Exchange Vector

Notation for indicating substitution 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 mineral 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 forsterite, Mg2SiO4, towards fayalite, 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 feldspar substitution from anorthite 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.

Some or all content above used with permission from J. H. Wittke.

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