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whiteschist
A whiteschist is an uncommon metamorphic rock formed at high to ultra-high pressures. It has the characteristic mineral assemblage of kyanite + talc, responsible for its white colour. The name was introduced in 1973 by German mineralogist and petrologist Werner Schreyer. This rock is associated with the metamorphism of some pelites, evaporite sequences or altered basaltic or felsic intrusions.〔 Whiteschists form in the MgO–––– (MFASH) system. Rocks of this primary chemistry are extremely uncommon and they are in most cases thought to be the result of metasomatic alteration, with the removal of various mobile elements.〔
==Occurrence==
Whiteschists occur as lenses or tectonic slices on a metre to kilometre scale within nine orogenic belts around the world. There are two occurrences in Central Africa, one in Tasmania, one in the Norwegian Caledonides, two in the Alps and three in Asia. One of the most extensive outcrops of whiteschist occurs within the Lufilian Arc - Zambezi Belt orogen. This northwest-southeast trending zone extends for about 700 km. The whiteschist is found with rocks bearing the assemblages anthophyllite–cordierite–kyanite and garnet–staurolite–kyanite. Most occurrences were originally described from metasedimentary sequences, and thought to represent either evaporite or bentonite layers. However, whiteschists have now been described with protoliths ranging from metabasalt to granite.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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