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.
Occurrence
Whiteschists occur as lenses or tectonic slices on a metre to kilometre scale within nine
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 orogeny. 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
sequences, and thought to represent either evaporite or
bentonite layers. However, whiteschists have now been described with
ranging from
metabasalt to granite.
Formation
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 include , CaO, , MnO, , Rb, Ba, Th. Another feature of whiteschists is that iron and manganese only occur in their highest
oxidation state, indicating that the fluid responsible for the metasomatism was characterised by a high oxygen
fugacity. The main reaction involved in their formation is Mg-
Chlorite group + quartz → talc + kyanite, which has been used to define the stability of the whiteschist assemblage.
In at least one example however, the overall reaction
phlogopite + amphibole + plagioclase → kyanite + talc + quartz + Fe(
hematite) + Na, Ca, K, Mn (fluid) has been described from an altered
amphibolite, suggesting that the original reaction may be insufficient to describe the full stability range of the kyanite + talc assemblage under high oxygen fugacity conditions.
