1. Institut Charles Gerhardt Montpellier, UMR 5253 CNRS/UM2/ENSCM /UM1, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France, and Department of Geological Sciences, Chemical and Environmental Technologies, University of Urbino, Loc. Crocicchia, 61029 Urbino, Italy