Colour, properties:

Colour grey and black, caused by bitumen and graphite admixtures. Limonite provides brown, haematite reddish and chlorite greenish shades. There are also multi-coloured, striped or spotted types. The main constituents are clay minerals, quartz and mica, especially muscovite. Structure fine-grained to dense, excellent schistosity with flat separation surfaces.

Formation:

Through intensive transformation (metamorphosis) from clay and shale as a result of high pressures and temperatures. The ability to swell in water is an important distinguishing feature between sedimentary shale and metamorphic clay shale. Shale clays swell, clay shales do not. Clay shales are also harder and have no or strongly deformed fossils. Occurs in relatively small deposits.

Locations:

Fichtelgebirge, Thuringia, France (Ardennes), Great Britain (Wales, Lancashire)

Products:

Used as roof, wall and façade panels, for insulation and switchboards, formerly for school blackboards.