Phyllite
Phyllite (from ancient Greek φύλλον phýllon , German `` leaf '' ), phyllite slate , clay mica slate or Urtonschiefer is a finely crystalline, thin slate , mostly leafy metapelite with a sericite content of more than 50%, which in addition to mica also contains quartz , feldspar , chlorite , augite , May contain tourmalines and iron oxides as mineral phases.
Origin and occurrence
As a low-grade metamorphic rock of the green slate facies , phyllite emerged from clay slate through regional metamorphosis . Compared to clay slate, which is not or only very slightly overprinted metamorphically and therefore still belongs to the sedimentary rocks , phyllite is characterized by the absence of original (primary) clay minerals . The sericite ( muscovite ) part creates a silky sheen on the slate surfaces. The color ranges from dark gray to gray black, greenish gray and violet gray also occur.
Often, in addition to a main foliation , called S 1 - foliation , which is due to the first deformation event, handed down more than foliations consequences later tectonic events as surface structure in phyllite. In this case, the S 1 -foliation is usually folded . The position of the stratification (S S ) of the original claystone can only occasionally be determined in phyllites on the basis of noticeable changes in material and color.
In the course of regional metamorphosis, phyllite transforms into mica schist at higher pressures and temperatures (in the pressure-temperature field of the amphibolite facies ) when the rock-forming minerals of the phyllite enlarge through crystal growth to such an extent that they can be seen with a magnifying glass or with the naked eye are.
Phyllites occur in many areas of the world where clay stones and other fine-grain parent rocks have been deformed and heated under green-slate-facial conditions. Large parts of the central and eastern Ore Mountains as far as the neighboring Vogtland and Fichtel Mountains are made up of phyllites. A geological zone emerging in the southeast of the Rhenish Slate Mountains and the Harz Mountains is called the Northern Phyllite Zone, after the phyllites that dominate it . Phyllites are also frequently found in the Grauwackenzone , which stretches from west to east through the northern Alps . All of the phyllite deposits mentioned originated in the course of the Variscan mountain range over 300 million years ago.
Phyllite with quartz, as found in southern Finland, has been used as a grinding stone since the 16th century.
literature
- Roland Vinx: Rock determination in the field . Elsevier, Munich 2005, ISBN 3-8274-1513-6
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke, Stuttgart 1985, ISBN 3-432-94671-6
