Illite is a non-expanding,
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
-sized, micaceous mineral. Illite is a
phyllosilicateThe silicate minerals make up the largest and most important class of rock-forming minerals, comprising approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron (TOT) layers. The interlayer space is mainly occupied by poorly hydrated potassium cations responsible for the absence of swelling. Structurally illite is quite similar to
muscoviteMuscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl
22, or
236...
with slightly more
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
,
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
,
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
, and water and slightly less tetrahedral
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
and interlayer
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
. The chemical formula is given as (K,H
3O)(Al,Mg,Fe)
2(Si,Al)
4O
10[(OH)
2,(H
2O)], but there is considerable ion substitution. It occurs as aggregates of small monoclinic grey to white crystals. Due to the small size, positive identification usually requires x-ray diffraction analysis. Illite occurs as an alteration product of muscovite and
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
in
weatheringWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
and hydrothermal environments; it may be a component of
sericiteSericite is a fine grained mica, either muscovite, illite, or paragonite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars in areas that have been subjected to hydrothermal alteration typically associated with copper, tin, or other hydrothermal ore deposits...
. 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,
glauconiteGlauconite is a phyllosilicate mineral.It can also be referred to as an iron silicate. It crystallizes with monoclinic geometry. The name is derived from the Greek glaucos meaning 'gleaming' or 'silvery', to describe the appearance of the blue-green color, presumably relating to the sheen and...
, in sediments can be differentiated by x-ray analysis.
The cation exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of
kaoliniteKaolinite is a clay mineral with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...
, typically around 20 – 30 meq/100 g.
Illite was first described for occurrences in the Maquoketa
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
in
Calhoun County, IllinoisCalhoun County is located in the U.S. state of Illinois. As of the 2000 census, the population was 5,084; the population was 5,167 at the 2007 official US Census estimate. The county seat is Hardin. It is part of the St. Louis metropolitan area.-History:...
, USA, in 1937. The name was derived from its type location in Illinois.
Illite is also called hydromica or hydromuscovite.
BrammalliteBrammallite is a sodium rich analogue of illite. First described in 1943 for an occurrence in Llandybie, Carmarthenshire, Wales, it was named for British geologist and mineralogist Alfred Brammall ....
is a sodium rich analogue. Avalite is a
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
bearing variety which has been described form
Mt. AvalaAvala is the mountain overlooking the city of Belgrade, the capital of Serbia. It is situated in the south-eastern corner of the city and provides a great panoramic view to Belgrade, Vojvodina and Šumadija, as the surrounding area on all sides is mostly a lowlands...
,
BelgradeBelgrade Belgrade Belgrade (Serbian Cyrillic: Београд, Serbian Latin: Beograd (meaning "White City" in Serbian) is the capital and largest city of Serbia. The city lies on two international waterways, at the confluence of the Sava and Danube rivers, where Central Europe's Pannonian Plain meets...
,
SerbiaSerbia , officially the Republic of Serbia , is a country located in both Central and Southeastern Europe. Its territory covers the southern part of the Pannonian Plain and central part of the Balkans...
.
Illite crystallinity
The crystallinity of illite has been used as an indicator of metamorphic grade in clay-bearing rocks metamorphosed under conditions between
diagenesisIn geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
and low-grade.With increasing temperature illite is thought to undergo a transformation into
muscoviteMuscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl
22, or
236...
.