1. Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, PNC-CAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, Environmental Dynamics & Simulation, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-96, Richland, Washington 99352, and Stanford Synchrotron Radiation Laboratory, SLAC, MS 69, 2575 Sand Hill Road, Menlo Park, California 94025