NEW GENETIC ASPECTS OF "SILICEOUS EARTH" AND SIMILAR ROCKS IN SOUTHERN GERMANY

On the Mesozoic platform in Southern Germany the formation of silica rich fine grained rocks are frequently observed. The content of amorphous or nano-sized crystalline silica and clay minerals makes siliceous earth a unique material for polishing purposes or as filler in rubber etc. Formerly, the usage for ceramic production as a clayey material was common.

Very similar rocks occur in different sites in Central Europe and they are called siliceous earth, tripel, opuka (in Czech Republic) or gaize (in France). They look quite similar, but only few investigations were conducted to distinguish them or characterize the conditions of formation of these “strange” rocks. So far, siliceous sponges were regarded as a major source of the silica material and therefore the term “spongiolite” is widespread. So far, very little investigations were undertaken to characterize those materials by mineralogical means or by high resolution scanning electron microscopy.

Investigations on siliceous earth from the Cretaceous deposits in the vicinity of the town of Neuburg in Bavaria (Germany) revealed a very complex history of the formation of the siliceous earth. A detailed microcopic study of the material shows, that dissolution processes, reprecipitation and replacement processes occurred. Carbonatic material as well as silicic material such as quartz were dissolved and the whole original substances were replaced by nano-sized crystallized silica.

The formation occured obviously under very special chemical conditions, which favoured the dissolution and precipitation of silica very near together. Isotope analyses show, that the clay minerals formed under relatively moderate temperatures of 5-30 °C during the Upper Cretaceous to Middle Miocene.

Typical for the occurences of siliceous rich formations are silicified blocks. They are very resistant to chemical weathering and are enriched on the surface. They can be regarded as “petrified” orginal material, from which the loose siliceous earth formed as a weathering product. The preserved structures and materials within them are “keys” to the understanding of the earthy material.