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]

Abstract

The single-crystal Raman spectra of minerals brandholzite and bottinoite, formula M[Sb(OH)6]2•6H2O, 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 © 2011 John Wiley & Sons, Ltd.