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