1. Institut für Geowissenschaften, Kiel University, Olshausenstrasse 40, 24118 Kiel, Germany, Department of Crystallography, Nizhny Novgorod State University, Gagarin av. 23/3, 603950 Nizhny Novgorod, Russia, Department of Crystallography, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia, and Mineralogisch-Petrographisches Institut, Hamburg University, Grindelallee 48, 20146 Hamburg, Germany