Paragneiss

Earth´s crust is constantly moving and changing shape. Not only lithospheric plates, but also much smaller blocks are interacting and sliding over and past each other. Some blocks are lifting up, others sinking down into depths were they experience increased temperature and pressure. These elevated temperatures and pressures transform the rocks into metamorphic rocks. Under the conditions of moderate pressures and temperatures (around 750 MPa and 600 °C) original sedimentary rocks get transformed into paragneisses.

Gneisses can be identified by typical color banding. Light-coloured bands alternate with dark bands, differing in mineralogical composition. Light bands are dominated by quartz and feldspars, whereas dark bands have a high proportion of biotite. Paragneisses can be further subdivided according to presence of other minerals, like amphibole, garnet, staurolite, and blue kyanite. In the presented sample, biotite is accompanied by a colourless aluminium silicate mineral – sillimanite. The exact name of this particular paragneiss is therefore biotite–sillimanite paragneiss.

Paragneisses are common in Dyleň part of the Český les Highland, in northern part of Slavkovský les Highland – around Rovná and Horní Slavkov, and in Teplá Highland between Bochov and Toužim.

Sample donated by Lesy České republiky, s.e. – Regional directorate Western Bohemia.