Mineralogical and geochemical alteration of low-grade metamorphic black slates due to oxidative weathering

Abstract

Low-grade metamorphic black slates of Silurian and Lower Devonian ages (from the Thuringisches Schiefergebirge in Germany) were investigated to identify mineralogical and geochemical alterations that Occur during the oxidative weathering black slates. The slates exhibit ail intense total organic carbon decrease (> 90 wt%) due to oxidative weathering. The organic matter in black state consists mainly of non-pyrolysable material with only minor portions of pyrolysable material. In contrast to the non-pyrolysable organic matter, the concentration of pyrolysable organic matter is not decreased during weathering. Heating experiments have yielded information about a potential protection of pyrolysable organic matter against weathering, which is probably caused by a structural configuration or the intergrowth of organic matter and illite. The slates consist mainly of illite. Iron oxide (goethite) and phosphate encrustations were formed oil slate surfaces Lis a result of oxidative and acid weathering of both iron sulfide and monazite. Kaolinite often Occurs Lis ail older encrustation beneath the iron oxide encrustations. Encrustations of iron oxide and phosphate are major sinks for trace elements, released during oxidative weathering. (c) 2009 Elsevier GmbH. All rights reserved.