Bernardevansite, Al2(Se4+O3)3⋅6H2O, dimorphous with alfredopetrovite and the Al-analogue of mandarinoite, from the El Dragón mine, Potosí, Bolivia

Abstract A new mineral species, bernardevansite (IMA2022-057), ideally Al2(Se4+O3)3⋅6H2O, has been discovered from the El Dragón mine, Potosí Department, Bolivia. It occurs as aggregates or spheres of radiating bladed crystals on a matrix consisting of Co-bearing krut'aite–penroseite. Associated minerals are Co-bearing krut'aite–penroseite, chalcomenite and ‘clinochalcomenite’. Bernardevansite is colourless in transmitted light, transparent with white streak and vitreous lustre. It is brittle and has a Mohs hardness of 2½–3. Cleavage is not observed. The measured and calculated densities are 2.93(5) and 2.997 g/cm3, respectively. Optically, bernardevansite is biaxial (+), with α = 1.642(5), β = 1.686(5) and γ = 1.74(1) (white light). An electron microprobe analysis yielded an empirical formula (based on 15 O apfu) (Al1.26Fe3+0.82)Σ2.08(Se0.98O3)3⋅6H2O, which can be simplified to (Al,Fe3+)2(SeO3)3⋅6H2O. Bernardevansite is the Al-analogue of mandarinoite, Fe3+2(SeO3)3⋅6H2O or dimorphous with P$\bar{6}$2c alfredopetrovite. It is monoclinic, with space group P21/c and unit-cell parameters a = 16.5016(5), b = 7.7703(2), c = 9.8524(3) Å, β = 98.258(3)°, V = 1250.21(6) Å3 and Z = 4. The crystal structure of bernardevansite consists of a corner-sharing framework of M3+O6 (M = Al and Fe) octahedra and Se4+O3 trigonal pyramids, leaving large voids occupied by the H2O groups. There are two unique M3+ positions: M1 is octahedrally coordinated by (4O + 2H2O) and M2 by (5O + H2O). The structure refinement indicates that Al preferentially occupies M1 (= 0.692Al + 0.308Fe) over M2 (= 0.516Al + 0.484Fe). The substitution of the majority of Fe in mandarinoite by Al results in a significant reduction in its unit-cell volume from 1313.4 Å3 to 1250.21(6) Å3 for bernardevansite. The discovery of bernardevansite begs the question whether the Fe3+ end-member, Fe3+2(SeO3)3⋅6H2O, has two polymorphs as well, one with P21/c symmetry, as for mandarinoite and the other P$\bar{6}$2c, as for alfredopetrovite.


Introduction
Bernardevansite, ideally Al 2 (Se 4+ O 3 ) 3 ⋅6H 2 O, is a new mineral species from the El Dragón mine, Antonio Quijarro Province, Potosí Department, Bolivia. It is named in honour of Dr Bernard W. Evans (b. 1934, Fig. 1 [1965][1966][1967][1968][1969] and a Professor at the University of Washington in Seattle from 1969-2001. Bernard's major research interests included petrology, mineralogy, geochemistry and electron microprobe analysis, with outstanding contributions to the crystal chemistry and thermodynamics of amphiboles in particular and metamorphic minerals in general. In his over 50 years academic career, he has received numerous awards or honours, such as the Tennant Prize for Geology, King's College, London (1955), the Mineralogical Society of America (MSA) Award (1970), U.S. Senior Scientist Award, Humboldt Foundation (1988-89), the President of MSA (1993-94), Fulbright Scholar, France (1995 and the Roebling Medal of MSA (2008). Dr Evans has gladly accepted the proposed naming. The new mineral and its name (symbol Bev) have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA2022-057, Yang et al., 2023). The co-type samples have been deposited at the University of Arizona Alfie Norville Gem and Mineral Museum (Catalogue # 22712) and the RRUFF Project (deposition # R210010) (http://rruff.info). This paper describes the physical and chemical properties of bernardevansite, and its crystal structure determined from single-crystal X-ray diffraction data, illustrating its structural relationships with mandarinoite and alfredopetrovite.

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}, 110} 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/cm 3 and the calculated density is 2.997 g/cm 3 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, 1981). Bernardevansite is insoluble in water or hydrochloric acid.
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 ( 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.

Alfredopetrovite
Bernardevansite Mandarinoite (3) 16.5016 (5) 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 (1991) ( Table 7). The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Crystal structure description and discussion
Bernardevansite, Al 2 (SeO 3 ) 3 ⋅6H 2 O, is isostructural with mandarinoite, Fe 3+ 2 (SeO 3 ) 3 ⋅6H 2 O (Hawthorne, 1984), rather than with the Al end-member P 62c alfredopetrovite, Al 2 (SeO 3 ) 3 ⋅6H 2 O (Morris et al., 1992;Kampf et al., 2016a). In other words, it is dimorphous with alfredopetrovite. The crystal structure of bernardevansite consists of a corner-sharing framework of M 3+ O 6 (M = Al and Fe) octahedra and Se 4+ O 3 trigonal pyramids, leaving large voids that are occupied by the H 2 O groups (Fig. 7). There are three unique Se positions in bernardevansite, each of which is coordinated to three O atoms to form characteristic SeO 3 trigonal pyramids. There are two unique M 3+ positions: M1 is octahedrally coordinated by (4O + 2H 2 O) and M2 by (5O + H 2 O). The structure refinement indicates that Al preferentially occupies M1 (= 0.692Al + 0.308Fe) over M2 (= 0.516Al + 0.484Fe). There are three distinct H 2 O molecules (O13, O14 and O15) in the structure that are not bonded to any non-H cation (Table 7), in addition to three H 2 O 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, 1984) (Table 6).  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 Å 3 to 1250.21(6) Å 3 , 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, 1984), 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 VI Al 3+ (0.535 Å) is smaller than that of VI Fe 3+ (0.645 Å) (Shannon, 1976). A survey of the literature appears to suggest that, for a structure with two or more octahedral sites, Al 3+ is likely to be favoured by the site coordinated with more H 2 O molecules. This is indeed the case for bernardevansite, as the M1 site is coordinated by (4O + 2H 2 O) and M2 by (5O + H 2 O). Another typical example is coquimbite, which contains three distinct octahedral sites (M1, M2 and M3), with M1 coordinated by (6H 2 O), M2 by (6O 2-) and M3 by (3H 2 O + 3O 2-). All structure determinations on coquimbite have shown that Al 3+ is predominately or exclusively ordered into the M1 site (e.g. Demartin et al., 2010;Yang and Giester, 2018;Mauro et al., 2020 and references therein).
According to the Raman spectroscopic studies on hydrous materials containing (SeO 3 ) 2-(e.g. Wickleder et al., 2004;Frost et al., 2006;Frost and Keeffe, 2008;Djemel et al., 2013;Wolak et al., 2013;Kasatkin et al., 2014;Mills et al., 2014;Kampf et al., 2016b), 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 -  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.