:Illite

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

{{Infobox mineral

| name = Illite

| category = Phyllosilicate minerals, mica group, muscovite variety

| boxwidth =

| boxbgcolor =

| image = Illite.jpg

| imagesize = 260px

| alt =

| caption =

| formula = {{Chem2|(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]}}

| IMAsymbol = Ilt{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-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 =

| references = {{cite journal |last1=Drits |first1=V. A. |last2=Zviagina |first2=B. B. |last3=McCarty |first3=D. K. |last4=Salyn |first4=A. L. |title=Factors responsible for crystal-chemical variations in the solid solutions from illite to aluminoceladonite and from glauconite to celadonite |journal=American Mineralogist |date=1 February 2010 |volume=95 |issue=2–3 |pages=348–361 |doi=10.2138/am.2010.3300|bibcode=2010AmMin..95..348D |s2cid=62881038 }}}}

File:Illstruc.jpg

Illite, also called hydromica or hydromuscovite,{{Cite web|url=http://www.mindat.org/min-9603.html|title=Hydromuscovite: Mineral information, data and localities|website=www.mindat.org|access-date= 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.{{Cite web|url=https://pubs.usgs.gov/of/2001/of01-041/htmldocs/clays/illite.htm|title=USGS OFR01-041: Illite Group Minerals|website=USGS Coastal and Marine Geology Program|access-date=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)]}},{{Cite web|url=http://webmineral.com/data/Illite.shtml|title=Illite Mineral Data|website=webmineral.com|access-date=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.{{Cite journal|url=https://books.google.com/books?id=KhAJAQAAIAAJ|title=Crystal Structures of Clay Minerals and Their X-ray Identification|vauthors=Brindley GW, Brown G|journal=Mineralogical Society Monograph|publisher=Mineralogical Society|year=1980|edition=3rd|location=UK|isbn=9780903056083|issn=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.{{Cite web|url=http://www.mindat.org/min-2011.html|title=Illite: Mineral information, data and localities|website=www.mindat.org|access-date=3 Apr 2019}}

Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described from Mt. Avala, Belgrade, Serbia.{{Cite web|url=https://www.mindat.org/min-435.html|title=Avalite: Mineral information, data and localities|website=www.mindat.org|access-date=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.