Illite: Properties and Occurrences

An illite is a group of closely related non-expanding clay minerals. It is a clay mineral of a group resembling micas, with a lattice structure which does not expand on absorption of water. It is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. 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.

Structurally, illite is quite similar to muscovite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminum and interlayer potassium. The chemical formula is given as (K, H3O)(Al,  Mg, Fe)2(Si, Al)4O10[(OH)2,(H2O)], but there is considerable ion (isomorphic) substitution.

General Information

  • Category: Mica- phyllosilicates
  • Formula: (K, H3O)(Al, Mg, Fe)2(Si, Al)4O10[(OH)2,(H2O)]
  • Crystal system: Monoclinic
  • Crystal class: Prismatic (2/m) (same H-M symbol)

Fig: illite – clay minerals


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.

  • Color: Grey-white to silvery-white, greenish-gray
  • Crystal habit: Micaceous aggregates
  • Cleavage: {001} Perfect
  • Mohs scale hardness: 1 – 2
  • Luster: Pearly to dull
  • Streak: white
  • Diaphaneity: Translucent
  • Specific gravity: 2.6 – 2.9
  • Optical properties: Biaxial (-)


It occurs as aggregates of small monoclinic grey to white crystals. Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal 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.

Illite is also called hydromica or hydromuscovite. Brammallite is a sodium-rich analogue. Avalite is a chromium bearing variety which has been described from Mt. Avala, Belgrade, Serbia.


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