Phyllit

What is written on the plate:

Phyllit

If clayey rocks are subjected to increased pressure and temperature over long periods of time, their structure and composition change (metamorphism). The clay minerals are transformed into colorless muscovite and green chlorite at 300° C at a depth of 10-20 km. Changes in tectonic pressure cause new foliations (schists). This phyllite has several foliations. It originated from Ordovician
phycode schists. Relictively, the thin sandy layers can still be recognized.

Berga Formation, Ordovician, Lehna Mill, Thuringian Slate Mountains, ca. 490 Ma

Phyllit

Location: Lehnamühle, Thuringian Slate Mountains

Age: 490 million years

Lehnamühle

Image: Google Maps

Phyllite, also known as phyllite schist, clay mica schist or urton schist, is a fine crystalline, thin-slipped rock of metamorphism. It is characterized by a high sericite content of more than 50%, but also contains other mineral phases such as mica, quartz, feldspar, chlorites, augite, tourmalines and iron oxides.

Phyllite is formed from clay shale during regional metamorphism. Compared to clay shale, which still belongs to the sedimentary rocks, phyllite has no original clay minerals. The high sericite (muscovite) content gives the slate surfaces a silky luster. Phyllite varies in color from dark gray to gray-black, with possible shades of greenish gray and purplish gray.

Phyllite often shows multiple foliations due to different tectonic events. The main foliation, also called S1 foliation, occurs during the first deformation event and is usually folded. The original foliation (SS) of the mudstone can only occasionally be determined from changes in material and color.

At higher pressures and temperatures, phyllite transforms to mica schist as part of the amphibolite-facies pressure-temperature field. During this process, the rock-forming minerals of the phyllite enlarge through crystal growth to the point where they are visible to the naked eye or with a magnifying glass.

Phyllites occur worldwide in areas where clayey source rocks have been metamorphically altered under greenschist facies conditions. Some important occurrences are located in the central and eastern Erzgebirge, Vogtland and Fichtelgebirge. In the southeast of the Rhenish Slate Mountains and the Harz Mountains there is a geological zone called the Northern Phyllite Zone. Phyllites also occur frequently in the graywacke zone, which extends from west to east through the northern Alps. All of the above phyllite deposits were formed over 300 million years ago during the Variscan mountain building.

Phyllite is used in various applications due to its properties. For example, phyllite with quartz has been used as a grinding stone in southern Finland since the 16th century.

Overall, phyllite is a fascinating metamorphic rock with its characteristic schistosity, silky luster, and variety of mineral phases contained. Its formation and occurrence make it an important rock in geology.

Location of the phyllite at the Lehna mill

Image: Thomas Voigt