Sample description and experimental methods

Occurrence

Bernardevansite was found on a specimen (Fig. 2) collected from the El Dragón mine (19°49’15”S, 65°55’00”W), Antonio Quijarro Province, Potosí Department, Bolivia. Associated minerals are Co-bearing krut'aite–penroseite (matrix), chalcomenite and ‘clinochalcomenite’ (not IMA-approved). Detailed descriptions on the geology and mineralogy of the El Dragón mine have been given by Grundmann et al. (Reference Grundmann, Lehrberger and Schnorrer-Köhler1990, Reference Grundmann, Lehrberger and Schnorrer-Köhler2007) and Grundmann and Förster (Reference Grundmann and Förster2017). This mine exploited a telethermal deposit consisting of a single selenide vein hosted in sandstones and shales. The major ore mineral is krut'aite, CuSe₂, varying in composition to penroseite, NiSe₂. Later solutions rich in Bi, Pb and Hg resulted in the crystallisation of minerals such as clausthalite, petrovicite, watkinsonite, and the recently described minerals eldragónite, Cu₆BiSe₄(Se₂) (Paar et al., Reference Paar, Cooper, Moëlo, Stanley, Putz, Topa, Roberts, Stirling, Raith and Rowe2012), grundmannite, CuBiSe₂ (Förster et al., Reference Förster, Bindi and Stanley2016), hansblockite, (Cu,Hg)(Bi,Pb)Se₂ (Förster et al., Reference Förster, Bindi, Stanley and Grundmann2017), cerromojonite, CuPbBiSe₃ (Förster et al., Reference Förster, Bindi, Grundmann and Stanley2018) and nickeltyrrellite, CuNi₂Se₄ (Förster et al., Reference Förster, Ma, Grundmann, Bindi and Stanley2019). Oxidation produced a wide range of secondary Se-bearing minerals, such as favreauite, PbBiCu₆O₄(SeO₃)₄(OH)⋅H₂O (Mills et al., Reference Mills, Kampf, Housley, Christy, Thorne, Chen and Steele2014), alfredopetrovite, Al₂(Se⁴⁺O₃)₃⋅6H₂O (Kampf et al., Reference Kampf, Mills, Nash, Thorne and Favreau2016a), petermegawite Al₆(Se⁴⁺O₃)₃[SiO₃(OH)](OH)₉⋅10H₂O (Yang et al., Reference Yang, Gu, Jenkins, Gibbs, McGlasson and Scott2022a), franksousaite PbCu(Se⁶⁺O₄)(OH)₂ (Yang et al., Reference Yang, McGlasson, Gibbs and Downs2022b) and the new mineral bernardevansite, described herein.

Fig. 2. The specimen on which the new mineral bernardevansite, indicated by the blue arrow, was found (R210010).

Physical and chemical properties and Raman spectra

Bernardevansite occurs as aggregates or spheres of radiating bladed crystals (Figs 3,4,5) on a matrix consisting of Co-bearing krut'aite–penroseite. Individual crystals of bernardevansite are found up to 0.10 × 0.03 × 0.01 mm, with elongation along [001] and common crystal forms {100}, {110}, $\bar{1}$10} and {001}. Bernardevansite is colourless in transmitted light and transparent with white streak, and has a vitreous lustre. It is brittle and has a Mohs hardness of 2½–3. Cleavage was not observed. The density measured by flotation in heavy liquids is 2.93(5) g/cm³ and the calculated density is 2.997 g/cm³ on the basis of the empirical chemical formula and unit-cell volume from single-crystal X-ray diffraction data. Optically, bernardevansite is biaxial (+), with α = 1.642(5), β = 1.686(5), γ = 1.74(1) (determined in white light), 2V (meas.) = 84(2)° and 2V (calc.) = 87°. The pleochroism is very weak, from pale grey to grey, and dispersion was not observed. The calculated Gladstone-Dale compatibility index based on the empirical formula is 0.013 (superior) (Mandarino, Reference Mandarino1981). Bernardevansite is insoluble in water or hydrochloric acid.

Fig. 3. A microscopic view of aggregates or spheres of pale grey to colourless, radiating bladed bernardevansite crystals (R210010).

Fig. 4. A back-scattered electron image of aggregates of radiating bladed bernardevansite crystals (R210010).

Fig. 5. A back-scattered electron image of aggregates of bladed bernardevansite crystals (R210010).

The chemical composition was determined using a Shimadzu-1720 electron microprobe (WDS mode, 15 kV, 10 nA and a beam diameter of 2 μm). The standards used for the probe analysis are given in Table 1, along with the determined compositions (11 analysis points). The resultant chemical formula, calculated on the basis of 15 O apfu (from the structure determination), is (Al_1.26Fe³⁺_0.82)_Σ2.08(Se_0.98O₃)₃⋅6H₂O, which can be simplified to (Al,Fe³⁺)₂(SeO₃)₃⋅6H₂O.

Table 1. Chemical compositions (in wt.%) of bernardevansite.*

*Bernardevansite is prone to the electron beam damage, however this did not seem to affect the relative proportions of cations. The large variations in the Al₂O₃ and Fe₂O₃ contents result from the strong correlation between the two components.

S.D. – standard deviation

The Raman spectrum of bernardevansite (Fig. 6) was collected on a randomly oriented crystal with a Thermo Almega microRaman system, using a solid-state laser with a wavelength of 532 nm at 75 mW power and a thermoelectric cooled CCD detector. The laser is partially polarised with 4 cm^–1 resolution and a spot size of 1 μm.

Fig. 6. Raman spectra of bernardevansite, mandarinoite and alfredopetrovite.

X-ray crystallography

Both the powder and single-crystal X-ray diffraction data for bernardevansite were collected on a Rigaku Xtalab Synerg D/S 4-circle diffractometer equipped with CuKα radiation. Powder X-ray diffraction data were collected in the Gandolfi powder mode at 50 kV and 1 mA (Table 2) and the unit-cell parameters were refined using the program by Holland and Redfern (Reference Holland and Redfern1997): a = 16.535(1), b = 7.7762(5), c = 9.8841(6) Å, β = 98.337(7)° and V = 1257.5(5) ų.

Table 2. Powder X-ray diffraction data (d in Å, I in %) for bernardevansite.*

*The strongest lines are given in bold

All bernardevansite crystals examined are pervasively twinned on (100) with a twin law ($\bar{1}$ 0 ½, 0 $\bar{1}$ 0, 0 0 1). Single-crystal X-ray diffraction data were collected from a 0.03 × 0.02 × 0.01 mm fragment. The systematic absences of reflections suggest the unique space group P2₁/c. The structure was solved and refined using SHELX2018 (Sheldrick, Reference Sheldrick2015a, Reference Sheldrick2015b). No H atoms were located through the difference-Fourier syntheses. The refined Al/Fe ratios at the octahedral M1 and M2 sites are (0.692Al + 0.308Fe) and (0.516Al + 0.484Fe), respectively, yielding a total Al/Fe ratio of 1.208/0.792, which is very close to that (1.211/0.789, normalised) measured from the electron microprobe analysis. The structure was refined as a 2-component twin with a twin ratio of 0.81/0.19. Final refinement statistics for bernardevansite are listed in Table 3. Atomic coordinates and displacement parameters are given in Tables 4 and 5, respectively. Selected bond distances are presented in Table 6. The bond-valence sums were calculated using the parameters given by Brese and O'Keeffe (Reference Brese and O'Keeffe1991) (Table 7). The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 3. Summary of crystallographic data and refinement results for bernardevansite, mandarinoite and alfredopetrovite.

Table 4. Fractional atomic coordinates and equivalent isotropic displacement parameters (Ų) for bernardevansite.

Table 5. Atomic displacement parameters (Ų) for bernardevansite.

Table 6. Selected bond distances (Å) for bernardevansite, Al₂(SeO₃)₃⋅6H₂O and mandarinoite, Fe₂(SeO₃)₃⋅6H₂O.*

*Notes: M = Fe and (Al,Fe) for mandarinoite and bernardevansite, respectively.

References: (1) Hawthorne (Reference Hawthorne1984); (2) this study.

Table 7. Bond-valence sums for bernardevansite.*

*Note: The bond valence sums for M1 and M2 were calculated based on (0.692 Al + 0.308 Fe³⁺) and (0.516 Al + 0.484 Fe³⁺), respectively.

Crystal structure description and discussion

Bernardevansite, Al₂(SeO₃)₃⋅6H₂O, is isostructural with mandarinoite, Fe³⁺₂(SeO₃)₃⋅6H₂O (Hawthorne, Reference Hawthorne1984), rather than with the Al end-member P $\bar{6}$2c alfredopetrovite, Al₂(SeO₃)₃⋅6H₂O (Morris et al., Reference Morris, Harrison, Stucky and Cheetham1992; Kampf et al., Reference Kampf, Mills, Nash, Thorne and Favreau2016a). In other words, it is dimorphous with alfredopetrovite. The crystal structure of bernardevansite consists of a corner-sharing framework of M ³⁺O₆ (M = Al and Fe) octahedra and Se⁴⁺O₃ trigonal pyramids, leaving large voids that are occupied by the H₂O groups (Fig. 7). There are three unique Se positions in bernardevansite, each of which is coordinated to three O atoms to form characteristic SeO₃ trigonal pyramids. There are two unique M ³⁺ positions: M1 is octahedrally coordinated by (4O + 2H₂O) and M2 by (5O + H₂O). The structure refinement indicates that Al preferentially occupies M1 (= 0.692Al + 0.308Fe) over M2 (= 0.516Al + 0.484Fe). There are three distinct H₂O molecules (O13, O14 and O15) in the structure that are not bonded to any non-H cation (Table 7), in addition to three H₂O molecules (O10, O11 and O12) bonded to M cations. Although our structure determination failed to locate H atoms, all O–O distances for H-bonding in bernardevansite are consistent and comparable with those found in mandarinoite (Hawthorne, Reference Hawthorne1984) (Table 6).

Fig. 7. Crystal structure of bernardevansite. Green, yellow and grey polyhedra represent M1O₆, M2O₆ and SeO₃ groups, respectively. Purple and aqua spheres represent Se (Se1, Se2 and Se3) atoms and H₂O (O13, O14 and O15) groups that are not bonded to any non-H cation, respectively.

The substitution of the majority of Fe in mandarinoite by Al in bernardevansite results in a significant reduction in unit-cell volume from 1313.4 ų to 1250.21(6) ų, which motivated this investigation. Compared to mandarinoite, which has the identical average <M–O> bond distances (2.021 Å) for the two octahedral sites (Hawthorne, Reference Hawthorne1984), the <M–O> distance for the M1 site (1.942 Å) in bernardevansite is shorter than that for the M2 site (1.955 Å), consistent with the preference of Al at M1 over M2 (0.692 vs. 0.516), as the ionic radius of ^VIAl³⁺ (0.535 Å) is smaller than that of ^VIFe³⁺ (0.645 Å) (Shannon, Reference Shannon1976). A survey of the literature appears to suggest that, for a structure with two or more octahedral sites, Al³⁺ is likely to be favoured by the site coordinated with more H₂O molecules. This is indeed the case for bernardevansite, as the M1 site is coordinated by (4O + 2H₂O) and M2 by (5O + H₂O). Another typical example is coquimbite, which contains three distinct octahedral sites (M1, M2 and M3), with M1 coordinated by (6H₂O), M2 by (6O^2–) and M3 by (3H₂O + 3O^2–). All structure determinations on coquimbite have shown that Al³⁺ is predominately or exclusively ordered into the M1 site (e.g. Demartin et al., Reference Demartin, Castellano, Gramaccioli and Campostrini2010; Yang and Giester, Reference Yang and Giester2018; Mauro et al., Reference Mauro, Biagioni, Pasero, Skogby and Zaccarini2020 and references therein).

According to the Raman spectroscopic studies on hydrous materials containing (SeO₃)^2– (e.g. Wickleder et al., Reference Wickleder, Buchner, Wickleder, Sheik, Brunklaus and Eckert2004; Frost et al., Reference Frost, Weier, Reddy and Čejka2006; Frost and Keeffe, Reference Frost and Keeffe2008; Djemel et al., Reference Djemel, Abdelhedi, Ktari and Dammak2013; Wolak et al., Reference Wolak, Pawlowski, Polomska and Pietraszko2013; Kasatkin et al., Reference Kasatkin, Plášil, Marty, Agakhanov, Belakovskiy and Lykova2014; Mills et al., Reference Mills, Kampf, Housley, Christy, Thorne, Chen and Steele2014; Kampf et al., Reference Kampf, Mills and Nash2016b), we made the following tentative assignments of major Raman bands for bernardevansite. The broad bands between 2900 and 3500 cm^–1 and those between 1500 and 1750 cm^–1 are due to the O–H stretching and H–O–H bending modes in H₂O groups, respectively. The bands at 844 and 685 cm^–1 are ascribable to the Se⁴⁺–O symmetric and antisymmetric stretching vibrations, respectively, within the Se⁴⁺O₃ groups, whereas those from 320 to 570 cm^–1 originate from the O–Se⁴⁺–O bending modes. The bands below 320 cm^–1 are mainly associated with the rotational and translational modes of Se⁴⁺O₃ groups, as well as the M ³⁺–O interactions and lattice vibrational modes.

For comparison, the Raman spectra of alfredopetrovite, Al₂(Se⁴⁺O₃)₃⋅6H₂O and mandarinoite, Fe³⁺₂(Se⁴⁺O₃)₃⋅6H₂O, from the RRUFF Project (http://rruff.info/R210014 and http://rruff.info/R140742, respectively) are also plotted in Fig. 6. Evidently, the spectrum of bernardevansite is more similar to that of mandarinoite than to that of alfredopetrovite, pointing to the structural similarities between bernardevansite and mandarinoite.

Although the bernardevansite sample we studied here, (Al_0.61Fe³⁺_0.39)₂(SeO₃)₃⋅6H₂O, is isostructural with mandarinoite, its chemistry is closer to that of alfredopetrovite, the Al end-member of the Fe³⁺₂(SeO₃)₃⋅6H₂O–Al₂(SeO₃)₃⋅6H₂O system, as illustrated in Fig. 8. This raises an interesting question about its ideal chemical formula. Should it be expressed as (1) an Fe-bearing formula, (Al_1–xFe_x)₂(SeO₃)₃⋅6H₂O, where 0 < x < 0.5, or (2) an Fe-free end-member formula, Al₂(SeO₃)₃⋅6H₂O. The first Fe-bearing formula requires that Fe is essential to stabilise the P2₁/c mandarinoite-type structure and there is no complete solid-solution series between Al₂(SeO₃)₃⋅6H₂O and Fe₂(SeO₃)₃⋅6H₂O. This formula appears to be consistent with synthetic experiments, as several hydrothermal syntheses of Al selenites conducted thus far have revealed only the hexagonal form of Al₂(SeO₃)₃⋅6H₂O and no monoclinic form (Morris et al., Reference Morris, Harrison, Stucky and Cheetham1992; Ratheesh et al., Reference Ratheesh, Suresh, Nayar and Morris1997 and references therein). In contrast, the second Fe-free formula implies that Al₂(SeO₃)₃⋅6H₂O possesses two polymorphs: a monoclinic P2₁/c mandarinoite-type form and a hexagonal P $\bar{6}$2c alfredopetrovite form. Regardless of its ideal chemical formula, the discovery of bernardevansite begs the question whether the Fe³⁺ end-member, Fe³⁺₂(SeO₃)₃⋅6H₂O, has two polymorphs as well, one with P2₁/c symmetry, as for mandarinoite, and the other P $\bar{6}$2c, as for alfredopetrovite.

Fig. 8. A concept of classification of M ³⁺(SeO₃)₃⋅6H₂O minerals (M = Al and Fe).