Illite is a group of closely related non-expanding clay
minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate
, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina
octahedron (O) – silica tetrahedron (T) layers.
The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar to muscovite
with slightly more silicon
, and water and slightly less tetrahedral aluminium
and interlayer potassium
. The chemical formula is given as ,
but there is considerable ion (isomorphic) substitution. It occurs as aggregates of small monoclinic
grey to white crystals. Due to the small size, positive identification usually requires x-ray diffraction
or SEM-EDS (automated mineralogy
) analysis. Illite occurs as an altered product of muscovite and feldspar
environments; it may be a component of sericite
. It is common in sediments, soils, and argillaceous sedimentary rocks
as well as in some low grade metamorphic rocks
. The iron-rich member of the illite group, glauconite
, in sediments can be differentiated by x-ray analysis.
The cation-exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of kaolinite, typically around 20 – 30 meq/100 g.
Illite was first described for occurrences in the Maquoketa shale in Calhoun County, Illinois, US, in 1937. The name was derived from its type location in Illinois.]
Illite is also called hydromica or hydromuscovite. Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described form Mt. Avala, Belgrade, Serbia.
The crystallinity of illite has been used as an indicator of metamorphic grade in clay-bearing rocks metamorphosed under conditions between diagenesis and low-grade metamorphism.
With increasing temperature, illite is thought to undergo a transformation into muscovite.
Category:Clay minerals group
Category:Minerals in space group 12