Awhiteschist is an uncommonmetamorphic rock formed at high to ultra-high pressures. It has the characteristic mineral assemblage ofkyanite +talc, responsible for its white colour. The name was introduced in 1973 by German mineralogist and petrologistWerner Schreyer.[1] This rock is associated with themetamorphism of somepelites,evaporite sequences or alteredbasaltic orfelsicintrusions.[2][3][4] Whiteschists form in the MgO–Fe
2O
3–Al
2O
3–SiO
2–H
2O (MFASH) system.[5] Rocks of this primary chemistry are extremely uncommon and they are in most cases thought to be the result ofmetasomatic alteration, with the removal of various mobile elements.[3]
Whiteschists occur as lenses or tectonic slices on a metre to kilometre scale within nineorogenic belts around the world. There are two occurrences in Central Africa, one inTasmania, one in the Norwegian Caledonides, two in the Alps and three in Asia.[3][6] One of the most extensive outcrops of whiteschist occurs within theLufilian Arc -Zambezi Beltorogen. This northwest-southeast trending zone extends for about 700 km. The whiteschist is found with rocks bearing the assemblagesanthophyllite–cordierite–kyanite andgarnet–staurolite–kyanite.[7] Most occurrences were originally described frommetasedimentary sequences, and thought to represent either evaporite orbentonite layers. However, whiteschists have now been described withprotoliths ranging frommetabasalt to granite.[3]
Whiteschists have a chemistry that only very rarely occurs as the primary composition of rocks. This implies that they can only form under conditions where other chemical components have been removed by large scale metasomatism, strongly altering the original rock composition. The mobile components that may be removed includeNa
2O, CaO,K
2O, MnO,P
2O
5, Rb, Ba, Th. Another feature of whiteschists is that iron and manganese only occur in their highestoxidation state, indicating that the fluid responsible for the metasomatism was characterised by a high oxygenfugacity. The main reaction involved in their formation is Mg-chlorite + quartz → talc + kyanite, which has been used to define the stability of the whiteschist assemblage.[3] In at least one example however, the overall reactionphlogopite + amphibole +plagioclase → kyanite + talc + quartz + Fe(hematite) + Na, Ca, K, Mn (fluid) has been described from an alteredamphibolite, suggesting that the original reaction may be insufficient to describe the full stability range of the kyanite + talc assemblage under high oxygen fugacity conditions.[3]