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Title: Illite  
Author: World Heritage Encyclopedia
Language: English
Subject: Kaolinite, Mica, Soil, List of countries by bentonite production, Cotta Sandstone
Publisher: World Heritage Encyclopedia


Category Mica - Phyllosilicates
(repeating unit)
Strunz classification 09.EC.60
Dana classification 71.02.02d.02
Color Grey-white to silvery-white, greenish-gray
Crystal habit Micaceous aggregates
Crystal system Monoclinic - Prismatic, 2/m
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 (-)
Refractive index nα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
References [1][2]
Structure of Illite mica - USGS.

Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron (TOT) layers.[3] The interlayer space is mainly occupied by poorly hydrated potassium cations responsible for the absence of swelling, even though its structure is constituted by TOT layers. Structurally illite is quite similar to muscovite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium. The chemical formula is given as (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)],[2] but there is considerable ion 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 alteration 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.[3]

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, USA, in 1937. The name was derived from its type location in Illinois.[1]

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.[4]

Illite crystallinity

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.[5] With increasing temperature, illite is thought to undergo a transformation into muscovite.[6]


  • Mitchell J.K. (1993) Fundamentals of soil behavior. Second edition. John Wiley and Sons, Inc., New York. 437 pp, see Chapter 3, Soil Mineralogy, p. 32. ISBN 978-0-471-46302-3
  1. ^ a b Mindat
  2. ^ a b
  3. ^ a b Illite group, USGS
  4. ^ Mindat - avalite
  5. ^ M. Frey; Douglas Robinson (26 January 1999). Low-Grade Metamorphism. Wiley. pp. 61–107.  
  6. ^ Gharrabi, M., Velde, B. & Sagon, J.-P. 1998. The transformation of illite to muscovite in pelitic rocks : Constraints from X-ray diffraction, Clays and clay minerals, 46, 79-88.
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