[1] Free Univ Berlin, Inst Kristallog, D-14195 Berlin, Germany
[2] Univ Witwatersrand, Dept Chem, ZA-2050 Johannesburg, Johannesburg, South Africa
[3] Univ Bonn, Mineral Petrol Inst, D-53115 Bonn, Germany
[4] Univ Hamburg, Inst Mineral & Petrog, D-20146 Hamburg, Germany