Atomic Substitution/Isomorphism

According to Goldschmidt's Rules atomic substitution is controlled by:
  1. the size (i.e., radii) of the ions

  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)

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.

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

Questions to: rapril@mail.colgate.edu
Copyright 1997 © Colgate University.

Home page