Re-Examination of Yushkinite: Chemical Composition, Optical Properties and Interlayer Charge Transfer

Y. Moëlo1, O. Rouer2, L. Cario1 and B. Cervelle3
1 Laboratoire de Chimie des Solides, Institut des Matériaux Jean Rouxel, UMR 6502 CNRS-Université de Nantes, 2, rue de la Houssinière, BP 32 229, 44 322 Nantes cedex 03, France
2 CNRS-CRSCM, 1A, rue de la Férollerie, 45 071 Orléans cedex, France
3 Institut Francilien des Géosciences, Université de Marne-La-Vallée, 5, Bd. Descartes, Champs-sur-Marne, 77 454 Marne-La-Vallée cedex 2, France

Abstract: The hydroxy-sulphide yushkinite from the type deposit has been re-examined by electron probe microanalysis and reflectance measurement. The EPMA indicates minor Cu (up to 1.5 wt.%) and confirms an excess of V. Taking into account its composite structure (brucite-like hydroxide layer alternating with a berndtite-type sulphide layer) the proposed structural formula is, with the addition of minor intercalated Cu: [(Mg0.71Al0.36V0.03)Σ1.10(OH)2.18O0.02]Cu0.02 [VS2]. At the crystal scale, some crystal edges give a variable excess of Si and Al (in the 1/1 ratio), indicating a late topotactic intergrowth with a layered alumino-silicate (or a composite silicate-sulphide). New specular reflectance data are given in air and oil from 400 to 800 nm, with the polarization plane parallel or perpendicular to the elongation. The refractive index n and absorption coefficient k are given; no absorption band exists in the perpendicular position, with constant n close to 1.9, and k to 0.3. In a parallel position, a pseudo-transparency window is centred around 500–520 nm (∼2.4 eV). The main absorption band, centred around 730 nm (∼1.7 eV), corresponds probably to a transition t2g → eg related to intra-layer V-V bonding. Charge transfer from the brucite-type layer to the VS2 one, strictly controlled through H-bonding, induces a mean formal valency of V close to 3.6. Yushkinite is compared with other synthetic layered hydroxy-sulphides, as well as brucite-type intercalated compounds.

Keywords: yushkinite • hydroxy-sulphide • vanadium • electron microprobe data • specular reflectance data • charge transfer • hydrogen bonding

Mineralogical Magazine; December 1999 v. 63; no. 6; p. 879-889; DOI: 10.1180/002646199548989
© 1999, The Mineralogical Society
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