1. Environmental Geochemistry Group, LGIT-IRIGM, University of Grenoble and CNRS, BP 53, 38041 Grenoble Cedex 9, France, Environmental Biology and Institute of Material Sciences, Limburgs Universitair Centrum, Universitaire Campus, building D, B-3590 Diepenbeek, Belgium, and Laboratoire de Cristallographie, CNRS, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France