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Illite
A variety ofMuscovite
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Illite is a very commonmica andclay mineral, typically found as extremely fine-grained masses of grayish-white to silvery-gray, sometimes greenish-gray, material. The literature on "illite" is very large as it is a widespread component of many sediments, especially marine; despite this it is poorly defined and variably considered a mineral, series or variety.
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 toglauconite." They defined it as"Dioctahedral interlayer-deficient micas." with K dominant over Na. They also suggest it may be an interlayer structure with randomsmectite layers between mica sheets. If not a mixture, considering that there is no defined end member to the "series", and K>0.6 in most cases, it could best be considered a K-deficient variety ofmuscovite.
This clay-like mineral is essentially a K-deficientmuscovite but may contain randomly sequencedmontmorillonite/beidellite layers (Schultz, 1978). Illite is dioctahedral, although some references are known which incorrectly refer to "illite" as including a similar alteration sequence of trioctahedral micas, although some concomitant Mg and Fe substitution occurs in octahedral sites.
The formula K0.65Al2.0Al0.65Si3.35O10(OH)2 is only an arbitrary point in this large compositional space, which is just regarded as "representative" by the Mica Subcommittee of IMA CNMMN. However, the K content probably does not get much lower, and it may be always K-dominant. The composition range of illite is described more appropriately by the following formula: K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2. In the octahedral layer (Mg+Fe)is less than Al, and it may grade into glauconite with increasing Fe and Mg. Hydronium may occur in the interlayer region and tetrahedral Al-Si ratios also vary; both may contribute to charge balance.
Illite is sometimes considered an alteration product of muscovite, wherein it 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. (Paragonite, the Na analog of muscovite, alters in a more or less similar manner as muscovite, and leads to a Na-deficient variety calledbrammallite.)
Schultz (1978) studied the the development of samples that were 20-60% illite that was randomly interstratified with varying smectite layers (i.e.beidellite ormontmorillonite). 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.
The Kübler index (KI), also called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade ofpelites in very low-grade metamorphic environments, using the degree of crystallinity of fine grained micas, mostly illite-muscovite (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 toglauconite." They defined it as"Dioctahedral interlayer-deficient micas." with K dominant over Na. They also suggest it may be an interlayer structure with randomsmectite layers between mica sheets. If not a mixture, considering that there is no defined end member to the "series", and K>0.6 in most cases, it could best be considered a K-deficient variety ofmuscovite.
This clay-like mineral is essentially a K-deficientmuscovite but may contain randomly sequencedmontmorillonite/beidellite layers (Schultz, 1978). Illite is dioctahedral, although some references are known which incorrectly refer to "illite" as including a similar alteration sequence of trioctahedral micas, although some concomitant Mg and Fe substitution occurs in octahedral sites.
The formula K0.65Al2.0Al0.65Si3.35O10(OH)2 is only an arbitrary point in this large compositional space, which is just regarded as "representative" by the Mica Subcommittee of IMA CNMMN. However, the K content probably does not get much lower, and it may be always K-dominant. The composition range of illite is described more appropriately by the following formula: K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2. In the octahedral layer (Mg+Fe)is less than Al, and it may grade into glauconite with increasing Fe and Mg. Hydronium may occur in the interlayer region and tetrahedral Al-Si ratios also vary; both may contribute to charge balance.
Illite is sometimes considered an alteration product of muscovite, wherein it 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. (Paragonite, the Na analog of muscovite, alters in a more or less similar manner as muscovite, and leads to a Na-deficient variety calledbrammallite.)
Schultz (1978) studied the the development of samples that were 20-60% illite that was randomly interstratified with varying smectite layers (i.e.beidellite ormontmorillonite). 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.
The Kübler index (KI), also called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade ofpelites in very low-grade metamorphic environments, using the degree of crystallinity of fine grained micas, mostly illite-muscovite (Frey, 1987; Guggenheim et al., 2012).
Unique IdentifiersMineral Symbols
Colour:
Gray-white to silvery-white, greenish-gray, sometimes stained other hues.
Streak:
White
Hardness:
1 - 2 onMohs scale
Tenacity:
Elastic
Cleavage:
Perfect
Perfect on {001}.
Perfect on {001}.
Fracture:
Micaceous
Density:
2.79 - 2.8 g/cm3 (Measured) 2.61 g/cm3 (Calculated)