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J. Bourdier, F-45067 Orl6ans cedex, France. (jeanlouis .bourdier @ univ-orleans, fr) F. Holtz, M. Pichavant, and B. Scaillet, Centre de Recherches sur la Synth?se et chimie des Min6raux, CNRS, 1A rue de la F6rollerie, F- 45071 Orl6ans