Clay minerals

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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, 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 a clay mineral (kaolinite) (SEM image, x1340 magnification) Kaolinite - USGS bws00008.jpg
Hexagonal sheets of a clay mineral (kaolinite) (SEM image, ×1340 magnification)

Clays form flat hexagonal sheets similar to the micas. 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.

Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification and study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, and SEM-EDS or automated mineralogy processes. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.

Occurrence

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, [2] including Echus Chasma, Mawrth Vallis, the Memnonia quadrangle and the Elysium quadrangle. Spectrography has confirmed their presence on asteroids including the dwarf planet Ceres [3] and Tempel 1 [4] as well as Jupiter's moon Europa. [5]

Classification

Clay minerals can be classified as 1:1 or 2:1, this originates because they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.

Clay minerals include the following groups:

Mixed blue layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to a R1 ordered illite-smectite, for example. This type would be ordered in an ISISIS 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. [11]

History

Knowledge of the nature of clay became better understood in the 1930s with advancements in x-ray diffraction technology necessary to analyze the molecular nature of clay particles. [7] Standardization in terminology arose during this period as well [7] with special attention given to similar words that resulted in confusion such as sheet and plane. [7]

Structure

Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO
4 tetrahedra and/or AlO
4 octahedra. The sheet units have the chemical composition (Al,Si)
3O
4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared vertices are on the same side of the sheet.

In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium 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.

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.

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+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.

Biomedical applications of clays

As most of the clays are made from minerals, they are highly biocompatible and have interesting biological properties. Due to disc-shape and charged surfaces, clay interact with a range of macromolecules such as drugs, protein, polymers, DNA, etc. Some of the applications of clays include drug delivery, tissue engineering, and bioprinting.

Mortar applications

Clay minerals can be incorporated in lime-metakaolin mortars to improve mechanical properties. [12]

See also

Related Research Articles

Kaolinite Layered silicate clay 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
4) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.

Mineral Element or chemical compound that is normally crystalline and that has been formed as a result of geological processes

A mineral is, broadly speaking, a solid chemical compound that occurs naturally in pure form. Minerals are most commonly associated with rocks due to the presence of minerals within rocks. These rocks may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are usually excluded, but some minerals are often biogenic or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.

Silicate class of chemical compounds, salts and esters of silicic acids

In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO(4−2x)−
4−x]
n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−
4, metasilicate SiO2−
3, and pyrosilicate Si
2O6−
7. The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate.

Gibbsite 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).

Sekaninaite cyclosilicate, mineral

Sekaninaite ((Fe+2,Mg)2Al4Si5O18) is a silicate mineral, the iron-rich analogue of cordierite.

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.

Dickite 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.

Montmorillonite smectite, phyllosilicate mineral

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.

Illite degradation product of muscovite to montmorillonite

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. 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
3O)(Al,Mg,Fe)
2(Si,Al)
4O
10[(OH)
2,(H
2O)], 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.

Tetrahedral-octahedral honeycomb Quasiregular space-filling tesselation

The tetrahedral-octahedral honeycomb, alternated cubic honeycomb is a quasiregular space-filling tessellation in Euclidean 3-space. It is composed of alternating regular octahedra and tetrahedra in a ratio of 1:2.

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. Minerals 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.

Nontronite smectite, 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).

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.

Iberulite

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.

Pimelite mineral that was discredited 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.

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.

Clay minerals are one of the most diverse minerals but all have a commonalty of crystal or grain sizes below 2 µm. Chemically clays are defined by crystal structure and chemical composition. Sometimes fine grain sediments are mistakenly described as clays, this is actually a description of the “clay-size fraction” rather than the mineralogy of the sediment. There are three crystallographic clay groups: platy clays (phyllosilicates), fibrous clay minerals, and amorphous clay. Phyllosilicates are the most abundant clays and are categorized based on the layering of a tetrahedral and an octahedral layer. For most clays, the octahedral layer is centered with Al3+, Fe3+, or Mg(OH)2, but sometimes Zn2+, Li+, and Cr3+ can substitute as well. Si4+ is normally the center of the tetrahedral layer but Al3+ will often partially substitute and create a charge imbalance. Two-layer clays are composed of a tetrahedral layer and an octahedral layer (T-O) while three-layer clays contain an octahedral layer sandwiched by two tetrahedral layers (T-O-T). When substitution of Al3+ for Si4+ creates a charge imbalance, an interlayer cation will fill in between tetrahedral layers to balance the charge of the clay.

Zigrasite is a phosphate mineral with the chemical formula of MgZr(PO4)2(H2O)4. Zigrasite was discovered and is only known to occur in the Dunton Quarry at Oxford County, Maine. Zigrasite was specifically found in the giant 1972 gem tourmaline-bearing pocket at the Dunton Quarry. Zigrasite is named after James Zigras who originally discovered and brought the mineral to attention.

References

  1. ↑ Kerr PF (1952). "Formation and Occurrence of Clay Minerals". Clays and Clay Minerals. 1 (1): 19–32. doi: 10.1346/CCMN.1952.0010104 .
  2. ↑ Georgia Institute of Technology (20 Dec 2012). "Clays on Mars: More plentiful than expected". Science Daily . Retrieved 22 Mar 2019.
  3. ↑ Rivkin AS, Volquardsen EL, Clark BE (2006). "The surface composition of Ceres: Discovery of carbonates and iron-rich clays" (PDF). Icarus . 185 (2): 563–567. doi:10.1016/j.icarus.2006.08.022.
  4. ↑ Napier WM, Wickramasinghe JT, Wickramasinghe NC (2007). "The origin of life in comets". Int. J. Astrobiol. 6 (4): 321–323. doi:10.1017/S1473550407003941.
  5. ↑ Greicius T (26 May 2015). "Clay-Like Minerals Found on Icy Crust of Europa". NASA .
  6. 1 2 3 4 "The Clay Mineral Group". Amethyst Galleries. 1996. Archived from the original on 27 Dec 2005. Retrieved 22 Feb 2007.
  7. 1 2 3 4 Bailey SW (1980). "Summary of recommendations of AIPEA nomenclature committee on clay minerals". Am. Mineral. 65: 1–7.
  8. ↑ Agle DC, Brown D (12 Mar 2013). "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars". NASA . Retrieved 12 Mar 2013.
  9. ↑ Wall M (12 Mar 2013). "Mars Could Once Have Supported Life: What You Need to Know". Space.com . Retrieved 12 Mar 2013.
  10. ↑ Chang K (12 Mar 2013). "Mars Could Once Have Supported Life, NASA Says". The New York Times . Retrieved 12 Mar 2013.
  11. ↑ Moore DM, Reynolds Jr RC (1997). X-Ray Diffraction and the Identification and Analysis of Clay Minerals (2nd ed.). Oxford: Oxford University Press. ISBN   9780195087130. OCLC   34731820.
  12. ↑ Andrejkovičová, S.; Velosa, A.L.; Ferraz, E.; Rocha, F. (2014). "Influence of clay minerals addition on mechanical properties of air lime–metakaolin mortars". Construction and Building Materials. 65: 132–139. doi:10.1016/j.conbuildmat.2014.04.118.
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