A whiteschist is an uncommon

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

formed at high to ultra-high pressures. It has the characteristic mineral assemblage of

kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks gener ...

talc Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...

, 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 Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

of some

pelite A pelite ( Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...

sequences or altered

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

ic 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 Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...

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 belt An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collec ...

s around the world. There are two occurrences in Central Africa, one in

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

, 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 The Lufilian Arc (or Lufilian Belt) is part of a system of orogenic belts in southern Africa formed during the Pan-African orogeny, a stage in the formation of the Gondwana supercontinent. It extends across eastern Angola, the Katanga Province of ...

- Zambezi Belt

orogen An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collecti ...

. This northwest-southeast trending zone extends for about 700 km. The whiteschist is found with rocks bearing the assemblages

anthophyllite Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of antho ...

–

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-temperat ...

–

and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

– staurolite–

. Most occurrences were originally described from

metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and e ...

ary sequences, and thought to represent either evaporite or

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

layers. However, whiteschists have now been described with

protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and ...

s 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 In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

, indicating that the fluid responsible for the metasomatism was characterised by a high oxygen

fugacity In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas whic ...

. 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. In at least one example however, the overall reaction

phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica. Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O ...

+ amphibole +

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

→ 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.

References

{{Reflist Metamorphic petrology Schist