Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New AI-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia

Abstract

A new mineral species, petermegawite, ideally Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, was discovered at the El Dragón mine, Potosí Department, Bolivia. It occurs as aggregates of bladed or tabular crystals. Associated minerals are Co-bearing krut′aite–penroseite (matrix), chalcomenite, ‘clinochalcomenite’ (not IMA approved), molybdomenite, lepidocrocite, goethite, ahlfeldite, and calcite. Petermegawite is colorless in transmitted light and transparent with a white streak and vitreous luster. It is brittle and has a Mohs hardness of 2–2½. Cleavage is perfect on {001}. The measured and calculated densities are 2.27(5) and 2.32 g/cm3, respectively. Optically, petermegawite is biaxial (+), with α = 1.545(5), β = 1.554(5), γ = 1.567(5) (white light). An electron microprobe analysis yielded an empirical formula (based on 10 non-H cations pfu) of Al6.00[(Se0.89S0.11)Σ1.00O3]3[(Si0.90Al0.07)Σ0.97O2.81(OH)1.19](OH)9⋅10H2O, which can be simplified to Al6[(Se,S)O3]3[(Si,Al)O3(OH)](OH)9⋅10H2O. Petermegawite is orthorhombic, space group Cmc21 with unit-cell parameters a = 16.2392(2), b = 10.96370(10), c = 15.3367(2) Å, V = 2730.57(5) Å3, and Z = 4. Its crystal structure is characterized by clusters of six-membered rings of edge-sharing AlO6 octahedra with an SiO3(OH) tetrahedron situated at the center of each ring and three Se4+O3 triangular pyramids appended outside the ring. These clusters are joined together by hydrogen bonds to form heteropolyhedral layers parallel to (001). The H2O molecules that are not bonded to any non-H cations are sandwiched between the layers. The cluster formed by a six-membered AlO6 octahedral ring with an SiO3OH tetrahedron at the center of the ring in petermegawite has the composition [Al6(SiO3OH)O6(OH)9(H2O)6]6−. It is topologically identical to the clusters of composition [Al6(AsO4)O7(OH)9(H2O)5]8− that are found in bettertonite and penberthycroftite. As a new polyoxometalate building block, this type of cluster may be expressed with a general chemical formula [TM6X25], where T = tetrahedrally coordinated cation, M = octahedrally coordinated cation, and X = O, OH, H2O.