Siliceous slates

Siliceous slate, also known as lydite, is a rock that belongs to the group of sedimentary rocks and was formed in the sea. It is a dark gray to black variety of radiolarite formed during the Paleozoic era (about 541 million to 251.9 million years ago).

The term "siliceous schist" was once commonly used, but nowadays the name "lydite" is increasingly used to avoid confusion with other schistose rocks.

Siliceous slate is characterized by its dark color, which can range from dark gray to slate gray to black. In some specimens, white veins can be observed running through the rock and appearing in varying thicknesses. These veins are composed of quartz, while the dark part of the siliceous shale is mainly bituminous. Other constituents of the rock include mica, calcite, and small amounts of clay minerals. The color variations and layered patterns in the siliceous shale result from the changing input of the mineral constituents during the formation of the rock.

This rock exhibits a conchoidal-sharp fracture and is difficult to disaggregate. It is a fine-grained rock type with a dense structure. In the past, the rock was used as a grinding stone due to its hardness. In Germany, deposits of siliceous slate are found in the Ore Mountains, Harz Mountains, Sauerland, Thuringian Slate Mountains and Vogtland.

Kieselschiefer is a weakly metamorphically overprinted radiolarite. It is important to note that not every radiolarite is a lydite, as the lydite has undergone metamorphic overprinting. This metamorphism often makes the fossil content difficult to recognize. Lydite forms thin-bedded, dense rocks with conchoidal fracture.

Location of the rock siliceous slate near the village Schmiedefeld

Image: Thomas Voigt