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 rock. ...

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-temperatu ...

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 alumino ...

chlorite The chlorite ion, or chlorine dioxide anion, is the 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 salts of chlorous ac ...

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

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 crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

. Fruchtschiefer occurs at 500 °C. Knotenschiefer is characterised by small nodules, 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 extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

as a result of the growth in grain size during

metamorphism 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 ...

. 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 distinctive cross-shaped black inclusions of graphite. The presence of the cross caused this mineral be used as a gem. Chiastolite spec ...

increasingly occur.

References

* Walter Maresch, Olaf Medenbach: ''Steinbachs Naturführer. Gesteine.'' S. 252. Mosaik Verlag, München 1987, {{ISBN, 3-576-10699-5.
''Fleckschiefer.''
Otto Lueger: ''Lexikon der gesamten Technik und ihrer Hilfswissenschaften.'' Bd. 4, Stuttgart, Leipzig 1906, S. 82. Metamorphic rocks