Illite

Illite is a very common mica and clay 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 to glauconite." They defined it as "Dioctahedral interlayer-deficient micas." with K dominant over Na. They also suggest it may be an interlayer structure with random smectite 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 of muscovite.

This clay-like mineral is essentially a K-deficient muscovite but may contain randomly sequenced montmorillonite/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 K_0.65Al_2.0Al_0.65Si_3.35O₁₀(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: K_0.6-0.85(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 called brammallite.)

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.

The Kübler index (KI), also called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade of pelites in very low-grade metamorphic environments, using the degree of crystallinity of fine grained micas, mostly illite-muscovite (Frey, 1987; Arkai & Töth, 1983; Eberl & Velde, 1989; Kübler, 1990; Kübler & Goy-Eggenberger, 2001; Guggenheim et al., 2012). Gharrabi (1998) write: "Metamorphic illites (probably muscovites) show no smectite interlayers in any fraction. In the transition from sedimentary and diagenetic to metamorphic illites, new grains of smectite-free illite are formed at the expense of the older minerals. This suggests that the new metamorphic minerals are recrystallized phases. Metamorphism of illites then produces new mica phases."