GeoClassroom Physical Geology Historical Geology Structure Lab Mineralogy Petrology

Atomic Substitution/Isomorphism

According to Goldschmidt's Rules atomic substitution is controlled by:

  1. the size (i.e., radii) of the ions
    • free substitution can occur if size difference is less than ~15%
    • limited substitution can occur if size difference is 15 - 30%
    • little to no substitution can occur if size difference is greater than 30%
  2. the charge of the ions --> cannot differ by more than 1

Isomorphs: minerals with different chemical compositions; same crystal structure (belong to same crystal class)

Polymorphs: minerals with same chemical composition; different crystal structures

When the chemical composition of a mineral varies because of atomic substitution, the mineral is said to exhibit "Solid Solution"

Solid Solution is defined as "a mineral structure in which specific atomic site(s) are occupied in variable proportions by two or more different elements." (Klein & Hurlbut, p.233)


The Olivine group represents a complete solid solution series

Compositions range from a 100% Mg-rich "end member" (forsterite) to a 100% Fe-rich "end member" (fayalite), with all mixtures of these two elements possible (e.g., 90% Mg and 10% Fe)

  • complete solid solution series because Fe and Mg have same charge and similar ionic radius

Plagioclase Feldspars also display a complete solid solution series ranging from a 100% Na-rich end member (albite) to a 100% Ca-rich end member (anorthite), with all intermediate compositions possible.

  • Because Na has a charge of +1 and Ca has a charge of +2, a double substitution must occur to maintain the charge balance. The other cations in the feldspar composition are Al (charge = +3) and Si (charge = +4). (Al and Si are close enough in size for substitution to occur)For each substitution of Ca (+2) for Na (+1), an equal amount of Al (+3) is substituted for Si (+4)
      e.g., a 50/50 mixture would have the composition:
          Ca0.5Na 0.5Al 1.5Si 2.5 O 8

      charge balance: (+2 x 0.5) + (+1 x 0.5) + (+3 x 1.5) + (+4 x 2.5) = +16

            which is balanced by -16 from oxygen

In contrast Alkali Feldspars display only a limited solid solution series because the radius of K (1.33 Å) is significantly larger than that of Na (0.97 Å)

Some carbonates show a limited solid solution series (e.g., calcite & magnesite)

  • to maintain the calcite-type structure, large cations (e.g., Ca radius = 1.08 Å) cannot be freely replaced by small ones (e.g., Mg radius = 0.66 Å); if too much Mg is substituted, the structure destabilizes and collapses
  • dolomite: 50/50 mix of Ca and Mg
    *structurally completely different from calcite - has alternating Ca-rich and Mg-rich layers

All Pages Copyright © GeoClassroom. All Rights Reserved.