Substitution

Replacement of one ionAtom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. Click on Term to Read More or ionic group for another in the same structural site in a mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More yielding a solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with. Most substitution in minerals is of cations which are smaller and essentially sit in a latticeRegularly spaced array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated at regular interval in three dimensions with the same orientation. The smallest division of the lattice which can still be used to represent the entire structure is called the unit Click on Term to Read More of oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More anions. Anionic substitution does occur in halides. Substitutions are classified based on the exact nature of the processes, which include: simple substitutionSubstitution in which the cations or anions replacing one another have the same charge and similar radii (within ~15%). Simple substitutions may result in complete or partial solid solution. The following simple substitutions are commonly complete: Fe2+ (0.78 Å) ↔ Mg2+ (0.72 Å), Fe2+ (0.78 Å) ↔ Mn2+ (0.83 Å),, coupled substitutionSubstitution in which the charges of substituting ions are not same and charge balance is achieved by a second substitution on a different crystallographic site. The most common example in the solid solution series of the plagioclase feldspars: anorthite, CaAl2Si2O8, to albite, NaAlSi3O8. Here, there are two substitutions taking place: Click on Term to Read More, and interstitial substitutionSubstitution in which interstitial ions are added or removed. This is most common in cyclosilicate minerals, which have channel-like structures (e.g., beryl or zeolites) or in clay minerals, which can accommodate ions between their silicate sheets. The charge of interstitial ions is often balanced by substitutions in the crystal lattice. Click on Term to Read More.

Four factors control whether substitution will occur: (1) ionic radii; (2) ionic charges; (3) temperature; and (4) natures of the bonds formed. Substitution is more likely if the two ions or ionic groups are similar in size. We can calculate the size difference as a percentage:

Substitution is common if sizes are within 15 %, limited if sizes are between 15–30 % and unlikely if sizes differ by >30 %. Similarly, substitution is more likely if the charge difference is 0 or 1; larger differences make substitution unlikely. At higher temperatures, the sites into which the ions must fit are larger (bonds lengthen: thermal expansion) and substitution is easier. Increasing temperature effectively makes the size constraint less rigid, providing an additional ~10% leeway. Lastly, substitution is unlikely if the types of bonds formed by the two ions are very different (essentially a function of electronegativity). The chart below applies to terrestrial rocks.