(mineralogy) A group of gray, green, or yellowish-brown micalike clay minerals found in argillaceous sediments; intermediate in composition and structure between montmorillonite and muscovite.
Illite
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(′i′līt)
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A clay-size, micaceous mineral; a common component of soil, sediments, sedimentary rocks, and hydrothermal deposits. Illite is considered to possess a smaller layer charge and potassium (K) content than muscovite. It is characterized by the ideal formula Al2(Si3.2Al0.8)O10(OH)2K0.8.
Illite frequently is interlayered with smectite, and can be considered the nonswelling end member in an illite-smectite mixed-layer clay series. Pure end-member illite, with no interlayer smectite, is rare.
Illitic clays are used for manufacturing structural clay products such as brick and tile. Some degraded illites (vermiculites) are used for molding sands. Illite may also be useful for storing certain types of radioactive wastes, because it is less subject to transformation by heat than are other common clays and because it is highly specific for the sorption of cesium. See also Clay, commercial; Clay minerals.
| Wikipedia: Illite |
| Illite | |
|---|---|
| General | |
| Category | Silicate mineral |
| Chemical formula | (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] |
| Strunz classification | 09.EC.60 |
| Dana classification | 71.02.02d.02 |
| Identification | |
| 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 |
| 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] |
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. 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 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.[5]With increasing temperature illite is thought to undergo a transformation into muscovite.[6]
References
- 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 978-0471463023
- ^ a b http://www.mindat.org/min-2011.html Mindat
- ^ a b http://webmineral.com/data/Illite.shtml
- ^ a b http://pubs.usgs.gov/of/2001/of01-041/htmldocs/clays/illite.htm Illite group, USGS
- ^ http://www.mindat.org/min-435.html Mindat - avalite
- ^ Merriman, R.J. & Frey, M. 1998, Patterns of very low-grade metamorphism in metapelitic rocks. In: Frey, M. & Robinson, D. (Eds) Low-grade metamorphism, WileyBlackwell, 61-107.
- ^ 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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Mentioned in
- brammalite (mineralogy)
- degraded illite (mineralogy)
- sarospatakite (geology)
- potassium bentonite (geology)
- clay mineral (mineralogy)
- hydromica (geology)
- Glauconite (mineralogy and petrology)
- clay (in archaeology)
- Shale (mineralogy and petrology)
- Slate
- clay minerals
- French Green Clay
- Loess (geology and geodesy)
- Redbeds (geology and geodesy)
