The paper describes the Rossendorf beamline (ROBL) built by the Forschungszentrum Rossendorf at the ESRF. ROBL comprises two different and independently operating experimental stations: a radiochemistry laboratory for X-ray absorption spectroscopy of non-sealed radioactive samples and a general purpose materials research station for X-ray diffraction and re¯ectometry mainly of thin ®lms and interfaces modi®ed by ion beam techniques.

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The formation of colloids during the weathering of phyllite was investigated by exposing ground phyllite to Milli-Q water. Secondary mineral colloids of 10 1 -10 2 nm were detected in significant concentrations. At pH of about 8.5, the solution concentration of these colloids reached up to 10 mg/L (however, acidification to pH 4.0 prevented the formation of the colloids). The mineralogical composition of the secondary mineral colloids is assumed to be a mixture of ferrihydrite, manganese oxyhydroxides, aluminosilicates, amorphous Al(OH) 3 and gibbsite with possible additions of iron silicates and iron-alumino silicates. The colloids were stable over longer periods of time (at least several weeks), even in the presence of suspended ground rock. Direct formation of iron-containing secondary mineral colloids at the rock-water interface by the weathering of rock material is an alternative to the well-known mechanism of iron colloid formation in the bulk of water bodies by mixing of different waters or by aeration of anoxic waters. This direct mechanism is of relevance for colloid production during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in mine waste rock piles. Colloids produced by this mechanism, too, can influence the transport of contaminants such as actinides because these colloids have a large specific surface area and a high sorption affinity.

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