[1] Free Univ Berlin, Inst Geolog Wissensch, Malteserstr 74-100, D-12249 Berlin, Germany
[2] Univ Cologne, Inst Geol & Mineral, Zulpicher Str 49b, D-50674 Cologne, Germany
[3] Max Planck Inst Chem, Hahn Meitner Weg 1, D-55128 Mainz, Germany
[4] Imperial Coll London, Royal Sch Mines, Dept Earth Sci & Engn, Prince Consort Rd, Kensington London SW7 2AZ, England
[5] Univ Firenze, Dipartimento Sci Terra, Via Pira 4, I-50121 Florence, Italy