17 - Lecture notes for GEOL 3010

Compositional variations in minerals - Purity of composition in a mineral is more the exception than the rule.

Most minerals display variability in composition. Compositional variation is a result of the substitution of one ion or ionic compound for another ion or ionic compound into similar structural sites. The reasons for chemical variation in minerals is best understood by the study of mineral thermodynamics.

This is termed solid-solution is defined as: a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions.

Conditions for solid-solution

1. Similarity of ion size.

The relative size of two different ions in the same valence state can be evaluated by the ratio of the one ionic radius to the other . For example ionic radius of Fe3+ is 0.65 when in octahedral coordination. The ionic radius of Al3+ is 0.54 when in octahedral coordination. 0.65/0.54 = 1.20. It could also be said that the Fe3+ ion is 20% larger than the Al3+ ion. This is determine by taking the difference between the ionic radii and dividing by the radius of smaller ion. (0.65 - 0.54)/0.54 = 20%

Some common examples;

ion A

ion B

Ratio A/B

 Relative size difference

Fe2+ = 0.74 Mg2+ = 0.66



Al3+ = 0.51 Si4+ = 0.42



OH- = 1.36 (O2-) F- = 1.33



Mn2+ = 0.80 Fe2+ = 0.74



A rule of thumb for determining the likelihood for ionic substitution :

Ranges: <15% common, 15-30% limited, >30% unlikely

2. The charges of ions involved must be the same. If not, then there must be a compensating substitution in another site so as to maintain electric neutrality.

3. In general, the higher the temperature during crystallization, the greater the chance for substitution.

Three types of solid-solution mechanisms

1. Substitutional - Most common type of solid solution.

The solid-solution example of the fosterite - fayalite olivine series. (i.e., Mg for Fe).

If minerals are formed at high temperature, thermal disorder is prevalent. In this case, a greater range in differences are allowed. A common example is the feldspar Sanidine KAlSi3O8 which incorporates Na.

K+ = 1.33 Na+ = 0.97 size difference = 37%

Recall a mineral must always be electrically neutral. If a divalent substitutes for a monovalent cation, then a compensating charge change must be made somewhere in the structure. In this case, the type of substitution is known as a coupled-cationic substitution.

2A2+ <---> B+ + C3+

Plagioclase series

NaAlSi3O8+ Ca2+ + Al3+ <----> CaAl2Si2O8 + Na1+ + Si4+

2. Interstitial solid-solutions

Certain structures have large interstices or voids (e.g., ring-silicates such as beryl; sheet silicates such as clay minerals and tectosilicates such as the zeolites).

See for example the beryl structure.

Water and carbon dioxide are a common examples of neutral compounds occupying an interstitial site with varying amounts.

2. Omission solid-solutions

In some cases, sites within a mineral structure will be unoccupied or unfilled. This phenomenon can be caused by several conditions. Structural defects including 1) point, 2) line and 3) planer defects.

Schottky and Frenkel defects.

Best known example of omission solid-solution is pyrrhotite. Pyrrhotite is an Fe and S structure with Fe2+octahedrally coordinated by S.

R = Fe2+/S2- = 0.74/1.84= 0.40 ---> C.N. = 6

If all sites are full, then the structural formula is FeS. In some cases, the Fe site is left vacant. Vacancies can occur with nearly 20% Fe missing from the structure. Because this is variable the formula of pyrrhotite is typically given as Fe(1-x)S.

Recall that the structure must be electrically neutral. The compensation is made through a redox state of the iron.

3 Fe2+ ---> 2 Fe3+

The formula is then written as:

Fe2+(1-3x) Fe3+(2x)__(x) S

__ denotes a cation vacancy.

The mixed iron valence states is what imparts the ferrimagnetic property of pyrrhotite.