Illite, also called hydromica or hydromuscovite,[6] is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina octahedron (O) – silica tetrahedron (T) layers.[5] The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are 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)],[3] but there is considerable ion (isomorphic) 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 altered 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.[7]
Illite was first described for occurrences in the Maquoketa shale in Calhoun County, Illinois, US, in 1937. The name was derived from its type location in Illinois.[2]
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.[9] With increasing temperature, illite is thought to undergo a transformation into muscovite.[10]
References
Mitchell JK (1993). "Ch. 3: Soil Mineralogy". Fundamentals of soil behavior (2nd ed.). New York: John Wiley and Sons, Inc. p. 32. ISBN9780471463023.
^Drits, V. A.; Zviagina, B. B.; McCarty, D. K.; Salyn, A. L. (1 February 2010). "Factors responsible for crystal-chemical variations in the solid solutions from illite to aluminoceladonite and from glauconite to celadonite". American Mineralogist. 95 (2–3): 348–361. Bibcode:2010AmMin..95..348D. doi:10.2138/am.2010.3300. S2CID62881038.
^Gharrabi M, Velde B, Sagon JP (1998). "The Transformation of Illite to Muscovite in Pelitic Rocks: Constraints from X-ray Diffraction". Clays and Clay Minerals. 46 (1): 79–88. Bibcode:1998CCM....46...79G. doi:10.1346/CCMN.1998.0460109.
