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]
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, US, in 1937. The name was derived from its type location in Illinois.[2]
Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described from Mt. Avala, Belgrade, Serbia.[8]
Zipao 'jade' is an ornamental form of illite showing bands of red-purple and pale yellow-green. It may be carved into pendants and other ornaments.
^ ab"Illite: Mineral information, data and localities". www.mindat.org. Retrieved 3 Apr 2019.
^ ab"Illite Mineral Data". webmineral.com. Retrieved 3 Apr 2019.
^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. S2CID 62881038.
^ ab"USGS OFR01-041: Illite Group Minerals". USGS Coastal and Marine Geology Program. Retrieved 3 Apr 2019.
^"Hydromuscovite: Mineral information, data and localities". www.mindat.org. Retrieved 2023-08-27.
^Brindley GW, Brown G (1980). "Crystal Structures of Clay Minerals and Their X-ray Identification". Mineralogical Society Monograph (3rd ed.). UK: Mineralogical Society. ISBN 9780903056083. ISSN 0144-1485.
^"Avalite: Mineral information, data and localities". www.mindat.org. Retrieved 3 Apr 2019.
Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate,...
(especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica, sodium, calcium, magnesium...
Illite crystallinity is a technique used to classify low-grade metamorphic activity in pelitic rocks. Determining the "illite crystallinity index" allows...
presence of smectite clay minerals on the planet Mars. Illite group which includes the clay-micas. Illite is the only common mineral in this group. Chlorite...
when the dominant clay species is illite (a non-swelling clay). However, in pure clay mineralogy, the term illite is more appropriate than "K-bentonite"...
Weathering of the same kind of rock under alkaline conditions produces illite. Smectite forms by weathering of igneous rock under alkaline conditions...
kaolinite decreases, while the proportion of other clay minerals such as illite (in cooler climates) or smectite (in drier climates) increases. Such climatically...
converted to graphite. Slate is mainly composed of the minerals quartz, illite, and chlorite, which account for up to 95% of its composition. The most...
varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and kaolinite. Montmorillonite can be concentrated and transformed...
and aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or...
followed by interstratified illite and smectite. The alkaline nature of the clay can be attributed to calcite. Quartz, mica and illite are very stable substances...
Etsell, Thomas H. (16 July 2015). "Influence of Nonswelling Clay Minerals (Illite, Kaolinite, and Chlorite) on Nonaqueous Solvent Extraction of Bitumen"....
Stráská, Jitka; Chmelík, František (January 2016). "Study of microcracking in illite-based ceramics during firing". Journal of the European Ceramic Society....
presence of illite, chlorite, smectite and traces of mixed layer illite/chlorite. Meknes clays belong to illitic clays, characterized by illite (54 – 61%)...
clay content should be expansive clays, with the remainder non-expansive illite or kaolinite. Too much expansive clay results in uneven drying through the...
less stable minerals have been weathered to clay minerals like kaolinite, illite or smectite. Among the three major types of rock, fossils are most commonly...
of feldspars happens by hydrolysis and produces clay minerals, including illite, smectite, and kaolinite. Hydrolysis of feldspars begins with the feldspar...
saturated specimens of clay minerals increases in the order kaolinite <illite<smectite. The compression index Cc, which is defined as the change in void...
sediments to be then re-exposed as coastal plains called strandflats. Dating of illite clay from a strandflat of Bømlo, southern Norway, have shown that landscape...
table is only a rough guide to order of weathering. Some minerals, such as illite, are unusually stable, while silica is unusually unstable given the strength...
interlayer) while poorly hydrated potassium ions are "collapsers" like in illite (totally collapsed interlayer). As the interlayer space of smectites is...
found sandwiched between silicate sheets in important clay groups: the illite, kaolinite, and montmorillonite/smectite groups. The individual aluminium...
to calcium-rich clay (montmorillonite) rather than potassium-rich clay (illite). Further weathering, particularly in tropical climates, converts the montmorillonite...
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