Knotenschiefer is a variety of spotted

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

characterized by conspicuous subspherical or polyhedral clots that are often individual minerals such as

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present, and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-tempera ...

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

chlorite The chlorite ion, or chlorine dioxide anion, is the halite (oxyanion), halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as s ...

andalusite Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétherie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the sp ...

and others.Bucksch, Herbert (1997). ''Dictionary of Geotechnical Engineering'', Vol. 2., Springer-Verlag, Berlin. . Like fleckschiefer, fruchtschiefer and garbenschiefer, knotenschiefer is a variety of

contact metamorphic Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

. It is formed at temperatures of around 400 °C and its dark coloration is caused by

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

. Fruchtschiefer occurs at 500 °C. Knotenschiefer is characterised by small

nodules Nodule may refer to: *Nodule (geology), a small rock or mineral cluster *Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, p ...

, up to one centimetre in size, and nodular deposits of

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

as a result of the growth in grain size during

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

. The nodules consist of iron minerals, carbon substances and mica; as the metamorphic temperature rises, minerals such as

chiastolite Chiastolite ( ) is a variety of the mineral andalusite with the chemical composition Al2SiO5. It is noted for its distinctive cross-shaped black inclusions of graphite. The presence of these crosses caused the mineral to be used as a gem. Chias ...

increasingly occur.

References

* Walter Maresch, Olaf Medenbach: ''Steinbachs Naturführer. Gesteine.'' S. 252. Mosaik Verlag, München 1987, .
''Fleckschiefer.''
Otto Lueger: ''Lexikon der gesamten Technik und ihrer Hilfswissenschaften.'' Bd. 4, Stuttgart, Leipzig 1906, S. 82. Metamorphic rocks {{Metamorphic-rock-stub