Exchange Vector

Notation for indicating 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 that occurs in minerals; also called “exchange component.” Probably the most common vector in minerals is [FeMg-1], which indicates Fe ↔ Mg substitution. Fractions of this vector are added (or subtracted) to 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 end member to produce the range of compositions observed. In the case of simple olivines, [FeMg-1] can be thought of as a vector pointing from forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo), Mg₂SiO₄, towards fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo), Fe₂SiO₄, the magnitude of which tells us how far in this direction the mineral composition lies. Adding 1 unit of this exchange component to forsterite, yields [FeMg]SiO₄, or Fo50. Adding 2 units of this exchange component to forsterite takes us all the way to fayalite. Adding 0.5 units of this exchange component takes us to Fo75, etc…

More complicated mineral substitutions require multi-element exchange vectors. For example, [NaSiCa-1Al-1] describes the coupled feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. substitution from anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly to albite: Ca₂₊ + Al₃₊ ↔ Na₁₊ + Si₄₊. The exchange vector for the tschermak substitution, which occurs in a wide range of silicates is [AlviAlivMg-1Si-1], where, VI indicates aluminum in an octahedral site and IV indicates it in a tetrahedral site.