1. Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S. Geological Survey, 345 Middlefield Rosd, MS 901, Menlo Park, California 94025, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Rosd, MS 69, Menlo Park, California 94025