**Acknowledgements**

The author is grateful to *Bernd Köhler*, *Bernd Steinweg*, *Lutz Huischen*, *Timo Kluttig*, *Thilo Rennert* and *Kai Gockel* for field and laboratory assistance. Laboratory assistance was also given by *Heidi Biernath*, *Willi Gosda* and *Gerlind Wilde*. Technical support was provided by *Heinrich Wolfsperger* and Dr*. Norbert Krahmer*, Geologischer Dienst Nordrhein-Westfalen, Germany.
