Solid solutions

A solid solution is a single phase which exists over a range of chemical compositions. Some minerals are able to tolerate a wide and varied chemistry, whereas others permit only limited chemical deviation from their ideal chemical formulae. In many cases, the extent of solid solution is a strong function of temperature, with solid solution being favoured at high temperatures and unmixing and/or ordering favoured at low temperatures.

Types of solid solution:

1. Substitutional solid solution: chemical variation is achieved simply by substituting one type of atom in the structure by another.
2. Coupled substitution: this is similar to the substitutional solid solution, but in a compound cations of different valence are interchanged. To maintain charge balance, two coupled cation substitutions must take place.
3. Omission solid solution: chemical variation is achieved by omitting cations from cation sites that are normally occupied.
4. Interstitial solid solution: chemical variation is achieved by adding atoms or ions to sites in the structure that are not normally occupied.

Factors affecting the extent of solid solution:

1. Atomic/ionic size: If the atoms or ions in a solid solution have similar ionic radii, then the solid solution is often very extensive or complete. Generally, if the size difference .is less than about 15%, then extensive solid solution is possible. For example, Mg²⁺ and Fe²⁺ have a size mismatch of only about 7%, and complete solid solution between these two elements is observed in a wide range of minerals. However, there is a 32% size difference between Ca²⁺ and Mg²⁺, and we expect very little substitution of Mg for Ca to occur in minerals.
2. Temperature: High temperatures favour the formation of solid solutions, so that endmembers which are immiscible at low temperature may form complete or more extensive solid solutions with each other at high temperature. High temperatures promote greater atomic vibration and open structures, which are easier to distort locally to accommodate differently-sized cations. Most importantly, solid solutions have a higher entropy than the endmembers, due to the increased disorder associated with the randomly distributed cations, and at high temperatures, the –TS term in the Gibb’s free energy stabilises the solid solution.
3. Structural flexibility: Although cation size is a useful indicator of the extent of solid solution between two endmembers, much depends on the ability of the rest of the structure to bend bonds (rather than stretch or compress them) to accommodate local strains.
4. Cation charge: Heterovalent substitutions (i.e. those involving cations with different charges) rarely lead to complete solid solutions at low temperatures, since they undergo complex cation ordering phase transitions and/or phase separation at intermediate compositions. These processes are driven by the need to maintain local charge balance in the solid solution as well as to accommodate local strain.