Alfredopetrovite, a new selenite mineral from the El Dragón mine, Bolivia
Kampf, Anthony R.; Mills, Stuart J.; Nash, Barbara P.; Thorne, Brent; Favreau, Georges
European Journal of Mineralogy Volume 28 Number 2 (2016), p. 479 - 484
published: May 1, 2016
Alfredopetrovite, Al2 (Se4+ O3)3 6H2 O, is a new secondary selenite mineral from the El Dragón mine, Antonio Quijarro Province, Potosí 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, felsobányaite, 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 cm3. Alfredopetrovite is uniaxial (+), with ω = 1.554(2) and ε = 1.566(2) (white light), and exhibits no pleochroism. Electron microprobe analyses gave the empirical formula Al1.94 Cu0.07 Ni0.03 Co0.01 Se2.95 O15 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) Å , c = 10.721(2)Å , V = 722.0(5) Å 3and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d obs/Å (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. > 4σF]. The structure is comprised of fairly regular AlO6 octahedra and Se4+ O3 triangular pyramids. Three Se4+ O3 pyramids link two adjacent AlO6 octahedra forming a [Al(H2O)3]2 (Se4+ O3)3 cluster structural unit. These structural units are bonded to one another only via hydrogen bonds yielding a structure with relatively large channels along . The configuration of the cluster is similar to that of the distinctive unit in the NASICON structure, commonly referred to as a lantern unit.