Clay mineral

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Oxford Clay (Jurassic) exposed near Weymouth, England OxfordClay Weymouth.JPG
Oxford Clay (Jurassic) exposed near Weymouth, England

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al 2 Si 2 O 5(OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

Contents

Clay minerals form in the presence of water [1] 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.

Properties

Hexagonal sheets of the clay mineral kaolinite (SEM image, 1,340x magnification) Kaolinite - USGS bws00008.jpg
Hexagonal sheets of the clay mineral kaolinite (SEM image, 1,340× magnification)

Clay is a very fine-grained geologic material that develops plasticity when wet, but becomes hard, brittle and non–plastic upon drying or firing. [2] [3] [4] It is a very common material, [5] and is the oldest known ceramic. Prehistoric humans discovered the useful properties of clay and used it for making pottery. [6] The chemistry of clay, including its capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility. [7]

Because the individual particles in clay are less than 4 micrometers (0.00016 in) in size, they cannot be characterized by ordinary optical or physical methods. The crystallographic structure of clay minerals became better understood in the 1930s with advancements in the x-ray diffraction (XRD) technique indispensable to deciphering their crystal lattice. [8] Clay particles were found to be predominantly sheet silicate (phyllosilicate) minerals, now grouped together as clay minerals. Their structure is based on flat hexagonal sheets similar to those of the mica group of minerals. [9] Standardization in terminology arose during this period as well, [8] with special attention given to similar words that resulted in confusion, such as sheet and plane. [8]

Because clay minerals are usually (but not necessarily) ultrafine-grained, special analytical techniques are required for their identification and study. In addition to X-ray crystallography, these include electron diffraction methods, [10] various spectroscopic methods such as Mössbauer spectroscopy, [11] infrared spectroscopy, [10] Raman spectroscopy, [12] and SEM-EDS [13] or automated mineralogy [10] processes. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships. [14]

Occurrence

Clay minerals are common weathering products (including weathering of feldspar) and low-temperature hydrothermal alteration products. Clay minerals are very common in soils, in fine-grained sedimentary rocks such as shale, mudstone, and siltstone and in fine-grained metamorphic slate and phyllite. [9]

Given the requirement of water, clay minerals are relatively rare in the Solar System, though they occur extensively on Earth where water has interacted with other minerals and organic matter. Clay minerals have been detected at several locations on Mars, [15] including Echus Chasma, Mawrth Vallis, the Memnonia quadrangle and the Elysium quadrangle. Spectrography has confirmed their presence on celestial bodies including the dwarf planet Ceres, [16] asteroid 101955 Bennu, [17] and comet Tempel 1, [18] as well as Jupiter's moon Europa. [19]

Structure

View of tetrahedral sheet structure of a clay mineral. Apical oxygen ions are tinted pink. Mica T.png
View of tetrahedral sheet structure of a clay mineral. Apical oxygen ions are tinted pink.

Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner-sharing SiO4 tetrahedra or AlO4 octahedra. The sheet units have the chemical composition (Al, Si)3O4. Each silica tetrahedron shares three of its vertex oxygen ions with other tetrahedra, forming a hexagonal array in two dimensions. The fourth oxygen ion is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared oxygen ions are on the same side of the sheet. These unshared oxygen ions are called apical oxygen ions. [20]

In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet. [20]

Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite. [20]

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+ or by a lone octahedral sheet. The interlayer may also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers. [20]

Classification

Structure of clay mineral groups Clay Mineral Structure.jpg
Structure of clay mineral groups

Clay minerals can be classified as 1:1 or 2:1. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentinite. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite, and montmorillonite. The layers in 1:1 clays are uncharged and are bonded by hydrogen bonds between layers, but 2:1 layers have a net negative charge and may be bonded together either by individual cations (such as potassium in illite or sodium or calcium in smectites) or by positively charged octahedral sheets (as in chlorites). [9]

Clay minerals include the following groups:

Mixed layer clay variations exist for most of the above groups. [9] Ordering is described as a random or regular order and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to an R1 ordered illite-smectite, for example. This type would be ordered in an illite-smectite-illite-smectite (ISIS) fashion. R0 on the other hand describes random ordering, and other advanced ordering types are also found (R3, etc.). Mixed layer clay minerals which are perfect R1 types often get their own names. R1 ordered chlorite-smectite is known as corrensite, R1 illite-smectite is rectorite. [25]

Summary of Clay Mineral Identification Criteria—Reference Data for Clay Mineral Identification [26]
ClayKaoliniteDehydrated halloysiteHydrated halloysiteIlliteVermiculiteSmectiteChlorite
X-ray rf(001)77101010–1410–1814
Glycol (mg/g)1635606020030030
CEC (meq/100 g)31212251508540
K2O (%)0008–10000
DTA End. 500–660° + Sharp* Exo. 900–975° SharpSame as kaolinite but 600° peak slope ratio > 2.5Same as kaolinite but 600° peak slope ratio > 2.5End. 500–650° Broad. End. 800–900° Broad Exo. 950°0End. 600–750° End. 900°. Exo. 950°End. 610 ± 10° or 720 ± 20°

X-ray rf(001) is the spacing between layers in nanometers, as determined by X-ray crystallography. Glycol (mg/g) is the adsorption capacity for glycol, which occupies the interlayer sites when the clay is exposed to a vapor of ethylene glycol at 60 °C (140 °F) for eight hours. CEC is the cation exchange capacity of the clay. K2O (%) is the percent content of potassium oxide in the clay. DTA describes the differential thermal analysis curve of the clay.

Clay and the origins of life

The clay hypothesis for the origin of life was proposed by Graham Cairns-Smith in 1985. [27] [28] It postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in contact with an aqueous solution. The clay mineral montmorillonite has been shown to catalyze the polymerization of RNA in aqueous solution from nucleotide monomers, [29] and the formation of membranes from lipids. [30] In 1998, Hyman Hartman proposed that "the first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic acid and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hot spring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids." [31]

Biomedical applications of clays

The structural and compositional versatility of clay minerals gives them interesting biological properties. Due to disc-shaped and charged surfaces, clay interacts with a range of drugs, protein, polymers, DNA, or other macromolecules. Some of the applications of clays include drug delivery, tissue engineering, and bioprinting. [32]

Mortar applications

Clay minerals can be incorporated in lime-metakaolin mortars to improve mechanical properties. [33] Electrochemical separation helps to obtain modified saponite-containing products with high smectite-group minerals concentrations, lower mineral particles size, more compact structure, and greater surface area. These characteristics open possibilities for the manufacture of high-quality ceramics and heavy-metal sorbents from saponite-containing products. [34] Furthermore, tail grinding occurs during the preparation of the raw material for ceramics; this waste reprocessing is of high importance for the use of clay pulp as a neutralizing agent, as fine particles are required for the reaction. Experiments on the histosol deacidification with the alkaline clay slurry demonstrated that neutralization with the average pH level of 7.1 is reached at 30% of the pulp added and an experimental site with perennial grasses proved the efficacy of the technique. Moreover, the reclamation of disturbed lands is an integral part of the social and environmental responsibility of the mining company and this scenario addresses the community necessities at both local and regional levels. [35]

The tests which verify that clay minerals are present

The results of glycol adsorption, cation exchange capacity, X-ray diffraction, differential thermal analysis, and chemical tests all give data that may be used for quantitative estimations. After the quantities of organic matter, carbonates, free oxides, and nonclay minerals have been determined, the percentages of clay minerals are estimated using the appropriate glycol adsorption, cation exchange capacity, K20, and DTA data. The amount of illite is estimated from the K20 content since this is the only clay mineral containing potassium. [36]

Argillaceous rocks

Argillaceous rocks are those in which clay minerals are a significant component. [37] For example, argillaceous limestones are limestones [38] consisting predominantly of calcium carbonate, but including 10-40% of clay minerals: such limestones, when soft, are often called marls. Similarly, argillaceous sandstones such as greywacke, are sandstones consisting primarily of quartz grains, with the interstitial spaces filled with clay minerals.

See also

Related Research Articles

<span class="mw-page-title-main">Biotite</span> Group of phyllosilicate minerals within the mica group

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2. It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalysed dark micas in the field. Biotite was named by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica.

<span class="mw-page-title-main">Kaolinite</span> Phyllosilicate clay mineral

Kaolinite ( KAY-ə-lə-nyte, -⁠lih-; also called kaolin) is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6).

<span class="mw-page-title-main">Muscovite</span> Hydrated phyllosilicate mineral

Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(AlSi3O10)(F,OH)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.

<span class="mw-page-title-main">Clay</span> Fine grained soil

Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.

<span class="mw-page-title-main">Gibbsite</span> Form of aluminium hydroxide, mineral

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).

<span class="mw-page-title-main">Bentonite</span> Rock type or absorbent swelling clay

Bentonite is an absorbent swelling clay consisting mostly of montmorillonite which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

<span class="mw-page-title-main">Dickite</span> Phyllosilicate mineral

Dickite is a phyllosilicate clay mineral named after the metallurgical chemist Allan Brugh Dick, who first described it. It is chemically composed of 20.90% aluminium, 21.76% silicon, 1.56% hydrogen and 55.78% oxygen. It has the same composition as kaolinite, nacrite, and halloysite, but with a different crystal structure (polymorph). Dickite sometimes contains impurities such as titanium, iron, magnesium, calcium, sodium and potassium.

<span class="mw-page-title-main">Montmorillonite</span> Phyllosilicate group of minerals

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 resolve individual clay particles. Members of this group include saponite, nontronite, beidellite, and hectorite.

<span class="mw-page-title-main">Illite</span> Group of non-expanding clay minerals

Illite, also called hydromica or hydromuscovite, 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. 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)], 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 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.

<span class="mw-page-title-main">Smectite</span> Mineral mixture of phyllosilicates

A smectite is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.

Expansive clay is a clay soil that is prone to large volume changes that are directly related to changes in water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called vertisols. Soils with smectite clay minerals, including montmorillonite and bentonite, have the most dramatic shrink–swell capacity.

Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.

<span class="mw-page-title-main">Nontronite</span> Phyllosilicate mineral

Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe2O3 and less than ~12% Al2O3 (ignited basis). Nontronite has very few economic deposits like montmorillonite. Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.

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).

<span class="mw-page-title-main">Iberulite</span>

Iberulites are a particular type of microspherulites that develop in the atmosphere (troposphere), finally falling to the Earth's surface. The name comes from the Iberian Peninsula where they were discovered.

<span class="mw-page-title-main">Chamosite</span> Phyllosilicate mineral member of the chlorite group

Chamosite is the Fe2+end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore. Like other chlorites, it is a product of the hydrothermal alteration of pyroxenes, amphiboles and biotite in igneous rock. The composition of chlorite is often related to that of the original igneous mineral so that more Fe-rich chlorites are commonly found as replacements of the Fe-rich ferromagnesian minerals (Deer et al., 1992).

<span class="mw-page-title-main">Pimelite</span> Nickel-rich smectite deprecated as mineral species in 2006

Pimelite was discredited as a mineral species by the International Mineralogical Association (IMA) in 2006, in an article which suggests that "pimelite" specimens are probably willemseite, or kerolite. This was a mass discreditation, and not based on any re-examination of the type material. Nevertheless, a considerable number of papers have been written, verifying that pimelite is a nickel-dominant smectite. It is always possible to redefine a mineral wrongly discredited.

The soil matrix is the solid phase of soils, and comprise the solid particles that make up soils. Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential, but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.

Antigorite Monoclinic mineral

Antigorite is a lamellated, monoclinic mineral in the phyllosilicate serpentine subgroup with the ideal chemical formula of (Mg,Fe2+)3Si2O5(OH)4. It is the high-pressure polymorph of serpentine and is commonly found in metamorphosed serpentinites. Antigorite, and its serpentine polymorphs, play an important role in subduction zone dynamics due to their relative weakness and high weight percent of water (up to 13 weight % H2O). It is named after its type locality, the Geisspfad serpentinite, Valle Antigorio in the border region of Italy/Switzerland and is commonly used as a gemstone in jewelry and carvings.

References

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Works cited

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