Alfredopetrovite, a new selenite mineral from the El Dragón mine, Bolivia

Abstract

Alfredopetrovite, Al 2 (Se 4+ O 3 ) 3 · 6H 2 O, is a new secondary selenite mineral from the El Dragon mine, Antonio Quijarro Province, Potosi Department, Bolivia. The mineral occurs in vugs in a matrix of Co-rich krut’aite–penroseite, dolomite and goethite. Associated minerals are: ahlfeldite, allophane, calcite, chalcomenite, favreauite, felsőbanyaite, malachite and molybdomenite. Crystals occur in drusy/scaly coatings and compact balls, the latter to 0.5 mm in diameter. Individual crystals are up to about 0.1 mm across. The Mohs hardness of alfredopetrovite is 2½; it has no cleavage, curved fracture and a vitreous lustre. The calculated density based on the empirical formula is 2.504 g cm −3 . Alfredopetrovite is uniaxial (+), with ω = 1.554(2) and e = 1.566(2) (white light), and exhibits no pleochroism. Electron microprobe analyses gave the empirical formula Al 1.94 Cu 0.07 Ni 0.03 Co 0.01 Se 2.95 O 15 H12.16, based on 15 O apfu . Alfredopetrovite is hexagonal, space group P 6 ¯ 2 c , with the unit-cell parameters: a = 8.818(3) A, c = 10.721(2) A, V = 722.0(5) A 3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [ d obs /A ( I ) ( hkl )]: 7.63(55)(100), 6.22(55)(101), 5.37(26)(002), 4.398(40)(110,102), 3.404(100)(112), 2.783(50)(211), 2.606(22)(203), and 1.6609(26)(410,322,314,116). The crystal structure was refined to R 1 = 0.0268 for 240 observed reflections [ F o > 4σ F ]. The structure is comprised of fairly regular AlO 6 octahedra and Se 4+ O 3 triangular pyramids. Three Se 4+ O 3 pyramids link two adjacent AlO 6 octahedra forming a [Al(H 2 O) 3 | 2 (Se 4+ O 3 ) 3 cluster structural unit. These structural units are bonded to one another only via hydrogen bonds yielding a structure with relatively large channels along [001]. The configuration of the cluster is similar to that of the distinctive unit in the NASICON structure, commonly referred to as a lantern unit .