Colour, characteristics:

Colour predominantly grey, but also brownish, reddish, greenish. Usually light-coloured when freshly broken, appearing considerably darker when polished. Main constituents are feldspars and quartz; secondary constituents biotite, muscovite, hornblende, cordierite, garnets, sillimanite. Coarse-grained structure with distinct schistosity, i.e. a parallel structure recognisable by interlocking minerals. Gneisses with coherent mica layers are sensitive to frost.

Formation:

Through intensive transformation (metamorphosis) of silica-rich rocks as a result of high pressures and temperatures, e.g. granite, rhyolite, sandstone, shale. Gneisses formed from magmatites are called orthogneiss, those from sedimentites are called paragneiss. The mineral composition of orthogneiss is only slightly altered compared to the original rock. The schistosity of gneiss is caused in particular by the biotite, which, as a result of one-sided pressure, is aligned with its longitudinal axis perpendicular to this direction. Occurs in large rock complexes.

Locations:

Bavarian Forest, Bohemian Forest, Ore Mountains, Central Alps, France (Vosges, Massif Central, Brittany), Scandinavia, USA (Appalachians)

Products:

Used for building blocks, crushed stone and chippings; thin-split grades for wall panels, formerly also roof panels.