Schnett, Thuringian Slate Mountains​
Image: IGW

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Quartz phyllites

Quartz phyllites originated from impure, quartz-rich mudstones.
The metamorphic, intensely folded rock was deformed several times, so that the former stratification can no longer be identified. The age of the original rocks is not yet known. Precambrian or Ordovician sediments of the Gondwana shelf are possible. During the Variscan mountain building they were Between Gondwana and the Middle German crystalline zone.

Core zone complex, Precambrian or Ordovician, Schnett, Thuringian Slate Mountains, ca. 600 or 490 Ma


Location: Schnett, Thuringian Slate Mountains

Age: 600 - 490 Millionen Jahre

Thuringian Forest
Thuringian Forest
Image: Google Maps

Quartz phyllite is a metamorphic rock belonging to the phyllite group. Phyllite is a fine crystalline, thin-slipped rock with a characteristic leafy structure. The name "phyllite" comes from the Greek word "phýllon", meaning "leaf", and refers to the leaf-like texture of the rock.

Quartz phyllite consists mainly of quartz, mica and especially sericite, with the sericite content usually exceeding 50%. In addition to quartz and mica, feldspar, chlorite, augite, tourmaline and iron oxides may also occur as mineral phases in the rock.

This rock is formed by the regional metamorphism of clay shale in the green shale facies. Compared to the clay shale, which still belongs to the sedimentary rocks and is only slightly metamorphically overprinted, the phyllite is characterized by the absence of original clay minerals. The high sericite (muscovite) content gives the slate surfaces a silky luster. The color of the quartz phyllite varies from dark gray to gray-black, with possible shades of greenish gray and purplish gray.

The quartz phyllite usually shows a main foliation called S1 foliation, which is due to the first deformation event. In addition to the S1 foliation, other foliations may occur in the phyllite that are indicative of later tectonic events. These shales are often folded, and the original layering of the mudstone can only occasionally be determined from conspicuous changes in material and color.

At higher pressures and temperatures, quartz phyllite can be transformed into mica schist when the rock-forming minerals of the phyllite enlarge by crystal growth and become visible to the naked eye. Quartz phyllites occur worldwide, especially in areas where clayey source rocks have been metamorphically overprinted under greenschist facies conditions. Examples of occurrences of phyllites, including quartz phyllites, are the central and eastern Erzgebirge, the adjacent Vogtland and Fichtelgebirge, the Northern Phyllite Zone in the Rhenish Slate Mountains and Harz, and the Greywacke Zone in the Northern Alps .

The quartz phyllite is also used economically. In southern Finland, quartz phyllite has been used as a grindstone since the 16th century.

Sources: Wikipedia