1. Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC), UMR 7590, CNRS, Université Paris 6 & 7, IPGP, 140, rue de Lourmel, 75015 Paris, France, Surface & Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025