ABSTRACT
In the oxidized top of both the Grafenbergand the Haltern-layers, accumulations of Fe(III) hydroxides are documented (Banning et al., 2009). In the course of oxidation, these primarily amorphous minerals “aged”, i.e. they evolved into more crystalline phases like goethite. Iron concretions developed which can be significantly enriched in As with up to 1860 μg/g in the Grafenberg-layers and up to 204 μg/g in the Haltern-layers. Reduced Fe phases in the lower part of these units are suspected to be the primary As source.
