Substitution (mineralogy)

In mineralogy, substitution ( exchange ) refers to the replacement of one chemical element in the crystal structure with another. The underlying chemical reaction is called a substitution or exchange reaction . A distinction is made between simple substitution , also called diadochie , and coupled substitution .

Basics

To describe the chemical composition of minerals , idealized empirical formulas are usually set up, e.g. B. Mg ₂ SiO ₄ for forsterite . In reality, however, other elements will always be found in natural crystals in chemical analyzes , in this example iron (Fe). Whether an element (in minerals practically always in the form of an ion ) can be inserted into a crystal lattice depends on its radius, its electrical charge and its binding character. In general it can be said that elements with the same ion charge and a similar ion radius are interchangeable in the crystal lattice of a mineral. If an element in the crystal can be completely replaced by another (or several other) elements, one speaks of a mixed crystal row (a mixed crystal system) with two (or more) end link compositions. These extreme compositions usually have their own names. If, in the above example, all of the Mg were replaced by Fe, a mineral with the formula Fe ₂ SiO _{4 would be obtained} ; this mineral is called fayalite . All compositions between the Mg end link forsterite and the Fe end link fayalite can be given in the form of a mole fraction . The corresponding mixed crystal series is referred to as the olivine series with the formula (Mg, Fe) ₂ SiO ₄ . In the literature, the percentage of an end link is often given, e.g. B. Fo ₇₀ for a mixed crystal with 70% forsterite (Fo) and 30% fayalite. Whether and to what extent a theoretically possible substitution takes place in individual cases depends heavily on the geological conditions and can provide information about the formation conditions of the mineral and the rock ( petrogenesis ).

Substitution types

Depending on the number of lattice sites of a crystal affected by the exchange, a distinction is made between two types of substitution, the simple and the coupled substitution.

Simple substitution

Here an element A with the charge n is replaced by an element B with an identical charge n on the same lattice site (Diadoch replacement). The generalized substitution response is

A ⁿ ↔ B ⁿ

Examples

Mg ²⁺ ↔ Fe ²⁺

Mg ²⁺ ₂ SiO ₄ ↔ Fe ²⁺ ₂ SiO ₄ (forsterite ↔ fayalite; olivine series)

K ⁺ ↔ Na ⁺

K ⁺ AlSi ₃ O ₈ ↔ Na ⁺ AlSi ₃ O ₈ (orthoclase ↔ albite; alkali feldspar series)

Coupled substitution

In the case of a coupled substitution, an element A with the charge n is replaced by an element B with a different charge m . In order to keep the overall crystal electrically neutral, an element C of charge m must also be exchanged for an element D of charge n . The elements A and B as well as the elements C and D usually each occupy one grid position. The general substitution reaction is

A ⁿ + C ^m ↔ B ^m + D ⁿ

A coupled substitution can be formulated for any number of elements and is the normal case in natural systems (real chemical formula). However, the total charge per formula unit always remains the same. Both cations and anions can be involved in the substitution.

Examples

Mg ²⁺ + Si ⁴⁺ ↔ 2 Al ³⁺

Mg ²⁺ ₇ Si ⁴⁺ ₈ O ₂₂ OH ↔ Mg ²⁺ ₆ Al ³⁺ ₂ Si ⁴⁺ ₇ O ₂₂ OH ( Tschermak exchange in amphiboles )

Na ⁺ + Si ⁴⁺ ↔ Ca ²⁺ + Al ³⁺

Na ⁺ Al ³⁺ Si ⁴⁺ ₃ O ₈ ↔ Ca ²⁺ Al ³⁺ ₂ Si ⁴⁺ ₂ O ₈ (albite ↔ anorthite; plagioclase series)

Frequent combinations

Due to the required properties of the substituting elements in terms of ionic charge and radius, certain combinations of mutually substituting elements are often observed. With simple substitutions :

• Mg ²⁺ (ionic radius in the corresponding compound = 0.86 Å ) and Fe ²⁺ (0.75 Å) and Mn ²⁺ (0.81 Å) are more or less completely interchangeable in many minerals (e.g. olivine , Pyroxene , amphibole , grenade , mica )
• K ⁺ (1.65 Å) and Na ⁺ (1.32 Å) e.g. B. in feldspars , zeolites , amphiboles
• Ca ²⁺ (1.26 Å), Sr ²⁺ (1.40 Å), Ba ²⁺ (1.56 Å) and Eu ²⁺ (1.31 Å) as well
• Fe ³⁺ (0.69 Å) and Al ³⁺ (0.68 Å) e.g. B. in amphiboles, mica, epidote group
• Nb ⁵⁺ (0.78 Å) and Ta ⁵⁺ (0.78 Å) e.g. B. in columbite
• F ^- (1.19 Å), Cl ^- (1.67 Å) and OH ^- (1.23 Å) e.g. B. in mica, amphiboles and zeolites

In the case of coupled substitutions , the following elements can be exchanged on a grid space (further replacements are necessary to balance the charge):

• Fe ³⁺ , Al ³⁺ , Cr ³⁺ for Fe ²⁺ , Mg ²⁺
• Al ³⁺ , P ⁵⁺ for Si ⁴⁺
• O ²⁻ for OH ^-

literature

• Nesse WD (2000): Introduction to Mineralogy - Oxford University Press, ISBN 0-19-510691-1

See also

• Mixed crystal