Coupled substitution is the geological process by which two elements simultaneous substitute into a crystal in order to maintain overall electrical neutrality and keep the charge constant. In forming a solid solution series, ionic size is more important than

ionic charge An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, as this can be compensated for elsewhere in the structure.

Ionic size

To make a geometrically stable structure in a mineral, atoms must fit together in terms of both their size and charge. The atoms have to fit together so that their electron shells can interact with one another and they also have to produce a neutral molecule. For these reasons the sizes and electron shell structure of atoms determine what element combinations are possible and the geometrical form that various minerals take. Because electrons are donated and received, it is the ionic radius of the element that controls the size and determines how atoms fit together in minerals.

Examples

* Coupled substitutions are common in the

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

minerals where substitutes for in

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

sites. For example, when a

plagioclase feldspar Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...

series forms,

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. I ...

( Na Al Si₃ O₈) can change to

anorthite Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon. Mineralo ...

( Ca Al₂ Si₂ O₈) by having replace . However, this leaves a negative charge that has to be balanced by the (coupled) substitution of for . * Despite being nicknamed

fool's gold The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

, pyrite is sometimes found in association with small quantities of

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

. Gold and

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

occur as a coupled substitution in the pyrite structure. In the

Carlin–type gold deposit Carlin–type gold deposits are sediment-hosted disseminated gold deposits. These deposits are characterized by invisible (typically microscopic and/or dissolved) gold in arsenic rich pyrite and arsenopyrite. This dissolved kind of gold is c ...

s, arsenian pyrite contains up to 0.37% gold by weight. * The possible replacement of by in

Corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pres ...

. * and in

Haematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

* →

Diopside Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...

(MgCaSi₂O₆) →

Jadeite Jadeite is a pyroxene mineral with composition sodium, Naaluminium, Alsilicon, Si2oxygen, O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, bu ...

: (NaAlSi₂O₆ or Diopside Aoste

Jadeite Sodium aluminum silicate Burma 3025

* 2 → 2 As in the

Spinel group The spinels are any of a class of minerals of general formulation which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a cubic close-packed lattice and the cations ...

s * The site being filled to maintain charge does not have to be a substitution. It can also involve filling a site that is normally vacant in order to achieve charge balance. For example, in the

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

mineral

Tremolite 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 ...

- (Ca₂(Mg_5.0-4.5Fe²⁺_0.0-0.5)Si₈O₂₂(OH)₂), replaces then can go into a site that is normally vacant to maintain charge balance. This new mineral would then be

edenite Edenite is a double chain silicate mineral of the amphibole group with the general chemical composition NaCa2Mg5(Si7Al)O22(OH)2. Edenite is named for the locality of Edenville, Orange County, New York, where it was first described. Occurrence ...

a variety of

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rocks ...

. *

Bityite Bityite is considered a rare mineral, and it is an endmember to the margarite mica sub-group found within the phyllosilicate group. The mineral was first described by Antoine François Alfred Lacroix in 1908, and later its chemical composition wa ...

’s structure consists of a coupled substitution it exhibits between the sheets of polyhedra; the coupled substitution of beryllium for

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

within the tetrahedral sites allows a single lithium substitution for a vacancy without any additional octahedral substitutions.Lin, J-C. and Guggenheim, S. (1983) The crystal structure of a Li,Be-rich brittle mica: a dioctaheral-trioctahedral intermediate. American Mineralogist, 68, 130-142. The transfer is completed by creating a tetrahedral sheet composition of Si₂BeAl. The coupled substitution of lithium for vacancy and the beryllium for the tetrahedral aluminium maintains all the charges balanced; thereby, resulting in the trioctahedral end member for the

margarite Margarite is a calcium rich member of the mica group of the phyllosilicates with formula: Ca Al2(Al2 Si2) O10(O H)2. It forms white to pinkish or yellowish gray masses or thin laminae. It crystallizes in the monoclinic crystal system. It typical ...

sub-group of the

phyllosilicate 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 Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

group.Guggenheim, S. (1984) The brittle micas. Reviews in Mineralogy, 13, 61-104. * Ferrogedrite is related to

anthophyllite Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthoph ...

amphibole and

gedrite Gedrite is a crystal belonging to the orthorhombic ferromagnesian subgroup of the amphibole supergroup of the double chain inosilicate minerals with the ideal chemical formula . Gedrite is the magnesium (Mg) rich endmember of a solid solution ser ...

through coupled substitution of (Al, Fe³⁺) for (Mg, Fe²⁺, Mn) and Al for Si.Deer, William Alexander, Robert Andrew Howie, and Jack Zussman. Rock-forming minerals. 2B. Double-chain silicates. Vol. 2. Geological Society, 1997.

References

{{reflist Chemical reactions Minerals Crystallography