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{{Short description|Group of non-expanding clay minerals}}

factoids

DATE=2021URL=HTTPS://WWW.CAMBRIDGE.ORG/CORE/JOURNALS/MINERALOGICAL-MAGAZINE/ARTICLE/IMACNMNC-APPROVED-MINERAL-SYMBOLS/62311F45ED37831D78603C6E6B25EE0AVOLUME=85PAGES=291–320BIBCODE=2021MINM...85..291WDOI-ACCESS=FREE, | molweight = | strunz = 9.EC.60| dana = 71.02.02d.02| system = Monoclinic| class = Prismatic (2/m) (same H-M symbol)| symmetry = C2/m (no. 12)| color = Grey-white to silvery-white, greenish-gray| colour = | habit = Micaceous aggregates| twinning = | cleavage = {001} Perfect| fracture = | tenacity =| mohs = 1–2| luster = Pearly to dull| streak = white| diaphaneity = Translucent| gravity = 2.6–2.9| density = | polish = | opticalprop = Biaxial (−)| refractive = nα = 1.535 – 1.570 nβ = 1.555 – 1.600 nγ = 1.565 – 1.605| birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow =| fluorescence= | absorption =| melt = | fusibility = | diagnostic = | solubility = | other = | alteration = LAST2=ZVIAGINA LAST3=MCCARTY LAST4=SALYN TITLE=FACTORS RESPONSIBLE FOR CRYSTAL-CHEMICAL VARIATIONS IN THE SOLID SOLUTIONS FROM ILLITE TO ALUMINOCELADONITE AND FROM GLAUCONITE TO CELADONITE DATE=1 FEBRUARY 2010 ISSUE=2–3 DOI=10.2138/AM.2010.3300S2CID=62881038, }}(File:Illstruc.jpg|thumb|Structure of illite mica – USGS)Illite, also called hydromica or hydromuscovite,WEB,www.mindat.org/min-9603.html, Hydromuscovite: Mineral information, data and localities, www.mindat.org, 2023-08-27, 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.WEB,pubs.usgs.gov/of/2001/of01-041/htmldocs/clays/illite.htm, USGS OFR01-041: Illite Group Minerals, USGS Coastal and Marine Geology Program, 3 Apr 2019, 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 {{Chem2|(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2*(H2O)]}},WEB,webmineral.com/data/Illite.shtml, Illite Mineral Data, webmineral.com, 3 Apr 2019, 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.JOURNAL,books.google.com/books?id=KhAJAQAAIAAJ, Crystal Structures of Clay Minerals and Their X-ray Identification, Brindley GW, Brown G, Mineralogical Society Monograph, Mineralogical Society, 1980, 3rd, UK, 9780903056083, 0144-1485, 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.WEB,www.mindat.org/min-2011.html, Illite: Mineral information, data and localities, www.mindat.org, 3 Apr 2019, Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described from Mt. Avala, Belgrade, Serbia.WEB,www.mindat.org/min-435.html, Avalite: Mineral information, data and localities, www.mindat.org, 3 Apr 2019, 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.

Illite crystallinity

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.BOOK,books.google.com/books?id=n4Sxrlq1g58C&pg=PA61, Low-Grade Metamorphism, Frey M, Robinson D, Wiley, 1999, 9780632047567, 61–107, With increasing temperature, illite is thought to undergo a transformation into muscovite.JOURNAL, 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, 10.1346/CCMN.1998.0460109, 1998CCM....46...79G,

References

• BOOK, Fundamentals of soil behavior, Mitchell JK, John Wiley and Sons, Inc., 1993, 9780471463023, 2nd, New York, 32, Ch. 3: Soil Mineralogy,

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External links

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{{Phyllosilicates}}{{Clay minerals}}