Mica slate
Mica schist is a collective term for slate metamorphic rocks . They differ from gneiss in the receding feldspar and the frequent appearance of certain characteristic minerals. The starting rocks are mainly clay-rich rocks, i.e. claystones or clayey sandstones , they belong to the group of metapelites or metapsammites .
Appearance and composition
Mica schists are medium to coarse-grained rocks with mostly clearly visible metamorphic , parallel structures (metamorphic parallel texture). The proportion of layered silicates (especially muscovite and biotite , subordinate also paragonite ) is over 50%, the feldspar proportion below 20%. In addition to quartz , garnet , staurolite , tourmaline and thistle can also occur in other mixtures . The mineral grains can usually be seen with the naked eye or at least with a magnifying glass; in some cases they can be several centimeters long ( fruit slate , knotted slate ).
The color of the mica slate is light to dark gray, but due to weathering it can also appear brownish or reddish. The density is between 2.60 and 3.05 g / cm³.
Depending on the occurrence of the other parts of the mixture, a distinction is made between the types of mica slate, such as quartz mica slate, garnet mica slate, staurolite mica slate and others. Mica schists differ from phyllites in the size of the minerals, which in the case of phyllite can only be seen under the microscope.
Occurrence
Mica schists are found around the world where the pressure and temperature conditions are sufficient for their formation. For this reason, they are often found in the area of the cratons or the inner zones of mountains .
Mica slate is a common rock in the Alps , for example in the Austrian Central Alps or in the Valais . In Germany there are deposits in the Ore Mountains , in the Bavarian Forest on the Osser , in the Vorspessart (near Aschaffenburg ) and in the Taunus . Other European deposits are located approximately in Spain ( Sierra Nevada ), in France ( Massif Central , the Massif des Maures , Brittany ...) or over a large area on the surface pending in Norway , Sweden and Scotland . Outside Europe, for example, they occur in many mountains, for example in the Andes or the Himalayas , and form large areas in the Brazilian and Canadian shields, for example .
use
In the building industry, mica slate is sometimes used as flat building blocks, for example as floor coverings. High quality show & a. the quarries of Fischbach (Taunus) . Good varieties outside Germany come, for example, from Fåvang in Norway or until 1964 as Fexer plates from the Fex valley in Switzerland.
In the Middle Ages , mica slate was used to make millstones , which rarely had to be sharpened.
literature
- Martin Okrusch, Siegfried Matthes: Mineralogy: An introduction to special mineralogy, petrology and deposit science . Springer, Berlin 2005, ISBN 3-540-23812-3 , pp. 332 f .
- Walter Schumann: The great BLV stone and mineral guide . revised New edition, 7th edition. BLV Buchverlag, Munich 2007, ISBN 978-3-8354-0212-6 , p. 312 .
- Walter Schumann: Minerals and Rocks . BLV Buchverlag, Munich 1991, p. 116 .
- Schists . In: Encyclopædia Britannica . 11th edition. tape 24 : Sainte-Claire Deville - Shuttle . London 1911, p. 327 (English, full text [ Wikisource ]).
Web links
- Garnet-thistle-staurolite-mica schist. Description of a mica slate, course polarization microscopy of rocks . Institute for Geology and Mineralogy at the University of Cologne
Individual evidence
- ↑ Hans Murawski: Geological Dictionary . 8th edition. Ferd. Enke Verlag, Stuttgart 1983, ISBN 3-432-84108-6 , p. 264 .