Knotenschiefer
Encyclopedia
Knotenschiefer is a variety of spotted slate
characterized by conspicuous subspherical or polyhedral clots that are often individual minerals such as cordierite
, biotite
, chlorite
, andalusite
and others.
Like fleckschiefer, fruchtschiefer
and garbenschiefer, knotenschiefer is a variety of contact metamorphic slate
. It is formed at temperatures of around 400 °C and its dark coloration is caused by graphite
. Fruchtschiefer occurs at 500 °C. Knotenschiefer is characterised by small nodules
, up to one centimetre in size, and nodular deposits of mica
as a result of the growth in grain size during metamorphism
. The nodules consist of iron minerals, carbon substances and mica; as the metamorphic temperature rises, minerals such as andalusite
or chiastolite
increasingly occur.
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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
characterized by conspicuous subspherical or polyhedral clots that are often individual minerals such as cordierite
Cordierite
Cordierite or iolite 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: 2 to 2...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
, andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
and others.
Like fleckschiefer, fruchtschiefer
Fruchtschiefer
Fruchtschiefer is a local variety of contact metamorphic rock that is derived from argillite.- Formation and mineral content :Fruchtschiefer is formed under very low pressure and temperatures of ca. 500 °C...
and garbenschiefer, knotenschiefer is a variety of contact metamorphic slate
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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
. It is formed at temperatures of around 400 °C and its dark coloration is caused by graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
. Fruchtschiefer occurs at 500 °C. Knotenschiefer is characterised by small nodules
Nodule (geology)
A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock...
, up to one centimetre in size, and nodular deposits of mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
as a result of the growth in grain size during metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
. The nodules consist of iron minerals, carbon substances and mica; as the metamorphic temperature rises, minerals such as andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
or chiastolite
Chiastolite
The mineral chiastolite is a variety of andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite. In areas around Georgetown, California, metamorphosed sediments contained andalusite and chiastolite in a graphite rich metasediment...
increasingly occur.