Illite - Encyclopedia

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : (K,H3O)Al2(Si3Al)O10(H2O,OH)2
    Rarity : Very common

Illite is one of the main minerals of the clay group. Its structure is close to that of a dioctahedral mica (muscovite) : it differs by less potassium and more silica, resulting in a more regular stacking of its layers. It is a ubiquitous mineral, abundant in all clay formations, sometimes in limestone. Illite is notably an essential component of soils, in which, associated with montmorillonite, it plays the important role of potassium fixer for agriculture. Illite can also be of hydrothermal origin and formed from feldspars which provide it with potassium. It is thus common in the alteration halos of many hydrothermal deposits. Its name is linked to its location of discovery : Gilead in Illinois (USA). Illite forms scaly or finely lamellar masses with a smooth feel, cryptocrystalline, white to yellowish, greenish or brownish in color. Illite is mainly intended for the building sector in the form of terracotta products (bricks, tiles, flues, terracotta tiles, etc.) or lightweight ceramic (clay foam for sound insulation, thermofoam). Illites are also used in the composition of chamottes, where they serve as degreasers in the making of ceramic pastes and have additional uses in the chemical industry (ion exchange material, saponifiers, degreasers) and in pharmacy.

Main photo : Illite pseudomorph after halite from Auburn, Nemaha County, Nebraska, USA © Don Windeler

Illite in the World and in France

Illite is found absolutely everywhere, however it never constitutes a spectacular or interesting sample for the collection.


No twins known for this mineral species, but can replace other dissolved species such as halite (pseudomorph).

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 1 to 2
Density : 2.8
Fracture : Micaceous
Streak : White

TP : Translucent
RI : 1.535 to 1.605
Birefringence : 0.030 to 0.035
Optical character : Biaxial -
Pleochroism : None
Fluorescence : None

Solubility : Nitric acid, hydrochloric acid

Magnetism : NoneRadioactivity : None


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