \left( {{\rm M}{\rm g}_{2.05}{\rm V}_{0.61}{\rm C}{\rm u}^{2 + }_{0.02} {\rm Z}{\rm n}_{0.01}{\rm T}{\rm i}_{0.01}} \right)_{\Sigma 3.00} \cr & ^{\rm Z} \left( {{\rm A}{\rm l}_{3.86}{\rm C}{\rm r}_{1.38}{\rm V}_{0.76}} \right)_{\Sigma 6.00}\left( {^{\rm T} {\rm S}{\rm i}_6{\rm O}_{18}} \right)\left( {^{\rm B} {\rm B}{\rm O}_3} \right)_3^{\rm V} \left( {{\rm OH}} \right)_3 \cr & ^{\rm W} \left[ {{\rm O}_{0.60}{\left( {{\rm OH}} \right)}_{0.23}{\rm F}_{0.17}} \right]_{\Sigma 1.00}} $$Both the empirical and ordered formulae are consistent with an oxy-tourmaline species belonging to the alkali group, subgroup 3 (Henry et al., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011): Na-dominant at the X position of the tourmaline general formula and oxygen-dominant at the W position with O2– > (OH+F)–. As V3+ is the dominant cation at Y and Al3+ is the dominant cation at Z along with relatively minor amounts of Mg required for formula electroneutrality (valency-imposed double-site occupancy; Bosi et al., Reference Bosi, Biagioni and Oberti2019a), its end-member composition is NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O. As a result, the sample studied can be classified as Cr-rich vanadio-oxy-dravite.
Discussion
Similar to other tourmalines from the Sludyanka crystalline complex (Lake Baikal), the vanadio-oxy-dravite sample studied is also strongly enriched in Cr. Karelia appears to be an important area of the world in hosting tourmalines highly enriched in both Cr and V. In particular, the first description of chromium-dravite, ideally NaMg3Cr6(Si6O18)(BO3)3(OH)3OH is from Karelia (Rumyantseva, Reference Rumyantseva1983). More precisely, the holotype chromium-dravite specimen occurs in micaceous metasomatic clay-carbonate rocks from the Velikaya Guba gold–copper–uranium occurrence, Zaonezhye peninsula, Lake Onega, Karelia Republic, Northern Region, Russia. The Velikaya Guba occurrence is close to (~18 km) the Tzarevskoye deposit where the sample studied was found. The empirical formula of chromium-dravite (Rumyantseva, Reference Rumyantseva1983) recast in its ordered form is as follows:
$$\eqalign{& ^{\rm X} ({\rm N}{\rm a}_{0.97}{\rm C}{\rm a}_{0.03})_{\Sigma 1.00}^{\;\;\;\,{\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\rm Y}} ({\rm M}{\rm g}_{2.57}{\rm V}_{0.22}{\rm Fe}_{0.16}^{3 + } {\rm M}{\rm n}_{0.03}{\rm T}{\rm i}_{0.02})_{\Sigma 3.00} \cr & ^{\rm Z} ({\rm C}{\rm r}_{4.71}{\rm Fe}_{0.92}^{3 + } {\rm A}{\rm l}_{0.37})_{\Sigma 6.00}[^{\rm T}({\rm S}{\rm i}_{5.81}{\rm A}{\rm l}_{0.19})_{\Sigma 6.00}{\rm O}_{18}){\rm ]} \cr & [^{\rm B}({\rm B}_{0.97}{\rm A}{\rm l}_{0.03})_{\Sigma 1.00}{\rm O}_3{\rm ]}_3({\rm OH})_3{\rm [}({\rm OH})_{0.77}{\rm O}_{0.23}{\rm ]}_{\Sigma 1.00}.} $$From a classification viewpoint (Henry et al., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011), 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. (Reference Bosi, Hatert, Hålenius, Pasero, Miyawaki and Mills2019b), 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 (Reference Henry and Dutrow2018) proposed two ternary diagrams for the [6]Al–V–Cr subsystem and [6]Al–Cr–Fe3+ subsystem of the [6]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 [6]Al–V–Cr–Fe3+ quaternary system, we have merged the diagrams [6]Al–Cr–V and [6]Al–Cr–Fe3+ through the edge [6]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., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011). 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., Reference Andreozzi, Bosi, Celata, Capizzi, Stagno and Beckett-Brown2020). 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.
Fig. 1. Plot of oxy-tourmaline compositions on the [6]Al–V–Cr–Fe3+ diagram, obtained using 69 data sets.
Fig. 2. Plot of hydroxy-tourmaline compositions on the [6]Al–Cr–Fe3+ diagram, obtained using 33 data sets.
The plotted data in these diagrams (for a total 109 data sets) are from: Peltola et al. (Reference Peltola, Vuorelainen and Hakli1968); Foit and Rosenberg (Reference Foit and Rosenberg1979); Nuber and Schmetzer (Reference Nuber and Schmetzer1979); Rumyantseva (Reference Rumyantseva1983); Gorskaya et al. (Reference Gorskaya, Frank-Kamenetskaya, Rozhdestvenskaya, Frank-Kamenetskii, Rumyantseva, Kozlov, Urusov and Pushcharovsky1987, Reference Gorskaya, Frank-Kamenetskaya, Frank-Kamenetskii, Frank-Kamenetskii and Shmakin1989); Cavarretta and Puxeddu (Reference Cavarretta and Puxeddu1990); Grice et al. (Reference Grice, Ercit and Hawthorne1993); Grice and Ercit (Reference Grice and Ercit1993); Ẑàĉek et al. Reference Ẑàĉek, Frýda, Petrov and Hyršl2000; Bosi et al. (Reference Bosi, Lucchesi and Reznitskii2004, Reference Bosi, Reznitskii and Skogby2012, Reference Bosi, Reznitskii and Sklyarov2013a,Reference Bosi, Skogby, Hålenius and Reznitskiib, Reference Bosi, Skogby, Reznitskii and Hålenius2014a,Reference Bosi, Reznitskii, Skogby and Håleniusb, Reference Bosi, Cámara, Ciriotti, Hålenius, Reznitskii and Stagno2017a,Reference Bosi, Reznitskii, Hålenius and Skogbyb); Ertl et al. (Reference Ertl, Rossman, Hughes, Ma and Brandstätter2008, Reference Ertl, Baksheev, Giester, Lengauer, Prokof'ev and Zorina2016); Arif et al. (Reference Arif, Henry and Moon2010), in which Fe was considered +3 as suggested by the authors; Baksheev et al. (Reference Baksheev, Prokof'ev, Yapaskurt, Vigasina, Zorina and Solov'ev2011); Lupulescu and Rowe (Reference Lupulescu and Rowe2011); Rozhdestvenskaya et al. (Reference Rozhdestvenskaya, Vereshchagin, Frank-Kamenetskaya, Zolotarev and Pekov2011); Cempírek et al. (Reference Cempírek, Houzar, Novák, Groat, Selway and Šrein2013); Reznitskii et al. (Reference Reznitskii, Clark, Hawthorne, Grice, Skogby, Hålenius and Bosi2014) and Vereshchagin et al. (Reference Vereshchagin, Rozhdestvenskaya, Frank-Kamenetskaya and Zolotarev2014).
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 [6]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., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2013). The only exception regards one of the two samples described by Bosi et al. (Reference Bosi, Reznitskii and Skogby2012) 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, Reference Foit and Rosenberg1979) is classified as V-rich oxy-dravite (Fig. 1).
The position of the chromium-dravite from the Velikaya Guba gold–copper–uranium occurrence (Rumyantseva, Reference Rumyantseva1983) 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, Reference Cavarretta and Puxeddu1990). 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., Reference Bosi, Skogby, Hålenius and Reznitskii2013b). 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).
Conclusions
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 [6]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 [6]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.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2020.77
Acknowledgements
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.
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