Granulite

Granulite rocks are brownish grey to dark grey, locally whitish and are usually banded. They are characterized by uniform grain-size of individual minerals that fit together like the puzzle pieces. Granulites occur rather scarcely on the Earth surface.

Either granites or volcanic rhyolites are transformed into granulites during regional metamorphosis – i.e. metamorphic process affecting larger area. The granulites exposed by erosion of the Ohře River near Stráž nad Ohří were formed from granites during Hercynian (Variscan) orogenesis. The metamorphic conditions were really enormous – pressures reached 2 to 4.5 GPa and temperatures exceeded 1000°C. Such conditions correspond to depths of at least 140 km. The extreme conditions are evidenced by the presence of coesite (a high-pressure modification of quartz) and also microdiamonds. The diamonds were discovered in this granulite in 2011.

Sample donated by town Stráž nad Ohří.