Substitutional Solid solution.
Recalling Day 1 and our definition of a mineral,
the chemical composition of minerals is fixed, but fixed within limits.
In other words, most minerals can have some variability to their composition,
and we can write mineral formulas in a way to express this variation. You
can think of a solid solution as one mineral that has the chemistries of
two or more ideal, end-member minerals 'dissolved' or mixed into it.
How does solid solution occur? As reviewed in Pauling's Rules, both the size and charge of ions determines how they will enter structures. Therefore, ions of similar size and charge might be expected to substitute for, or take the place of, one another in minerals. As long as radius-ratio considerations and the principle of local charge balance are met, these substitutions will not decrease the stability of the structure.
Solid solution may be complete to partial depending on radius ratio constraints. Complete binary solid solutions exist when a complete spectrum of chemiical compositions is possible between two 'pure' endmembers. Partial solid solutions exist where there is a mixing 'gap' between the end-members. Solid solutions may be simple, involving only one ion-pair, or coupled, and they can also involve substitutions of vaciencies to maintain charge balance.
Simple cationic
Example: Mg+2 = Fe+2
Since iron and magnesium have about the same ionic radius, 0.72 and 0.78
angstroms, respectively, and the same charge, they can substitute for one
another extensively in mineral structures. The radius ratio constraints
predict that both enter into octahedral coordination with oxygen (1.36
angstroms, thus the radius ratios are about 0.55). Important mineral groups
in which Fe-Mg solid solution is near-complete include: the olivines, (Fe,Mg)2SiO4,
and the orthopyroxenes, (Fe,Mg)SiO3.
Simple anionic
Br-1 = Cl-1
Coupled cationic
Na-1Si+4 = Ca+2Al+3
(plagioclase; sensitive function of pressure)
AlVIAlIV = Mg+2Si+4 (Tschermak exchange)
Omission
Vac-4 = Na-1Al+3 (alkali site in micas or amphiboles) Compositional variations
in minerals are a function of: ionic size: Substitution is possible if
a size difference is less than about 30% charge: electrical neutrality
must be met (Paulings rule #1) temperature and pressure
Effects of Temperature and Pressure Increasing pressure and temperature favors substitution of smaller cations into sites, i.e., Temperature and pressure changes alter the the constraints of radius ratio for coordination polyhedra.
Exsolution
Imagine a nice bubbling pot of beef stew, with all the ingredients
mixed; upon cooling, the stew will separate into different components -
and a fat-rich layer will 'exsolve', or come out of solid solution, on
the top. (Ready to scoop up and eat for breakfast - Mmmmm!) In our stew
example, convection (and maybe a spoon) tend to mix the fat throughout
the stew. This is a pretty good analogy for exsolution in minerals. At
high temperatures and pressures, the constraints of size and valence may
permit a given substitution to occur - however, as conditions change, that
substitution may not remain stable, and the mineral will tend to 'unmix'
and separate into different components. So, mineral exsolution refers
to the process whereby an originally homogeneous solid solution separates
into two or more minerals of distinct composition. Mineral components in
solution can 'exsolve', or come out of solution, upon cooling or a drop
in pressure.
Why would changing pressure and temperature affect the
stability of a given atomic stubstitution? Well, solids tend to expand
with increasing temperature, and the sites for cations will also tend to
expand. Therefore, we would predict that high temperatures would favor
larger cations in a given site. Similarly, solids contract under increasing
pressure, and thus we would expect sites to favor smaller cations at higher
pressures.