Sericite is the name given to very fine, ragged grains and
aggregate
Aggregate or aggregates may refer to:
Computing and mathematics
* collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
s of white (colourless)
micas
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
, typically made of
muscovite
Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...
,
illite, or
paragonite
Paragonite is a mineral, related to muscovite. Its empirical formula is . A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector Na+ K+ and apparent micas of intermediate composition is most c ...
.
Sericite is produced by the alteration of
orthoclase
Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
or
plagioclase feldspars in areas that have been subjected to
hydrothermal alteration typically associated with
copper,
tin, or other hydrothermal
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
deposits. Sericite also occurs as the fine mica that gives the sheen to
phyllite and
schistose
metamorphic rocks
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, causi ...
.
The name comes from
Latin ''sericus'', meaning "silken" in reference to the location from which silk was first utilized, which in turn refers to the silky sheen of rocks with abundant sericite.
birefringence
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringe ...
flakes covering other larger minerals)" class="center">
File:Granite pmg ss 2006.jpg, Granite in thin section under cross-polarized light in which feldspar crystals exhibit sericite alteration.
File:Staurolite garnet schist 3mm xp 2007.jpg, Staurolite-garnet schist in thin section under cross-polarized light with sericite.