Crystal chemistry of povondraite by single-crystal XRD, EMPA, Mössbauer spectroscopy and FTIR

Abstract

Five povondraite crystals from San Francisco Mine, Villa Tunari, Bolivia, have been structurally and chemically characterised by single-crystal X-ray diffraction and electron microprobe analysis. For the first time, this characterisation is accompanied by Mössbauer spectroscopic and single-crystal infrared spectroscopic data, which show the exclusive presence of Fe³⁺ at both the octahedrally-coordinated Y and Z sites as well as slight disorder of (OH) and O over the O(1) and O(3) sites.

The data obtained along with those for earlier-studied bosiite and oxy-dravite oxy-tourmalines show a complete substitution series described by the reaction ^YFe³⁺₃ + ^ZMg + ^ZFe³⁺₄ ↔ ^YAl₂ + ^YMg + ^ZAl₅ (i.e. Fe³⁺Al_–1) with variation of the structural parameters dominated by Fe³⁺ (or Al). Povondraite is the tourmaline member having the largest unit-cell parameters due to the larger size of Fe³⁺ relative to other trivalent cations (V > Cr > Al). In the tourmaline-supergroup minerals, the a and c unit-cell parameters vary from ~15.60 Å to ~16.25 Å and ~7.00 Å to ~7.50 Å, respectively. Their values increase with increasing Fe³⁺ or decreasing Al. End-member compositions related to the smallest and largest a and c parameters are, respectively, NaAl₃Al₆(Si₃B₃O₁₈)(BO₃)₃(OH)₃(OH) (synthetic tourmaline) and NaFe³⁺₃(Fe³⁺₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O (povondraite).

Introduction

Tourmalines are complex borosilicates whose general chemical formula may 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 = (OH)^– and O^2–; W = (OH)^–, F^– and O^2– (Henry et al., 2011). In this representation unitalicised letters X, Y, Z, T and B represent groups of cations hosted at the ^[9]X, ^[6]Y, ^[6]Z, ^[4]T and ^[3]B crystallographic sites (letters italicised), whereas V and W represent groups of anions accommodated at the [3]-coordinated O(3) and O(1) crystallographic sites, respectively.

Tourmaline has been studied extensively in terms of crystal structure and crystal chemistry (e.g. Foit, 1989; Grice and Ercit, 1993; Hawthorne and Henry, 1999; Ertl et al., 2002; Novák et al., 2004; Bosi and Lucchesi, 2007; Bosi, 2018; Henry and Dutrow, 2011; Henry et al., 2011; Cempírek et al., 2013; Bačík and Fridrichová, 2020). Results show that the tourmaline structure is remarkably flexible in a chemical sense, accommodating ions of a wide range of size and charge, which in turn leads to Mg–Fe–Al–Cr–V disorder over the Y and Z sites.

Povondraite, ideally NaFe³⁺₃(Fe³⁺₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O, was described by Grice et al. (1993). Until now, however, a chemical and structural study dealing with a statistically significant dataset of Fe³⁺-dominant oxy-tourmalines is missing. To explore the crystal-chemical aspects and their implications on the tourmaline supergroup, single-crystal structure refinements and electron microprobe data have been collected on five crystals from the type locality for povondraite: San Francisco Mine, Villa Tunari, Alto Chapare, Cochabamba, Bolivia (Walenta and Dunn, 1979; Grice et al., 1993; Žáček et al., 2000). All five single crystals were extracted from povondraite sample 110379 from the American Museum of Natural History, New York, USA. This is the first time that Mössbauer spectroscopic and single-crystal infrared spectroscopic data have been presented for this mineral.

Analytical methods and results

General comment

Initially, several crystal fragments of sample 110379 were analysed by electron microprobe; these proved to be chemically inhomogeneous, as reported in Hovis et al. (in press). More recent analyses, reported in the present study, were obtained on different relatively small fragments, which proved to be quite homogeneous as shown by the relatively low standard deviation values of the analysed elements (Table 4).

Single-crystal structure refinement (SREF)

Single-crystal X-ray diffraction (XRD) was undertaken on five crystal fragments of povondraite by mounting on a Bruker KAPPA APEX-II single-crystal diffractometer (Sapienza University of Rome, Earth Sciences Department) equipped with a CCD area detector (6.2 × 6.2 cm active detection area, 512 × 512 pixels) and a graphite-crystal monochromator using MoKα radiation from a fine-focus sealed X-ray tube. The sample-to-detector distance was 4 cm. A total of 1296 exposures (step = 0.4° and time/step = 20 s) covering a full reciprocal sphere were collected using ω and φ scan modes. Final unit-cell parameters were refined using the Bruker AXS SAINT program on reflections with I > 10 σ_I in the range 5° < 2θ < 75°. The intensity data were processed and corrected for Lorentz polarisation and background effects using the APEX2 software program of Bruker AXS. The data were corrected for absorption using a multi-scan method (SADABS). The absorption correction led to an improvement in R _int. No violation of R3m symmetry was detected.

Structure refinement was done using the SHELXL-2013 program (Sheldrick, 2015). Starting coordinates were taken from Grice et al. (1993). Variable parameters included scale factor, extinction coefficient, atom coordinates, site-scattering values (for X, Y and Z sites) and atomic-displacement factors. Each structure was refined as a two-component inversion twin. Fully ionised-oxygen scattering factor and neutral-cation scattering factors were used. In detail, the X site was modelled using the Na vs. K scattering factors. The occupancy of the Y site was obtained by considering the presence of Fe vs. Mg, and the Z site with Fe vs. Al. Although there is more Mg than Al content, the latter was preferred to ^ZMg, having produced slightly better statistical indices and standard uncertainties. The T, B and anion sites were modelled, respectively, with Si, B and O scattering factors and with a fixed occupancy of 1, as refinement with unconstrained occupancies showed no significant deviations from this value. The position of the H atom bonded to the oxygen at the O(3) site in the structure was taken from the difference-Fourier map and incorporated into the refinement model; the O(3)–H(3) bond length was restrained (by DFIX command) to be 0.97 Å with isotropic displacement parameter constrained to be equal to 1.2 times that obtained for the O(3) site. Table 1 lists crystal data, data-collection information, and refinement details. Table 2 gives the fractional atom coordinates and equivalent isotropic-displacement parameters of a typical povondraite crystal (Pov1). Table 3 reports selected bond lengths for all studied crystals. The crystallographic information files showing all structural data have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Table 1. Single-crystal X-ray diffraction data details for the povondraite crystals studied.*

* 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; diff. peak = maximum and minimum residual electron density. Space-group type = R3m and Z = 3. Radiation: MoKα = 0.71073 Å. Range for data collection (2θ) = 5°–75°. Data collection temperature = 293 K. Axis = phi-omega, frame width = 0.4°, time per frame = 20 s. Absorption correction method: multi-scan (SADABS, Bruker). Refinement method: full-matrix last-squares on F ². Structural refinement program: SHELXL-2013 (Sheldrick, 2015).

Table 2. Fractional atom coordinates and isotropic (*) or equivalent-isotropic displacement parameters (in Ų) for crystal Pov1.

* Isotropic-displacement parameters (U _iso) for H(3) constrained to have a U _iso 1.2 times the U _eq value of the O(3) oxygen atom, respectively.

Table 3. Selected bond lengths (Å) and cation site occupancy (s.o.) for the povondraite crystals studied.*

* Note: T-site occupancy = Si_1.00 and B-site occupancy = B_1.00

Electron microprobe analysis (EMPA)

The crystals used for X-ray diffraction refinement were analysed by wavelength dispersive spectrometry (WDS mode) using a Cameca SX50 instrument (CNR-Istituto di Geologia Ambientale e Geoingegneria, Rome, Italy) operating at an accelerating potential of 15 kV and a sample current of 15 nA, with a 10 μm beam diameter. The following standards, X-ray Kα lines and analyser crystals were used: jadeite (Na; TAP), periclase (Mg; TAP), orthoclase (K; PET), rutile (Ti; PET), wollastonite (Si, Ca; PET), metallic Zn and Mn (Zn, Mn; LIF), vanadinite (V; PET), fluorophlogopite (F; TAP), metallic Cr (Cr; PET), corundum (Al; TAP) and magnetite (Fe; LIF). The ‘PAP’ routine was applied (Pouchou and Pichoir 1991). The results (Table 4) represent mean values of several spot analyses. Vanadium, Cr, Mn and Zn were below detection limits (< 0.03 wt.%).

Table 4. Chemical composition for the povondraite crystals studied.*

* Notes: wt.% = weighted percent; apfu = atoms per formula unit (normalised to 31 anions); epfu = electrons per formula unit; b.d.l. = below detection limit; errors for oxides and fluorine are standard deviations (in brackets).

^a Calculated by stoichiometry, (Y+Z+T) = 15.00 apfu.

^b Fe oxidation state determined by Mössbauer spectroscopy.

Mössbauer spectroscopy

Several crystal fragments from povondraite sample 110379 were used for ⁵⁷Fe Mössbauer spectroscopy performed at the Swedish Museum of Natural History, Stockholm, Sweden, using a conventional spectrometer system operated in constant-acceleration mode. Data were collected over 1024 channels; these were folded and calibrated against the spectrum of α-Fe foil. The spectrum (Fig. 1) was fit using the software MossA (Prescher et al., 2012) with two absorption doublets consistent with Fe³⁺ (Table 5). No indications of absorption due to Fe²⁺ was observed. In line with the site population results from SREF (see below, Table 6), the two doublets can be related to the occurrence of Fe³⁺ at both the Y and Z sites. However, the low resolution of the two doublets does not allow a definite site assignment.

Fig. 1. Mössbauer spectrum of povondraite obtained at room temperature. The fitted absorption doublets assigned to Fe³⁺ are indicated in blue. Diamonds denote the measured spectrum and the black curve represents summed fitted doublets.

Table 5. Mössbauer parameters for povondraite obtained at room temperature.*

* δ = centre shift, ΔE _Q = quadrupole splitting, FWHM = full width at half-maximum.

Table 6. Site populations (atoms per formula unit) and mean atomic number (man) for the povondraite crystals studied.*

* Notes: All crystals have the B and O(2,4,5,6,7,8) sites fully occupied by B³⁺ and O^2–, respectively; obs = observed, calc = calculated from the site population.

The Mössbauer spectroscopy results are consistent with a synchrotron XANES study that reported 87–100% Fe as Fe³⁺ (Levy et al., 2018).

Single-crystal infrared spectroscopy

Polarised Fourier-transform infrared (FTIR) absorption spectra were measured on a 33 μm thick doubly polished single-crystal section oriented parallel to the c-axis. A Bruker Vertex spectrometer attached to a Hyperion 2000 microscope and equipped with a halogen lamp source, CaF₂ beamsplitter, ZnSe wiregrid polariser and InSb detector was used to collect spectra in the range 2000–13000 cm^–1 at a resolution of 4 cm^–1. Spectra recorded in polarised mode parallel to the crystallographic c-axis (E||c) show a weak band at 3440 cm^–1, a very intense band around 3550 cm^–1, a significant band at 3593 cm^–1 and a weak band at 3699 cm^–1 (Fig. 2). As observed typically for polarised tourmaline spectra in the (OH) range, the main band is off-scale for the E||c direction due to excessive absorption. Spectra obtained perpendicular to the c-axis show considerably weaker bands.

Fig. 2. Polarised FTIR spectra of povondraite, off-set vertically for clarity. The main band is truncated at ~2 absorbance units in the E||c direction due to excessive absorption. Note the comparatively low intensity of the band at ~3699 cm^–1 corresponding to very small (OH) contents at W [≡ the O(1) site]. Sample thickness = 33 μm.

Bands above 3600–3650 cm^–1 are normally considered to be due to (OH) at the W position [≡ O(1) site] (e.g. Gonzalez-Carreño et al., 1988; Bosi et al., 2015). For the samples studied, the comparatively weak intensity of the band at 3699 cm^–1 indicates low amounts of ^W(OH). On the basis of studies by Bosi et al. (2016), Watenphul et al. (2016) and Gatta et al. (2014), the main FTIR bands at ~3440 cm^–1, ~3550 and ~3593 cm^–1 are probably caused by the occurrence of the atomic arrangements 3[^Y(Fe³⁺)^Z(Fe³⁺,Al)^Z(Al)]–^O(3)(OH)₃, {2[^Y(Fe³⁺)^Z(Fe³⁺)^Z(Mg)]–[^Y(Fe³⁺)^Z(Fe³⁺)^Z(Fe³⁺)]}–^O(3)(OH)₃ and 3[^Y(Fe³⁺)^Z(Fe³⁺)^Z(Mg)]–^O(3)(OH)₃, respectively, whereas the band at ~3699 cm^–1 may be caused by the arrangements ^Y(Fe³⁺MgMg)–^O(1)(OH)–^X(Na,K).

Determination of number of atoms per formula unit (apfu)

In agreement with the structure-refinement results, the boron content was assumed to be stoichiometric (B³⁺ = 3.00 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 with the T site free of B³⁺ (e.g. Bosi and Lucchesi, 2007). Iron oxidation state was determined by Mössbauer spectroscopy, which shows the exclusive presence of Fe³⁺. In accordance with Pesquera et al. (2016), Li concentrations were considered insignificant as MgO > 2 wt.% in the povondraite crystals studied. The (OH) content and the formula were then calculated by charge balance with the assumption (T + Y + Z) = 15 apfu and 31 anions. The excellent agreement between the number of electrons per formula unit (epfu) derived from EMPA and SREF (within 1 epfu for all studied crystals) supports the stoichiometric assumptions.

Site populations

The povondraite site populations at the X, B, T, O(3) (≡ V) and O(1) (≡ W) sites of crystals Pov1,2,3,4,5 follow the standard site preference suggested for tourmaline (Henry et al., 2011) and are coherent with the information from FTIR absorption spectra. In particular, the presence of ~0.10 Al apfu at the T site is consistent with observed <T–O> distances ranging from 1.621–1.625 Å, which are larger than the expected value for <^TSi–O> = 1.619(1) Å (Bosi and Lucchesi, 2007). The Fe³⁺, Al and Mg site populations at the octahedrally coordinated Y and Z sites were optimised according to the procedure of Bosi et al. (2017b) and Wright et al. (2000), as well as by fixing the minor elements Ti⁴⁺ at the Y site. The resulting site populations are reported in Table 6, which also includes a comparison between the values of observed mean atomic number (as defined by Hawthorne et al., 1995) and those calculated from the site populations. The agreement between the refined and calculated values is very good and validates the distribution of cations over the X, Y, Z and T sites in the crystals studied. This site population is also supported by the comparison of weighted bond-valence sums (BVS) and weighted atomic valence (or mean formal charge) calculated from the site populations (Table 7). It is worth noting that the presence of ^W(OH) at the O(1) site, revealed by FTIR spectra, has been quantified by using the empirical equation ^W(OH) = {2 – [1.01⋅BVS(O1)] – 0.21 – F} of Bosi (2013). As a result, O and (OH) are partially disordered over the O(1) and O(3) sites.

Table 7. Weighted bond-valence sum (BVS, in valence units) and weighted atomic valence (WAV) calculated from site population for the povondraite crystals studied.*

* Note: The O(2,4,5,6,7,8) sites are fully occupied by O^2–. Bond valence parameters from Gagné and Hawthorne (2015).

Discussion

All the crystals studied can by identified as povondraite (Table 6). More specifically, they are consistent with oxy-tourmalines belonging to the alkali group (Henry et al., 2011), Na-dominant at the X position and oxy-dominant at the W position with O^2– > (F + OH) in the tourmaline general formula. The Y position is dominated by Fe³⁺ and the Z position requires a double site-occupancy (Fe³⁺₄Mg₂) for formula electroneutrality. Collectively, these constituents lead to the povondraite end-member NaFe³⁺₃(Fe³⁺₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O.

The five analysed crystals show a substitution series dominated by Fe³⁺ and Al, which leads to bosiite, ideally NaFe³⁺₃(Al₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O, by the substitution ^ZFe³⁺ ↔ ^ZAl, and to oxy-dravite, ideally Na(Al₂Mg)(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃O, by the substitution ^YFe³⁺₃ + ^ZMg ↔ 2^YAl₂ + ^YMg + ^ZAl. As a result, the comprehensive substitution reaction along the povondraite–bosiite–oxy-dravite series is: ^YFe³⁺₃ + ^ZMg + ^ZFe³⁺₄ ↔ ^YAl₂ + ^YMg + ^ZAl₅. The latter can be summarised as Fe³⁺Al_–1, as shown in Fig. 3 where the substitution of Fe³⁺ for Al defines a line having a slope of ~45°. Any deviation from this line may be ascribed to other substitutional mechanisms such as coupled substitutions related to O–(OH) at the O(1) (= W) and O(3) (= V) sites. A similar Fe³⁺Al_–1 substitution is reported in Žáček et al. (2000).

Fig. 3. Plot of Fe³⁺ vs. Al. The dashed black line is a linear regression based on 8 data points, which gives a slope of ~45°. Data show the occurrence of the Fe³⁺Al_–1 substitution in the povondraite–bosiite–oxy-dravite series. Black circles represent povondraite (Pov1-5) from this study, grey circle represents povondraite (Pov) from Grice and Ercit (1993), blue circle represents bosiite (Bos) from Ertl et al. (2016), red circle represents oxy-dravite (Odrv) from Bosi and Skogby (2013). Abbreviations list according to Warr (2021).

Sodium and K show similar variations, 0.79–0.89 apfu and 0.12–0.24 apfu, respectively, which are strongly correlated with each other (coefficient of determination, r ² = 0.997) and affect the <X–O> mean bond-length variation (2.729–2.744 Å). In particular, the relatively high K content is related to the increase in Fe³⁺ (Table 4). As noted by Bačík et al. (2008), the incorporation of a relatively large cation such as K (+ Na) into the povondraite structure should be favoured by the larger unit-cell of Fe³⁺ – relative to Al-dominant tourmalines such as bosiite or oxy-dravite (see below). This mechanism is different from that involved in maruyamaite (K- and Al-dominant tourmaline) in which the substitution K → Na occurs only under high-pressure conditions (Berryman et al., 2014; Lussier, et al., 2016).

As for the octahedrally coordinated cations, Mg varies from 1.84 to 2.02 apfu and occupies both the Y and Z sites, whereas Al varies from 0.18 to 1.50 apfu and is ordered at the Z site. Ferric iron varies from 5.44 to 7.07 apfu, showing a rather disordered distribution over the Y and Z sites.

Despite the significant ^YFe³⁺ variations in the present povondraite crystals, the <Y–O> values are practically constant; the increase in contents of the smaller cation ^YFe³⁺ (2.26–2.69 apfu), accompanied by decrease in contents of the larger cation ^YMg (0.31–0.66 apfu), do not produce any decrease in <Y–O> (2.043–2.045 Å). Therefore, we may infer that the accommodation of Fe at the Y site should produce a <Y–O> expansion that compensates for the differences in size between Fe³⁺ and Mg²⁺ substituent ions. This expansion may be shown by the smaller values of bond-valence sum at Y (2.69–2.74 vu) with respect to the weighted atomic valence at Y (2.81–2.90 vu) (Table 7), indicating that the Y-cation is underbonded and bond lengths in the YO₆ polyhedron are stretched (Bosi, 2014).

In general, the variation of the structural parameters is dominated by Fe³⁺ (or Al). No significant correlation occurs between <Y–O> and ^YFe³⁺ (Fig. 4a), whereas the <Z–O> variation (1.997–2.017 Å) is positively correlated with ^ZFe³⁺ (Fig. 4b) and negatively correlated with ^ZAl (not shown; r ² = 0.98). Similarly, the a- and c-parameter are positively related to ^ZFe³⁺ (Fig. 5).

Fig. 4. Plot of <Y–O> vs. Fe³⁺ at the Y site (a) and plot of <Z–O> vs. Fe³⁺ at the Z site (b). The latter shows a much stronger correlation between the parameters than does the former. The dashed black line is a linear regression (number of data = 8 data). Sources of data as in Fig. 3.

Fig. 5. Plot of a and c unit-cell parameters vs. Fe³⁺ at the Z site. The dashed black line is a linear regression (number of data = 8 data). Sources of data as in Fig. 3.

The a- and c-parameters show similar variations in the studied crystals, 16.1679(2)–16.2366(3) Å and 7.4122(1)–7.4688(2) Å, respectively, which are positively correlated (r ² = 0.98). Povondraite has relatively large unit-cell parameters with respect to other tourmalines due to the larger size of Fe³⁺ compared to other trivalent cations V³⁺ > Cr³⁺ > Al³⁺ (Bosi, 2018). The plot of a against c (Fig. 6) shows the variation of these parameters in the tourmaline-supergroup minerals (a range ~15.60–16.25 Å and c range ~7.00–7.50 Å) and their increase with increasing Fe³⁺ or decreasing Al. In particular, the smallest a- and c-parameters are those of synthetic Al-B-tourmalines, whose compositions lead to the end-members NaAl₃Al₆(Si₃B₃O₁₈)(BO₃)₃(OH)₃(OH) (Schreyer et al., 2000; Marler et al., 2002) and NaAl₃Al₆(Si₄B₂O₁₈)(BO₃)₃(OH)₃O (Kutzschbach et al., 2016), whereas the largest ones are of povondraite crystal pov5 of the present study.

Fig. 6. Plot of a against c showing the whole variation of the unit-cell parameters in the tourmaline-supergroup minerals. Plot obtained using 326 data sets with structure refinement. In detail, black circles represent povondraite from this study, a grey circle represents povondraite from Grice and Ercit (1993), a blue circle represents bosiite from Ertl et al. (2016), a red circle represents oxy-dravite from Bosi and Skogby (2013), white circles represent data from literature (see figure 3 of Bosi, 2018), grey squares represent samples from Marler et al. (2002), a grey triangle symbol represents samples from Kutzschbach et al. (2016), and grey diamonds represent the ideal value from Epprecht (1953). Text symbols: Elb = elbaite, Drv = dravite, Srl = schorl. Text symbols highlighted in pale green refer to holotypes data of ^Y(Fe²⁺,Fe³⁺), Cr- and V-dominant tourmalines: Ovdrv = oxy-vanadium-dravite (Bosi et al., 2013), Ocdrv = oxy-chromium-dravite (representing also vanadio-chromium-oxy-dravite; Bosi et al., 2012, 2014a), Capov = chromo-alumino-povondraite (Reznitskii et al., 2014), Vodv = Vanadio-oxy-dravite (Bosi et al., 2014b), Lcc = lucchesiite (Bosi et al., 2017a), Fbu = fluor-buergerite (Donnay et al., 1966), Adc = Adachiite (Nishio-Hamane et al., 2014), Ofoi = oxy-foitite (Bosi et al., 2017c), Foi = foitite (MacDonald et al., 1993), Osrl = oxy-schorl (Bačík et al., 2013). Abbreviations list according to Warr (2021).