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Illite shows a range of compositions (see below). Hydronium may occur in the interlayer region and contribute to charge balance and tetrahedral Al-Si ratios may vary.
Gray-white to silvery-white, greenish-gray, sometimes stained other hues.
Waxy, Greasy, Dull, Earthy
1 - 2
Specific Gravity:
2.79 - 2.8
Crystal System:
Member of:
Named in 1937 by Ralph E. Grim, R. H. Bray, and W. F. Bradley for one of the co-type localities, in Illinois, USA. In the time period the mineral was named, the probable supposition was that the leaching of interlayer K was a consistent feature throughout the mineral's structure and that the alteration was systematic. Grim et al. (1937) noted unusual optical properties of illite as well as a cation exchange capacity of 20-40 meq/100 grams. The literature on "illite" is among the largest in all of mineralogy as it is a widespread component of sediments. Schultz (1978) studied the the development of samples that were 20-60% illite that was randomly interstratified with varying smectite layers (i.e. beidellite or montmorillonite). Stixrude and Peacor (2002) further considered the model of illite relating to its transformation into montmorillonite via two mixed-layer models of crystallization and concluded that charge defect layers influenced the composition of adjacent layers, but which, thermodynamically, should not result in the formation of K-rectorite.
A variety of Muscovite

Mica Group.

This clay-like series is essentially a K-deficient muscovite, but frequently contains randomly sequenced montmorillonite/beidellite layers. Illite is dioctahedral, although some references are known which incorrectly refer to "illite" as a similar alteration sequence of trioctahedral micas, although some concomitant Mg substitution in octahedral sites has been suggested. Illite is typically found as extremely fine-grained masses of grayish-white to silvery-gray, sometimes greenish-gray, material. (Paragonite, the Na analog of muscovite, alters in a more or less similar manner as muscovite, and leads to a Na-deficient variety called brammallite.)

Illite is usually an alteration product of muscovite and can be regarded as the mechanism where muscovite may be eventually altered to montmorillonite. During the alteration from muscovite to illite, the structure of illite may become "turbostratified" and the resulting layers of K-filled and K-deficient are randomly mixed, in addition to varying Al-Si substitutions in tetrahedral layers for charge balance.
The Kübler index (KI), earlier called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade of pelites in very low-grade metamorphic environments (Frey, 1987; Guggenheim et al., 2012).

The "Nomenclature of the micas" paper (Rieder et al., 1998) says: "Illite. This name has been used relatively vaguely, and the Subcommittee found it suitable as a series name for a relatively large volume in compositional space, as a counterpart to glauconite." (...) "Dioctahedral interlayer-deficient micas."
The formula K0.65Al2.0◻Al0.65Si3.35O10(OH)2 is only a point in this large compositional space, which is regarded "representative" by the Mica Subcommittee of IMA CNMMN. The composition range of illite is described more appropriate by the following formula: K0.6-0.85Al2(Si,Al)4O10(OH)2 or even K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2.

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Classification of IlliteHide

Dana 7th ed.:

71 : PHYLLOSILICATES Sheets of Six-Membered Rings
2 : Sheets of 6-membered rings with 2:1 layers

Physical Properties of IlliteHide

Waxy, Greasy, Dull, Earthy
Gray-white to silvery-white, greenish-gray, sometimes stained other hues.
1 - 2 on Mohs scale
Perfect on {001}.
2.79 - 2.8 g/cm3 (Measured)    2.61 g/cm3 (Calculated)

Optical Data of IlliteHide

Biaxial (-)
RI values:
nα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
Measured: 5° to 25°, Calculated: 42° to 68°
Max Birefringence:
δ = 0.030 - 0.035
Image shows birefringence interference colour range (at 30µm thickness)
and does not take into account mineral colouration.
Surface Relief:

Chemical Properties of IlliteHide


Illite shows a range of compositions (see below). Hydronium may occur in the interlayer region and contribute to charge balance and tetrahedral Al-Si ratios may vary.

Age distributionHide

Recorded ages:
Phanerozoic : 402 Ma to 1.22 ± 0.06 Ma - based on 85 recorded ages.

Crystallography of IlliteHide

Crystal System:
Class (H-M)
Space Group:
Space Group Setting:
Cell Parameters:
Unit Cell Volume (calc):


First Recorded Occurrence of IlliteHide

Geological Setting of First Recorded Material:
The Maquoketa shale, Gilead, Calhoun Co., Illinois, USA; and at Takova, Yugoslavia.
Associated Minerals at First Recorded Locality:

Synonyms of IlliteHide

Other Language Names for IlliteHide

Simplified Chinese:伊利石

Varieties of IlliteHide

Al-illite-hydromicaVariety of Illite very low in K and high in water.
AmmersooiteA variety of Illite capable of fixing Potassium, from Dutch fields.
AvaliteA chromian variety of Illite.
Originally described from Mt Avala, Belgrade, Serbia.
BrammalliteA sodium-rich illite. [AmMin 29:73]
Illite JadeA dense variety of illite with reddish banding (caused by microscopic inclusions of hematite), which is used for carving and as an ornamental stone. In addition to hematite, the material usually also contains small quantities of impurities such as quartz,...
LeverrieriteA kaolinite-group clay.
Originally reported from Saint-Etienne, Loire, Rhône-Alpes, France.

Relationship of Illite to other SpeciesHide

Member of:
Other Members of this group:
Anandite(Ba,K)(Fe2+,Mg)3((Si,Al,Fe)4O10)(S,OH)2Mon. 2/m : B2/b
AnniteKFe2+3(AlSi3O10)(OH)2Mon. 2/m : B2/m
AspidoliteNaMg3(AlSi3O10)(OH)2Mon. 2/m : B2/m
BalestraiteKLi2V5+Si4O12Mon. 2 : B2
BiotiteA series or subgroup of the Mica Group.
BityiteLiCaAl2(AlBeSi2O10)(OH)2Mon. 2/m : B2/b
CeladoniteK(Mg,Fe2+)Fe3+(Si4O10)(OH)2Mon. 2/m : B2/m
ChromphylliteK(Cr,Al)2(AlSi3O10)(OH,F)2Mon. 2/m : B2/b
ClintoniteCa(Mg,Al)3(Al3SiO10)(OH)2Mon. 2/m : B2/m
FerroaluminoceladoniteK(Fe2+,Mg)(Al,Fe3+)(Si4O10)(OH)2Mon. 2/m : B2/m
FerroceladoniteK(Fe2+,Mg)(Fe3+,Al)(Si4O10)(OH)2Mon. 2/m : B2/m
FluorophlogopiteKMg3(AlSi3O10)(F,OH)2Mon. 2/m : B2/m
Glauconite(K,Na)(Mg,Fe2+,Fe3+)(Fe3+,Al)(Si,Al)4O10(OH)2Mon. 2/m : B2/m
HendricksiteK(Zn,Mg,Mn2+)3(AlSi3O10)(OH)2Mon. 2/m : B2/m
LepidoliteA Li-rich mica in, or close to, the so-called Polylithionite-Trilithionite series.Mon.
LuanshiweiiteKLiAl1.50.5(Si3.5Al0.5)O10(OH,F)2 Mon. 2/m : B2/b
MuscoviteKAl2(AlSi3O10)(OH)2Mon. 2/m : B2/b
NorrishiteKLiMn3+2(Si4O10)O2Mon. 2/m : B2/m
Oxykinoshitalite(Ba,K)(Mg,Ti,Fe3+,Fe2+)3((Si,Al)4O10)(O,OH,F)2Mon. 2/m : B2/m
OxyphlogopiteK(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2Mon. 2/m : B2/m
PhlogopiteKMg3(AlSi3O10)(OH)2Mon. 2/m : B2/m
PolylithioniteKLi2Al(Si4O10)(F,OH)2Mon. 2/m : B2/b
RoscoeliteK(V3+,Al)2(AlSi3O10)(OH)2Mon. 2/m : B2/b
ShirokshiniteKNaMg2(Si4O10)F2Mon. 2/m : B2/m
ShirozuliteK(Mn2+,Mg)3(AlSi3O10)(OH)2Mon. 2/m : B2/m
Suhailite(NH4)Fe2+3(AlSi3O10)(OH)2Mon. 2/m : B2/m
TrilithioniteK(Li1.5Al1.5)(AlSi3O10)(F,OH)2Mon. 2/m : B2/b
VoloshiniteRb(LiAl1.51.5)(Al0.5Si3.5)O10F2Mon. 2/m : B2/b
Wonesite(Na,K)(Mg,Fe,Al)6((Al,Si)4O10)2(OH,F)4Mon. 2/m : B2/m
YangzhumingiteKMg2.5(Si4O10)F2Mon. 2/m : B2/m

Common AssociatesHide

Associated Minerals Based on Photo Data:
Quartz5 photos of Illite associated with Quartz on
Chalcedony5 photos of Illite associated with Chalcedony on
Chrysoprase5 photos of Illite associated with Chrysoprase on
Albite2 photos of Illite associated with Albite on
Kaolinite2 photos of Illite associated with Kaolinite on
Leucite2 photos of Illite associated with Leucite on
Malachite2 photos of Illite associated with Malachite on
Azurite2 photos of Illite associated with Azurite on
Franklinite2 photos of Illite associated with Franklinite on
1 photo of Illite associated with on

Related Minerals - Dana Grouping (8th Ed.)Hide

71.2.2d.1HydrobiotiteK(Mg,Fe2+)6((Si,Al)8O20)(OH)4 · nH2OMon.
71.2.2d.3VermiculiteMg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4 · 8H2OMon. 2/m

Fluorescence of IlliteHide

Not fluorescent in UV

Other InformationHide

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

Illite in petrologyHide

References for IlliteHide

Reference List:
Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Grim, Ralph E., Bray, R. H., Bradley, W. F. (1937) The Mica in Argillaceous Sediments, American Mineralogist: 22: 813-829.
Gaudette, H.E. (196x) The Nature of Illite, Thirteenth Conference on Clay and Clay Minerals, p. 1-35.
Eslinger, E., Highsmith, P., Albers, D., deMayo, B. (1979) Role of iron reduction in the conversion of smectite to illite in bentonites in the disturbed belt, Montana. Clays and Clay Minerals: 27: 327-338.
Nadeau, P., Bain, D. (1986) Composition of some smectites and diagenetic illitic clays and implications for their origin. Clays and Clay Minerals: 34: 455-464.
Drits, V.A., Weber, F., Salyn, A.L., Tsipursky, S.I. (1993) X-Ray Identification of One-Layer Illite Varieties: Application to the Study of Illites around Uranium Deposits of Canada. Clays and Clay Minerals: 41: 389-398.
Rieder, M., Cavazzani, G., D'Yakonov, Y.S., Frank-Kamenetskii, V.A., Gottardi, G., Guggenheim, S., Koval, P.V., Müller, G., Neiva, A.M.R., Radaslovich, E.W., Robert, J.-L., Sassi, F.P., Takeda, H., Weiss, Z., Wones, D.R. (1998) Nomenclature of the micas. The Canadian Mineralogist: 36: 905-912.
Drits, V.A., Dainyak, L.G., Muller, F., Besson, G., Manceau, A. (1997), Isomorphous cation distribution in celadonites, glauconites and Fe-illites determined by infrared, Mössbauer and EXAFS spectroscopy. Clay Minerals: 32: 153-180.
Gaines, R.V., Skinner, H.C.W., Foord, E.E., Mason, B., Rosenzweig, A., King, V.T. (1997) Dana's New Mineralogy : The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana: 1472.
Cuadros, J., Altaner, S.P. (1998) Compositional and structural features of the octahedral sheet in mixed layer illite-smectite from bentonites. European Journal of Mineralogy: 10: 111-124.
Stixrude, L., Peacor, D.R. (2002) First Principles Study of Illite-Smectite and Implications for Clay Minerals Systems. Nature: 420: 165-168.
Guggenheim, S. et al. (2002) Clays Clay Minerals 50, 406-409.
Pironon, J., Pelletier, M., De Donato, P., Mosser-Ruck, R. (2003) Characterization of smectite and illite by FTIR spectroscopy of interlayer NH4 + cations. Clay Minerals: 38: 201-211.
Sainz-Diaz, C.I., Palin, E.J., Dove, M.T., Hernandez-Laguna, A. (2003) Monte Carlo simulations of ordering of Al, Fe and Mg cations in the noctahedral sheet of smectites and illites. American Mineralogist: 88: 1033-1045.
Sainz-Diaz, C.I., Palin, E.J., Hernandez-Laguna, A., Dove, M.T. (2003) Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulation. Physics and Chemistry of Minerals: 30: 382-392.
Dainyak, L.G., Drits, V.A., Zviagina, B.B., Lindgreen, H. (2006) Cation redistribution in the octahedral sheet during diagenesis of illite-smectites from Jurassic and Cambrian oil source rock shales. American Mineralogist: 91: 589-603.

Internet Links for IlliteHide

Localities for IlliteHide

This map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.

Locality ListShow

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