Interstitial Substitution

SubstitutionReplacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid solution. Most substitution in minerals is of cations which are smaller and essentially sit in a lattice of oxygen anions. Anionic substitution does occur in halides. Substitutions are classified based in which interstitialTerm applied to ions or atoms occupying sites between lattice points. 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 silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the sheets. The charge of interstitial ions is often balanced by substitutions in the crystal latticeAtoms or groups of atoms repeated at regular intervals in three dimensions with the same orientation. One may regard each atom or group of atoms as occurring at a point and the resulting collection of points is the space lattice or lattice of the crystal.. For example, for interstitial ions in the channels of beryl (R):