1. Department of Geological Sciences, Stanford University, Stanford, California 94305, USA
2. Cinvestav Unidad Saltillo, Apartado Postal 663, 25000 Saltillo, Coahuila, Mexico
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA