Occurrence and paragenesis of oxy-dravite in the Beluga occurrence

The Beluga property is located in the southern part of Baffin Island, Nunavut territory, Canada (Fig. 1). The locality is known chiefly due to the occurrence of sapphire-bearing calc-silicate pods. These pods are hosted in a shelf sequence of clastic rocks and carbonates along with garnet-bearing metapsammite, metapelite, metasemipelite and rarely orthoquartzite of the Lake Harbour Group, a Paleoproterozoic supracrustal suite (Scott, Reference Scott1997; Lepage and Rohtert, Reference Lepage and Rohtert2006; Corrigan et al., Reference Corrigan, Pehrsson, Wodicka and de Kemp2009; Belley et al., Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017; Belley and Groat, Reference Belley and Groat2020). The Lake Harbour Group rocks, considered as a cover sequence of the Meta Incognita microcontinent, were transformed by several stages of metamorphism (granulite and amphibolite facies, 810°C/8 kbar and 720°C/8 kbar, respectively) during convergent orogenesis (initiated ca. 1849–1835 Ma) in the Trans-Hudson orogeny (St-Onge et al., Reference St-Onge, Wodicka and Ijewliw2007). High-grade metamorphism was followed by final, localised retrograde overprints (ca. 1797–1785 Ma; St-Onge et al., Reference St-Onge, Wodicka and Ijewliw2007). The area has diverse geology and is an excellent source of gemstones; sapphires, spinel (Belley and Groat, Reference Belley and Groat2019) and lapis lazuli (Hogarth, Reference Hogarth1971) occur in gem quality specimens.

Figure 1. Geology of southern Baffin Island; the Beluga occurrence is located in the Kimmirut area (3), after Belley and Groat (Reference Belley and Groat2020).

At Beluga, uncommon oxy-dravite occurs as dark brown, equant to short-prismatic, idiomorphic crystals with maximal dimensions of ca. 4 × 3 cm (Fig. 2) in the retrograde albite–muscovite–corundum–calcite domains in the calc-silicate rock (Belley et al., Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017). Belley et al. (Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017) did not have sufficient textural evidence to clearly establish the associated stable mineral assemblage for oxy-dravite (i.e. either the prograde/peak metamorphic Di + Ne, high-temperature retrograde Phl + Pl + Scp, or the retrograde corundum-bearing assemblage; see Belley et al., Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017 for details). However, additional field work at Beluga by Belley (2022 field season, unpublished) revealed the persistent, and seemingly exclusive, association of oxy-dravite with the retrograde corundum-bearing assemblage (albite–calcite–muscovite–corundum–graphite with accessory pyrrhotite). Oxy-dravite crystals observed in contact with corundum occur on the latter mineral's surface, conforming to the euhedral shape of corundum, and have not been observed as inclusions in corundum. Examination of 55 corundum crystals (Belley unpublished data, 2023) revealed common inclusions of calcite, three inclusions of phlogopite (partly altered to muscovite–amesite symplectites with subordinate V-bearing TiO₂) and one inclusion of scapolite. These inclusions may indicate that, at least in the early stages of mineralisation, corundum formed a stable assemblage with calcite–phlogopite–scapolite – forming earlier than previously estimated by Belley et al. (Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017). Euhedral muscovite crystals occur on the surface of some corundum crystals and within fractures, thus it may not form a stable assemblage with the corundum. The textural relationships between corundum and oxy-dravite suggest that oxy-dravite probably formed simultaneously with, or later than, corundum.

Figure 2. Photo of the sample containing the brown Beluga oxy-dravite in association with a prismatic light blue corundum crystal and late euhedral calcite (sample from collection of P. Belley).

The Beluga calc-silicate rock protolith is interpreted to be dolomitic marl (Belley et al., Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017; Belley and Groat, Reference Belley and Groat2019). Though the boron isotopic signature of the oxy-dravite is consistent with a marine boron source, this does not necessarily indicate that the Beluga rock consists of meta-evaporites. Evaporite-associated minor and trace elements, such as Mg, Mg/Ca ratio, B, B/Al ratio, Li, F and Cl, were not in concentrations sufficiently elevated as to indicate a definitive evaporitic signature (Belley et al., Reference Belley, Dzikowski, Fagan, Cempírek, Groat, Mortensen, Fayek, Giuliani, Fallick and Gertzbein2017).

Experimental methods

The sample, a polished section of a crystal (ca. 6 mm in diameter) oriented perpendicular to the c axis, was analysed using a fully automated CAMECA SX-100 electron microprobe (EMP) in the Electron Microprobe Laboratory of the Department of Geological Sciences, Masaryk University (joint workplace of the Faculty of Science of Masaryk University and the Czech Geological Survey). The tourmalines were analysed in wavelength-dispersive mode, with accelerating voltage of 15 kV, beam current of 10 nA, and spot size of 5 μm. The following standards were used for measurement of major and minor elements in the tourmalines on Kα X-ray lines: sanidine (Si, Al, K); albite (Na); pyrope (Mg); titanite (Ti); chromite (Cr); vanadinite (Cl); fluorapatite (P); wollastonite (Ca); almandine (Fe); spessartine (Mn); ScVO₄ (V); gahnite (Zn); and topaz (F). Average detection limits and 1σ for individual elements are, respectively (in wt.%): 0.043/0.201 (Na); 0.041/0.334 (Si); 0.035/0.424 (Al); 0.024/0.124 (Mg); 0.031/0.038 (Ti); 0.027/0.022 (Cr); 0.017/0.015 (Cl); 0.033/0.027 (P); 0.037/0.055 (Ca); 0.024/0.022 (K); 0.068/0.125 (Fe); 0.063/0.053 (Mn); 0.048/0.043 (V); 0.093/0.079 (Zn); and 0.053/0.05 (F). The Al/Si ratio was verified on a kyanite secondary standard.

The tourmaline formulae were calculated on the basis of T + Z + Y = 15 atoms per formula unit (apfu), assuming B = 3 apfu, full occupancy and Li at the Y site, zero ^[4]B at the T site, and OH = (4 – ^V,WO – F) (Henry et al., Reference Henry, Novák, Hawthorne, Ertl, Dutrow, Uher and Pezzotta2011). All Fe was assumed to be divalent which is in agreement with bond-valence calculations. The assumption regarding Li is supported by the negligible Li contents detected by laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) analysis (<13 ppm). The tetrahedral bond length <T–O> ≈ 1.621 Å indicates full occupancy by Si or trace ^[4]Al at the T-site.

Trace-element analyses (Li, Be, B, P, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Y, Zr, REE, Th and U) were performed by LA-ICP-MS analysis at the Laboratory of Atomic Spectrochemistry of the Department of Chemistry (Faculty of Science, Masaryk University). The LA-ICP-MS equipment consists of an Analyte G2 (Teledyne CETAC Technologies) connected to a sector field ICP-MS spectrometer Element 2 (Thermo Fischer Scientific). An ArF* laser ablation device equipped with a HelEx II sample cell was operated at a wavelength of 193 nm. The ablated material was carried by He flow (0.65 L/min) and mixed with Ar (~1 L/min) prior to entering the IC-PMS spectrometer. Optimisation of LA-ICP-MS parameters was performed with the glass reference material NIST SRM 610 with respect to the maximum signal-to-background ratio and ²⁴⁸ThO⁺/²³²Th⁺ < 1. All element contents were normalised using Si determined from electron microprobe analysis (EMPA) as an internal standard. Data were processed using an in-house programmed optimisation spreadsheet in MS Excel. Analytical results and limits of detection (LOD) are reported in Table 1.

Table 1. Selected representative chemical compositions of the Beluga oxy-dravite (rim to core profile across a large crystal using EMPA and LA-ICP-MS, and the EMPA composition of the crystal used for SC-XRD).

Note: b.d.l – below detection limit; LOD – limits of element detection for LA-ICP-MS data; SC-XRD – single-crystal X-ray diffraction.

* calculated from stoichiometry, see Methods.

The crystal structure of an oxy-dravite fragment extracted from the rim of the studied crystal was determined with a Bruker X8 APEX II diffractometer with graphite-monochromated MoKα radiation at C-HORSE (the Centre for Higher Order Structure Elucidation, in the Department of Chemistry at University of British Columbia). The crystal-to-detector distance was 40 mm. The data were collected at room temperature in a series of φ and ω scans in 0.50° oscillations with 10 s exposures (see Table 2). Data collection and integration was done using the Bruker SAINT software package (Bruker, Reference Bruker2007). Data were corrected for absorption effects using the multi-scan technique (SADABS, Sheldrick, Reference Sheldrick2008) and also for Lorentz and polarisation effects.

Table 2. Crystal data, data collection and refinement parameters for near end-member oxy-dravite from Beluga.

Polarised single-crystal Raman spectra of the Beluga oxy-dravite (rim of the polished section used for EMPA) were collected at room temperature using a Horiba LabRAM–HR spectrometer equipped with an Olympus BX41 optical microscope. The Raman signal was collected in the range of 100–4000 cm⁻¹ using grating of 600 gr/mm and a 50× objective from four 60 s accumulations. The system was operated in the confocal mode with a beam diameter of ~1 μm. No visual damage to the analysed surface was observed after the excitation. Raman shift calibration was done using a Si wafer. The wavenumber accuracy was ~0.5 cm⁻¹ and the spectral resolution was ~2 cm⁻¹. Band fitting was completed after appropriate background correction, assuming combined Lorentzian-Gaussian band shapes using the Voight function.

Chemical composition

The tourmaline sample is weakly zoned, with only minor changes from the crystal core to its rims (Fig. 3; Table 2). The X-site is occupied predominantly by Na (0.87 to 0.91 apfu), its Ca content varies from 0.03 to 0.12 apfu, and vacancy contents are negligible (<0.06 pfu). The V- and W-sites are characterised by high OH (2.92–3.00 apfu) and O contents (0.80–0.95 apfu), with minor F (0.12–0.17 apfu) only.

Figure 3. Composition of the Beluga oxy-dravite in several compositional spaces compared to compositions from other studies (Ertl et al., Reference Ertl, Hughes, Brandstätter, Dyar and Prasad2003; Čopjaková et al., Reference Čopjaková, Škoda and Vašinová-Galiová2012; Bosi and Skogby, Reference Bosi and Skogby2013; Pieczka et al., Reference Pieczka, Ertl, Sek, Twardak, Zelek, Szeleg and Giester2018; Sȩk et al., Reference Sȩk, Włodek, Stachowitz, Woźniak and Pieczka2022). Note the high miscibility with vacancy-, Fe- and OH-dominated species.

The octahedral Y- and Z-sites contain dominant Al (6.87–7.31 apfu) and contain high Mg (1.73–1.98 apfu), and low contents of Fe²⁺ (0.13–0.18 apfu); Li is negligible (<13 ppm based on LA-ICP-MS). The tetrahedral site is occupied mostly by Si (5.73–5.95 apfu) with minor substituted Al (0.06–0.27 apfu).

Amounts of trace elements determined by LA-ICP-MS analyses are generally very low (Table 1 – EMPA and LA-ICP-MS). The only exceptions are Ti and V with maximum values up to 4495 ppm (up to 0.75 wt.% TiO₂) and 312 ppm (0.05 wt.% V₂O₃), respectively; they show a slight decrease from the crystal rims towards the core. Contents of Sc, P, Li, Cr, Mn, Co, Ni, Ga and Zn are close to their detection limits and approach tens of ppm in maximum values. The amounts of REE, Be, Cu and Zr are below their detection limits.

Crystal structure and bond valence refinement

The crystal structure of the oxy-dravite sample was refined from single-crystal X-ray diffraction data. The refinements were performed using the SHELXTL crystallographic software package (Sheldrick, Reference Sheldrick2008, Reference Sheldrick2015) of Bruker AXS. Scattering factors for neutral atoms were employed for the cations and ionic factors for O^2– were used for oxygen (Hovestreydt, Reference Hovestreydt1983). The structure of dravite after Foit and Rosenberg (Reference Foit and Rosenberg1979) was introduced as an initial model for refinement that was processed (by refining occupancy of Na at the X-site, and Fe vs. Al at the Y- and Z-sites) to a final R index of ~1.45% for an anisotropic displacement model. The H3-atom site was located in residual electron-density maps whereas the H1 site was impossible to locate. The H3 site isotropic displacement parameter was constrained to be equal to 1.2 times of that of the O3 site; its distance from the donor oxygen atom was constrained to 0.92(2) Å.

The mean <Z–O> bond length of 1.928 Å indicates that the Z-site is not fully occupied by Al (ideally <^ZAl–O> = 1.906 Å; Bosi and Andreozzi, Reference Bosi and Andreozzi2013) but partially replaced by Mg, suggesting ^Y,Z(Al–Mg) disorder. Occupancies of octahedral sites were refined using bond-valence optimisation (Table 3) which confirmed disorder of Mg and Al between the Y- and Z-sites, typical for Mg-rich tourmalines (e.g. Grice and Ercit, Reference Grice and Ercit1993; Hawthorne et al., Reference Hawthorne, MacDonald and Burns1993; Ertl et al., Reference Ertl, Hughes, Brandstätter, Dyar and Prasad2003; Bosi and Lucchesi, Reference Bosi and Lucchesi2004; Bosi and Skogby, Reference Bosi and Skogby2013; Bačík, Reference Bačík2015; Pieczka et al., Reference Pieczka, Ertl, Sek, Twardak, Zelek, Szeleg and Giester2018; Scribner et al., Reference Scribner, Cempírek, Groat, Evans, Biagioni, Bosi, Dini, Hålenius, Orlandi and Pasero2021; Bosi et al., Reference Bosi, Biagioni, Pezzotta, Skogby, Hålenius, Cempírek, Hawthorne, Lussier, Abdu, Day, Fayek, Clark, Grice and Henry2022; Sȩk et al., Reference Sȩk, Włodek, Stachowitz, Woźniak and Pieczka2022).

Table 3. Fractional atomic coordinates, isotropic/equivalent isotropic displacement parameters (Ų), and anisotropic atomic displacement parameters (Ų).

The occupancy of the T-site is dominated by Si but its content varies: 5.73–5.95 apfu. With the elongated <T–O> bond length, it is evident that some amount of Al³⁺ (or, less likely, a combination of Al³⁺ + B³⁺) is present in the T-site (e.g. Grice and Ercit, Reference Grice and Ercit1993; MacDonald and Hawthorne, Reference MacDonald and Hawthorne1995; Hawthorne, Reference Hawthorne2002; Ertl et al., Reference Ertl, Henry and Tillmanns2018). The final empirical structural formula, with hydrogen distribution adjusted between the V- and W-sites, using the formula of Bosi (Reference Bosi2013), is:

$$\eqalign{&{ ^{\rm X}( {\rm N}{\rm a}_{0.88}{\rm C}{\rm a}_{0.08}\Box _{0.03}{\rm K}_{0.01}) ^{\rm Y}( {{\rm A}{\rm l}_{1.49}{\rm M}{\rm g}_{1.31}{\rm F}{\rm e}_{0.15}{\rm T}{\rm i}_{0.04}{\rm Z}{\rm n}_{0.01}} )} \cr & {^{\rm Z} ( {{\rm A}{\rm l}_{5.42}{\rm M}{\rm g}_{0.58}} ) ^{\rm T}( {{\rm S}{\rm i}_{5.84}{\rm A}{\rm l}_{0.16}{\rm O}_{18}} ) ^{\rm B}{( {{\rm B}{\rm O}_3} ) _3}^{\rm V}( {{\rm O}{\rm H}_{2.95}{\rm O}_{0.05}} )} \cr & ^{\rm W} ( {{\rm O}_{0.84}{\rm O}{\rm H}_{0.01}{\rm F}_{0.15}).}} $$

For the final step of the structure refinement, atom fractions from the final empirical formula were fixed to the individual sites; final bond lengths (rare changes on third decimal place) were used again for the bond-valence optimisation (using values R ₀ and B from Gagné and Hawthorne, Reference Gagné and Hawthorne2015) which resulted in results identical to the previous step. Final atomic parameters and refined scattering values expressed as site occupancies are given in Table 4 and selected interatomic distances in Table 5. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Bond distances of the crystal studied (Å).*

* Symmetry codes: (iv) –y+⅔, x–y+⅓, z–⅔; (v) –x+y+⅓, –x+⅔, z–⅓

Table 5. Empirical weighted bond valences (in valence units) and other structural site parameters for the Beluga oxy-dravite.*

* Note: obs. = observed; cal. = calculated. ^§Includes 0.99 valence units contribution from H3.

Physical properties

The Beluga oxy-dravite is strongly pleochroic, light yellowish-brown colour in the omega direction and colourless in the epsilon direction. The Beluga oxy-dravite is uniaxial (–) with ω = 1.6453(5) and ɛ = 1.6074(18), measured using a low pressure Na-vapour lamp. The density calculated using the observed cell volume and the empirical composition is 3.069 g.cm^–3. The compatibility index reflecting the difference between the physical (K _p) and the chemical specific-refractivity (Mandarino, Reference Mandarino1981) is 0.016, corresponding to the superior category. A comparison of the Beluga oxy-dravite optical properties with those for other tourmaline Mg-bearing end-members is shown in Table 6.

Table 6. The comparison of oxy-dravite optical properties with other tourmaline Mg-bearing end-members.

* https://www.mindat.org/min-1318.html

Raman spectra

The Raman spectrum of the Beluga oxy-dravite (Fig. 4) was (after background correction) fitted using Pseudo-Voight peaks. The spectrum pattern generally corresponds well to that of oxy-dravite and other Mg-bearing tourmalines (Watenphul et al., Reference Watenphul, Burgdorf, Schlüter, Horn, Malcherek and Mihailova2016a, Reference Watenphul, Schlüter, Bosi, Skogby, Malcherek and Mihailova2016b; Watenphul et al., Reference Watenphul, Malcherek, Wilke, Schlüter and Mihailova2017). According to the equations provided by Watenphul et al. (Reference Watenphul, Schlüter, Bosi, Skogby, Malcherek and Mihailova2016b), there is a dependence of measured ω(ZO₆) (ω = Raman shift) on the content of non-aluminium cations (Mg and Fe³⁺) at the Z-site. The calculated values for the Beluga oxy-dravite are 0.98(19) apfu ^ZMg and 0 apfu ^ZFe³⁺; the error of the calculated ^ZMg value overlaps with the error of the structurally refined ^ZMg value (0.66±0.30 apfu). Despite the large difference, the result shows that the Raman calculation above is a valid tool for estimation of ^Y,ZMg disorder.

Figure 4. Polarised Raman spectra (black) and the main fitted bands (grey) for the Beluga oxy-dravite (orientation || c).

The Beluga oxy-dravite Raman peaks related to OH at the W-site (range 3711–3772 cm^–1) generally agree with data described by Watenphul et al. (Reference Watenphul, Burgdorf, Schlüter, Horn, Malcherek and Mihailova2016a). The most intense peak at 3739 cm^–1 is related to the ^YMg^YMg^YAl–^XNa configuration of atoms around the O1 atom whereas the peak at 3772 cm^–1 matches values reported for the ^YMg^YMg^YMg–^XNa configuration in dravite and uvite (Watenphul et al., Reference Watenphul, Burgdorf, Schlüter, Horn, Malcherek and Mihailova2016a). For the peak at 3711 cm^–1 we propose the configuration ^YMg^YAl^YAl–^XNa as it must be present in a significant amount due to the Mg–Al disorder between the Y- and Z-sites revealed by the structural refinement (below). The presence of peaks related to ^WOH indicates that the ^VOH must be present in a significant amount. In the disorder calculation (Bosi, Reference Bosi2013), ^WOH is negatively correlated to F content; the Raman results may suggest slight fluorine overestimation, possibly due to use of topaz as an EMPA standard (Ottolini et al., Reference Ottolini, Cámara and Bigi2000).

By comparison with Raman spectra of dravite (Watenphul et al., Reference Watenphul, Burgdorf, Schlüter, Horn, Malcherek and Mihailova2016a), the two most intense ^VOH-peaks of the Beluga oxy-dravite at 3531 and 3574 cm^–1 (Fig. 4) probably correspond to the 2^YMg^ZAl^ZAl–^YAl^ZAl^ZAl, and 3^YMg^ZAl^ZAl configurations, respectively. The middle peak at 3556 cm^–1 is closest to the 2^YMg^ZMg^ZAl–^YMg^ZAl^ZAl configuration (located at 3547 cm^–1 in uvite; Watenphul et al., Reference Watenphul, Burgdorf, Schlüter, Horn, Malcherek and Mihailova2016a) which corresponds with Mg–Al disorder between the Y- and Z-sites. Peaks at similar wavenumbers (3548–3560 cm^–1) were observed by Fantini et al. (Reference Fantini, Tavares, Krambrock, Moreira and Righi2014) in dravite, magnesio-foitite and uvite.