Illite is the name for a series of clay minerals . The name comes from the US state of Illinois , where Illite was first described. They are three- layer sheet silicates , consisting of an aluminum (Al) octahedron layer and two surrounding silicate (Si) tetrahedron layers. They are structurally very similar to mica and also arise from these and other silicates ( feldspar , foids and others).
The general formula is K 0.65 Al 2.0 Al 0.65 Si 3.35 O 10 (OH) 2 . In contrast to real mica like muscovite or biotite, there is a charge deficit in the intermediate layers, which is caused by the small amounts of potassium that are built into them. Illite forms during the chemical weathering of muscovite, but also of feldspars and other aluminum-rich minerals. Illite is a very common mineral in pelites and siltstones . Changes in the crystal lattice with rising temperatures and pressures are used to estimate the depth of submergence of sedimentary rocks, the so-called illite crystallinity .
- Blum, EHW: Soil Science in Key Words . Stuttgart 2007, p. 17 .