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Illite
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Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding 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) – 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 with slightly more , , , and water and slightly less tetrahedral and interlayer . The chemical formula is given as , but there is considerable ion (isomorphic) substitution. It occurs as aggregates of small 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 in and environments; it may be a component of . 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, , in sediments can be differentiated by x-ray analysis.

(1980). 9780903056083, Mineralogical Society. .

The cation-exchange capacity (CEC) of illite is smaller than that of but higher than that of , typically around 20 – 30 meq/100 g.

Illite was first described for occurrences in the Maquoketa in Calhoun County, Illinois, US, in 1937. The name was derived from its type location in Illinois.

is a sodium rich analogue. Avalite is a bearing variety which has been described from , , .

Zipao 'jade' is an ornamental form of illite showing bands of red-purple and pale yellow-green. It may be carved into pendants and other ornaments.


Illite crystallinity
The crystallinity of illite has been used as an indicator of metamorphic grade in clay-bearing rocks metamorphosed under conditions between and low-grade metamorphism.
(1999). 9780632047567, Wiley. .
With increasing temperature, illite is thought to undergo a transformation into .


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