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,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)],[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]

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.[1]

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]

CategoryMica- phyllosilicates
(repeating unit)
Strunz classification9.EC.60
Dana classification71.02.02d.02
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
ColorGrey-white to silvery-white, greenish-gray
Crystal habitMicaceous aggregates
Cleavage{001} Perfect
Mohs scale hardness1 - 2
LusterPearly to dull
Specific gravity2.6 - 2.9
Optical propertiesBiaxial (-)
Refractive indexnα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
Structure of Illite mica - USGS.[3]

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]


  • Mitchell J.K. (1993) Fundamentals of soil behavior. Second edition. John Wiley and Sons, Inc., New York. 437 pp, see Chapter 3, Soil Mineralogy, p. 32. ISBN 978-0-471-46302-3
  1. ^ a b Mindat
  2. ^ a b
  3. ^ Crystal Structures of Clay Minerals and Their X-ray Identification, George William Brindley, George Brown, 1980
  4. ^ a b Illite group, USGS
  5. ^ Mindat - avalite
  6. ^ M. Frey; Douglas Robinson (26 January 1999). Low-Grade Metamorphism. Wiley. pp. 61–107. ISBN 978-0-632-04756-7.
  7. ^ Gharrabi, M., Velde, B. & Sagon, J.-P. 1998. The transformation of illite to muscovite in pelitic rocks : Constraints from X-ray diffraction, Clays and clay minerals, 46, 79-88.
Argillaceous minerals

Argillaceous minerals may appear silvery upon optical reflection and are minerals containing substantial amounts of clay-like components (Greek: ἄργιλλος = clay). Argillaceous components are fine-grained (less than 2 µm) aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite. Claystone and shales are thus predominantly argillaceous.

The adjective "argillaceous" is also used to define rocks in which clay minerals are a secondary but significant component. For example, argillaceous limestones are limestones consisting predominantly of calcium carbonate, but including 10-40% of clay minerals: such limestones, when soft, are often called marls. Similarly, argillaceous sandstones are sandstones consisting primarily of quartz grains, with the interstitial spaces filled with clay minerals.

Argillic alteration

Argillic alteration is hydrothermal alteration of wall rock which introduces clay minerals including kaolinite, smectite and illite. The process generally occurs at low temperatures and may occur in atmospheric conditions. Argillic alteration is representative of supergene environments where low temperature groundwater becomes acidic.

Argillic assemblages include kaolinite replacing plagioclase and montmorillonite replacing amphibole and plagioclase. Orthoclase is generally stable and unaffected. Argillic grades into phyllic alteration at higher temperatures in an ore deposit hydrothermal system.Advanced argillic alteration occurs under lower pH and higher temperature conditions. Kaolinite and dickite occur at lower temperatures whereas pyrophyllite and andalusite occur under high temperature conditions (T > 300°C). Quartz deposition is common. Alunite, topaz, zunyite, tourmaline, enargite and tennantite may also occur. Greisen alteration is similar.


Bentonite (/ˈbɛntənʌɪt/) is an absorbent aluminium phyllosilicate clay consisting mostly of montmorillonite. It was named by Wilbur C. Knight in 1898 after the Cretaceous Benton Shale near Rock River, Wyoming.The different types of bentonite are each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminium (Al). Experts debate a number of nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, has been used to describe clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. In addition to montmorillonite and illite another common clay species that is sometimes dominant is kaolinite. Kaolinite-dominated clays are commonly referred to as tonsteins and are typically associated with coal.


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).

Believed to be a degradation product of paragonite, like illite it is a non-expanding, clay-sized, micaceous mineral. Brammallite is a phyllosilicate or layered silicate. Structurally, brammallite is quite similar to muscovite or sericite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium.

It occurs as aggregates of small monoclinic white crystals. Due to the small size, positive identification usually requires x-ray diffraction analysis.

Clay 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.

Clay minerals form in the presence of water and have been important to life, and many theories of abiogenesis involve them. They are important constituents of soils, and have been useful to humans since ancient times in agriculture and manufacturing.


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.Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rocks.

Geology of Eswatini

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.


Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al(OH)3 (but sometimes as α-Al(OH)3.). It is also sometimes called hydrargillite (or hydrargyllite).

Gibbsite is an important ore of aluminium in that it is one of three main phases that make up the rock bauxite.

Gibbsite has three named structural polymorphs or polytypes: bayerite (designated often as α-Al(OH)3, but sometimes as β-Al(OH)3), doyleite, and nordstrandite. Gibbsite and bayerite are monoclinic, whereas doyleite and nordstrandite are triclinic forms.

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.

List of countries by bentonite production

Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements, such as potassium, sodium, calcium, and aluminium. As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays.

Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, have been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium bentonite and calcium bentonite.

In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species, and sometimes dominant, are montmorillinite and kaolinite. Kaolinite dominated clays are commonly referred to as tonsteins and are typically associated with coal.

List of minerals

This is a list of minerals for which there are articles on Wikipedia.

Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species. Within a mineral species there may be variation in physical properties or minor amounts of impurities that are recognized by mineralogists or wider society as a mineral variety.

Mineral variety names and mineraloids are to be listed after the valid minerals for each letter.

For a complete listing (about 5,000) of all mineral names, see List of minerals (complete).

Marine clay

Marine clay is a type of clay found in coastal regions around the world. In the northern, deglaciated regions, it can sometimes be quick clay, which is notorious for being involved in landslides.

Marine clay is a particle of soil that is dedicated to a particle size class, this is usually associated with USDA’s classification with sand at 0.05mm, silt at 0.05-.002mm and clay being less than 0.002 mm in diameter. Paired with the fact this size of particle was deposited within a marine system involving the erosion and transportation of the clay into the ocean.

Soil particles become suspended when in a solution with water, with sand being affected by the force of gravity first with suspended silt and clay still floating in solution. This is also known as turbidity, in which floating soil particles create a murky brown color to a water solution. These clay particles are then transferred to the abyssal plain in which they are deposited in high percentages of clay. A soil is only considered a clay if it has above 55% total clay content. This is due to the way in which the clay reacts to things like water, heat and other chemicals.

Once the clay is deposited on the ocean floor it can change its structure through a process known as flocculation, process by which fine particulates are caused to clump together or floc. These can be either edge to edge flocculation or edge to face flocculation. Relating to individual clay particles interacting with each other. Clays can also be aggregated or shifted in their structure besides being flocculated.

Clay particles can self-assemble into various configurations, each with totally different properties.

This change in structure to the clay particles is due to a swap in cations with the basic structure of a clay particle. This basic structure of the clay particle is known as a silica tetrahedral or aluminum octahedral. They are the basic structure of clay particles composing of one cation, usually silica or aluminum surrounded by hydroxide anions, these particles form in sheets forming what we know as clay particles and have very specific properties to them including micro porosity which is the ability of clay to hold water against the force of gravity, shrink swell capacity and absorption capabilities.

When clay is deposited in the ocean, the presence of excess ions in seawater causes a loose, open structure of the clay particles to form, a process known as flocculation. Once stranded and dried by ancient changing ocean levels, this open framework means that such clay is open to water infiltration. Construction in marine clays thus presents a geotechnical engineering challenge. Where clay overlies peat, a lateral movement of the coastline is indicated and shows a rise in relative sea level

Swelling of marine clay has the potential to destroy building foundations in only a few years. Due to the changes in climatic conditions on the construction site, the pavement constructed on the marine clay (as subgrade) will have less durability and requires lot of maintenance cost. Some simple precautions, however, can reduce the hazard significantly.

The swapping of this positive cation with another is what makes different types of clays including Kaolinite, montmorillonite, smectite and illite. This happens in marine clays due the fact the oceans water is high in solution with cations making it very easy to overcome the clays negative net charge and swap the clays cation with a less positive one. These marine clays can be what are known as quick clays, which are notorious for its erosive properties. A great example of these quick clays is in the pacific northwest. They are known as blue goo which is a mix of clay and mélange (greenstone, basalt, chert, shale, sandstone, schists. uplifted through the accretionary wedge). These quick clays have a very high-risk factor associated with them if they are built upon, as they are very unstable due to the fact that liquefaction happens when it becomes saturated and literally flows, causing mass wasting events to happen. Other marine clays are used all around the world for many different uses, such as ceramics, building material, including adobe. Clay layers in soils which can be used as an impermeable layer are very important for dumps or chemical spills as they have a very high absorption capacity for heavy metals. For these clays to be available for human use they must have been eroded deposited on the ocean floor and then uplifted through means of tectonic activity to bring it to land.

During the construction of Marina Barrage in Singapore, marine clay was found at the site. Since marine clay was the cause of the Nicoll Highway collapse years previous, the construction team removed all the marine clay to ensure the stability of Marina Barrage. Later on, they found marine clay mixed with seawater even in the deeper underground.

Geotechnical problems posed by marine clay can be handled by various ground improvement techniques. Marine clay can be densified by mixing it with cement or similar binding material in specific proportions. Marine clay can be stabilised using wastes of various industries like porcelain industry and tree-cutting industries. This method is usually adopted in highways where marine clay is used as a subgrade soil.


Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm and a thickness of 0.96 nm; magnification of about 25,000 times, using an electron microscope, is required to "see" individual clay particles. Members of this group include saponite.

Montmorillonite is a subclass of smectite, a 2:1 phyllosilicate mineral characterized as having greater than 50% octahedral charge; its cation exchange capacity is due to isomorphous substitution of Mg for Al in the central alumina plane. The substitution of lower valence cations in such instances leaves the nearby oxygen atoms with a net negative charge that can attract cations. In contrast, beidellite is smectite with greater than 50% tetrahedral charge originating from isomorphous substitution of Al for Si in the silica sheet.

The individual crystals of montmorillonite clay are not tightly bound hence water can intervene, causing the clay to swell. The water content of montmorillonite is variable and it increases greatly in volume when it absorbs water. Chemically, it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron, and other cations are common substitutes, and the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and kaolinite.

Nankai Trough

The Nankai Trough 南海トラフ (Nankai Torafu, Southern Sea Trough) is a submarine trough located south of the Nankaidō region of Japan's island of Honshū, extending approximately 900 km offshore. The underlying fault, the Nankai megathrust, is the source of the devastating Nankai megathrust earthquakes, while the trough itself is potentially a major source of hydrocarbon fuel, in the form of methane clathrate.

In plate tectonics, the Nankai Trough marks a subduction zone that is caused by subduction of the Philippine Sea Plate beneath Japan, part of the Eurasian plate (Kanda et al., 2004). This plate boundary would be an oceanic trench except for a high flux of sediments that fills the trench. Within the Nankai Trough there is a large amount of deformed trench sediments (Ike, 2004), making one of Earth's best examples of accretionary prism. Furthermore, seismic reflection studies have revealed the presence of basement highs that are interpreted as seamounts that are covered in sediments (Ike, 2004). The northern part of the trough is known as the Suruga Trough, while to the east is the Sagami Trough. The Nankai trough runs roughly parallel to the Japan Median Tectonic Line.

Prehnite-pumpellyite facies

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.

In mafic rocks the assemblage is chlorite, prehnite, albite, pumpellyite and epidote.

In ultramafic rocks the assemblage is serpentine, talc, forsterite, tremolite and chlorite.

In argillaceous sedimentary rocks the assemblage is quartz, illite, albite, and stilpnomelane chlorite.

In carbonate sediments the assemblage is calcite, dolomite, quartz, clays, talc, and muscovite.


Sericite is a fine grained mica, similar to 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. Sericite also occurs as the fine mica that gives the sheen to phyllite and schistose metamorphic rocks.

The name comes from Latin sericus, meaning "silken" in reference to the location from which silk was first utilized, which in turn refers to the silky sheen of rocks with abundant sericite.


Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) 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.


Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning rock) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite. The clays often are cemented by iron oxide minerals, carbonaceous matter, or chlorite. Tonsteins form from volcanic ash deposited in swamps.Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world. They are often used as key beds to correlate the strata in which they are found. The regional persistence of tonsteins and relict phenocrysts indicate that they formed as the result of the diagenetic alteration of volcanic ash falls in an acidic (low pH) and low-salinity environment, consistent with a freshwater swamp. In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.The induration of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the ball clays found at Bovey Tracey which formed by the erosion of a nearby kaolinised granite. These deposits are generally softer, white, and plastic.


Zellige (Arabic: [zˈliʑ]; Arabic: الزليج‎; also zelige or zellij) is mosaic tilework made from individually chiseled geometric tiles set into a plaster base. This form of Islamic art is one of the main characteristics of Moroccan architecture. It consists of geometrically patterned mosaics, used to ornament walls, ceilings, fountains, floors, pools and tables.

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