Last updated
Category Mica- phyllosilicates
(repeating unit)
Strunz classification 9.EC.60
Dana classification71.02.02d.02
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
ColorGrey-white to silvery-white, greenish-gray
Crystal habit Micaceous aggregates
Cleavage {001} Perfect
Mohs scale hardness1 - 2
Luster Pearly to dull
Streak white
Diaphaneity Translucent
Specific gravity 2.6 - 2.9
Optical propertiesBiaxial (-)
Refractive index nα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
References [1] [2]
Structure of Illite mica - USGS. Illstruc.jpg
Structure of Illite mica - USGS.

Illite 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. [4] 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,H
O)], [2] 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. [4]

Clay A finely-grained natural rock or soil material that combines one or more clay minerals

Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3, MgO etc.) and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, and become hard, brittle and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.

Silicate minerals Rock-forming minerals with predominantly silicate anions

Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of the Earth's crust.

Muscovite true mica, phyllosilicate mineral; polytypes: 1M, 2A, 2M2, 3T

Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(AlSi3O10)(FOH)2, or (KF)2(Al2O3)3(SiO2)6(H2O). It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets) which are often highly elastic. Sheets of muscovite 5 meters × 3 meters (16.5 feet × 10 feet) have been found in Nellore, India.

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.

Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with other positively charged particles in the surrounding soil water. This is one of the ways that solid materials in soil alter the chemistry of the soil. CEC affects many aspects of soil chemistry, and is used as a measure of soil fertility, as it indicates the capacity of the soil to retain several nutrients (e.g. K+, NH4+, Ca2+) in plant-available form. It also indicates the capacity to retain pollutant cations (e.g. Pb2+).

Kaolinite phyllosilicate mineral

Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO
) linked through oxygen atoms to one octahedral sheet of alumina (AlO
) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.

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. [1]

Shale A fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Calhoun County, Illinois County in the United States

Calhoun County is a county in the U.S. state of Illinois. As of the 2010 census, the population was 5,089, making it Illinois’ third-least populous county. Its county seat and biggest community is Hardin, with a population of slightly less than 1,000. Its smallest community is Hamburg, with a population of 123. Calhoun County is at the tip of the peninsula formed by the courses of the Mississippi and Illinois rivers above their confluence and is almost completely surrounded by water. Calhoun County is sparsely populated; it has just five municipalities, all of them villages.

Illite is also called hydromica or hydromuscovite. Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described form Mt. Avala, Belgrade, Serbia. [5]

Brammallite 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 (1879–1954).

Chromium Chemical element with atomic number 24

Chromium is a chemical element with symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard and brittle transition metal. Chromium boasts a high usage rate as a metal that is able to be highly polished while resisting tarnishing. Chromium is also the main additive in stainless steel, a popular steel alloy due to its uncommonly high specular reflection. Simple polished chromium reflects almost 70% of the visible spectrum, with almost 90% of infrared light being reflected. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored.

Avala Mountain in Serbia

Avala is a mountain in Serbia, overlooking Belgrade. It is situated in the south-eastern corner of the city and provides a great panoramic view of Belgrade, Vojvodina and Šumadija, as the surrounding area on all sides is mostly lowlands. It stands at 511 metres (1,677 ft) above sea level, which means that it enters the mountain category just by 11 m (36 ft).

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. [6] With increasing temperature, illite is thought to undergo a transformation into muscovite. [7]

Diagenesis The change of sediments or existing sedimentary rocks into a different sedimentary rock

Diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks. It does not include changes from weathering. It is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification. This process excludes surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures. Hydrothermal solutions, meteoric groundwater, porosity, permeability, solubility, and time are all influential factors.

Related Research Articles

Feldspar A group of rock-forming tectosilicate minerals

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.

Tremolite Amphibole, double chain inosilicate mineral

Tremolite is a member of the amphibole group of silicate minerals with composition: ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades to dark green with increasing iron content. It has a hardness on Mohs scale of 5 to 6. Nephrite, one of the two minerals of the gemstone jade, is a green variety of tremolite.

Clay minerals group of minerals

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

Chlorite group triclinic, monoclinic and orthorhombic phyllosilicates

The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.

Lazulite phosphate mineral

Lazulite ((Mg,Fe2+)Al2(PO4)2(OH)2) is a blue, phosphate mineral containing magnesium, iron, and aluminium phosphate. Lazulite forms one endmember of a solid solution series with the darker iron rich scorzalite.

Allophane An amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid

Allophane is an amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al2O3·(SiO2)1.3-2·(2.5-3)H2O. Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be identified by its distinctive infrared spectrum and its X-ray diffraction pattern. It was first described in 1816 in Gräfenthal, Thuringia, Germany. Allophane is a weathering or hydrothermal alteration product of volcanic glass and feldspars and sometimes has a composition similar to kaolinite but generally has a molar ratio of Al:Si = 2. It typically forms under mildly acidic to neutral pH (5-7). Its structure has been debated, but it is similar to clay minerals and is composed of curved alumina octahedral and silica tetrahedral layers. Transmission electron micrographs show that it is generally made up of aggregates of hollow spherules ~3-5 nm in diameter. Allophane can alter to form halloysite under resilicating aqueous conditions and can alter to form gibbsite under desilicating conditions. A copper containing variety cupro-allophane has been reported.

Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.

Clastic rock type of sedimentary rock

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres. It is a metamorphic grade transitional between zeolite facies and greenschist facies representing a temperature range of 250 to 350 °C and a pressure range of approximately two to seven kilobars. The mineral assemblage is dependent on host composition.

Zussmanite (K(Fe2+,Mg,Mn)13[AlSi17O42](OH)14) is a hydrated iron-rich silicate mineral. Zussmanite occurs as pale green crystals with perfect cleavage.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Phengite high silica variety of muscovite

Phengite is a series name for dioctahedral micas of composition K(AlMg)2(OH)2(SiAl)4O10, similar to muscovite but with addition of magnesium. It is a non-IMA recognized mineral name representing the series between muscovite and celadonite.

Taranakite phosphate mineral

Taranakite is a hydrated alkali iron-aluminium phosphate mineral with chemical formula (K,Na)3(Al,Fe3+)5(PO4)2(HPO4)6·18H2O. It forms from the reaction of clay minerals or aluminous rocks with solutions enriched in phosphate derived from bat or bird guano or, less commonly, from bones or other organic matter. Taranakite is most commonly found in humid, bat inhabited caves near the boundary of guano layers with the cave surface. It is also found in perennially wet coastal locations that have been occupied by bird colonies. The type location, and its namesake, the Sugar Loaf Islands off Taranaki, New Zealand, is an example of a coastal occurrence.

Illite crystallinity

Illite crystallinity is a technique used to classify low-grade metamorphic activity in pelitic rocks. Determining the "illite crystallinity index" allows geologists to designate what metamorphic facies and metamorphic zone the rock was formed in and to infer what temperature the rock was formed. Several crystallinity indices have been proposed in recent years, but currently the Kübler index is being used due to its reproducibility and simplicity. The Kübler index is experimentally determined by measuring the full width at half maximum for the X-ray diffraction reflection peak along the (001) crystallographic axis of the rock sample. This value is an indirect measurement of the thickness of illite/muscovite packets which denote a change in metamorphic grade. The method can be used throughout the field of geology in areas such as the petroleum industry, plate tectonics.

Aluminoceladonite mica, phyllosilicate mineral

Aluminoceladonite is a low-temperature potassium dioctahedral mica mineral which is an end-member in the illite-aluminoceladonite solid solution series. The chemical formula for aluminoceladonite is K(Mg,Fe2+)Al(Si4O10)(OH)2.


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  1. 1 2 "Illite: Mineral information, data and localities". Retrieved 3 Apr 2019.
  2. 1 2 "Illite Mineral Data". Retrieved 3 Apr 2019.
  3. Brindley GW, Brown G (1980). Crystal Structures of Clay Minerals and Their X-ray Identification (3rd ed.). UK: Mineralogical Society. ISSN   0144-1485.
  4. 1 2 "USGS OFR01-041: Illite Group Minerals". USGS Coastal and Marine Geology Program. Retrieved 3 Apr 2019.
  5. "Avalite: Mineral information, data and localities". Retrieved 3 Apr 2019.
  6. Frey M, Robinson D (1999). Low-Grade Metamorphism. Wiley. pp. 61–107. ISBN   9780632047567.
  7. 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. doi:10.1346/CCMN.1998.0460109.