Fjo De Ridder,* Rik Pintelon, Johan Schoukens, Jacques Navez, Luc André and Frank Dehairs Department of Electricity and Instrumentation, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. Tel: 132 2629 36 64; Fax: 132 2 629 28 50; E-mail: federid@pop.vub.ac.be Section of Petrography-Mineralogy-Geochemistry, Royal Museum for Central Africa, B-3080 Tervuren, Belgium Analytical Che...