Monday, June 14, 2010

Gneiss Rock

Gneiss is a foliated metamorphic rock which is widely distributed around the world. It is formed by high-grade regional metamorphic processes from pre-existing formations that were originally sedimentary rocks. Gneissic rocks are usually medium- to coarse-foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, or albite gneiss.

Found over extensive metamorphic terrain, gneiss is a coarse-grained banded crystalline rock. Orthogneiss is formed by the metamorphism of igneous rocks; paragneiss results from the metamorphism of original sedimentary rocks. Pencil gneiss contains rod-shaped individual minerals or segregations of minerals, and augen gneiss contains large lenticular mineral grains or mineral aggregates having the appearance of eyes scattered through the rock.