1. Faculty of Geosciences, Department of Geology, Mineralogy and Geophysics, Ruhr-Universitaet Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany, Institut de Ciencia de Materials de Barcelona, Campus de la Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain, and Max-Planck-Institut fuer Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Muelheim a. d. Ruhr, Germany