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

Abstract

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.