Single-crystal Raman spectroscopy of brandholzite Mg[Sb(OH)6]2•6H2O and bottinoite Ni[Sb(OH)6]2• 6H2O and the polycrystalline Raman spectrum of mopungite Na[Sb(OH)6]

The single-crystal Raman spectra of minerals brandholzite and bottinoite, formula M[Sb(OH)(6)](2)center dot 6H(2)O, where M is Mg+2 and Ni+2, respectively, and the non-aligned Raman spectrum of mopungite, formula Na[Sb(OH)(6)], are presented for the first time. The mixed metal minerals comprise alternating layers of [Sb(OH)(6)](-1) octahedra and mixed [M(H2O)(6)](+2)/[Sb(OH)(6)](-1) octahedra. Mopungite comprises hydrogen-bonded layers of [Sb(OH)(6)](-1) octahedra linked within the layer by Na+ ions. The spectra of the three minerals were dominated by the Sb-O symmetric stretch of the [Sb(OH)(6)](-1) octahedron, which occurs at approximately 620 cm(-1). The Raman spectrum of mopungite showed many similarities to spectra of the di-octahedral minerals, supporting the view that the Sb octahedra give rise to most of the Raman bands observed, particularly below 1200 cm(-1). Assignments have been proposed on the basis of the spectral comparison between the minerals, prior literature and density functional theory (DFT) calculations of the vibrational spectra of the free [Sb(OH)(6)](-1) and [M(H2O)(6)](+2) octahedra by a model chemistry of B3LYP/6-31G(d) and lanl2dz for the Sb atom. The single-crystal spectra showed good mode separation, allowing most of the bands to be assigned to the symmetry species A or E. Copyright (C) 2010 John Wiley & Sons, Ltd.