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Solid Solution and Exsolution Here is a short presentation that might help you to understand solid solution and exsolution. The following diagrams illustrate a 2-dimensional model of crystal. Large circles represent anions and smaller circles represent cations. Pay attention to how the cations move (diffuse) through the crystal structure.
The grey spheres are anions, and the pink and green spheres are two kinds of cations. Notice that half the cations are pink and half are green. This represents a 50/50 solid solution.
At high temperatures, this mineral can exist with this 50/50 mix of cations. However, at lower temperatures, this mineral will no longer be stable. Instead, two new minerals will be stable – one with 75% green and 25% pink cations, and the other with 25% green and 75% pink cations. Both of these new minerals will also be solid solutions. How does this happen? The cations diffuse (move) from one region in the crystal to another. Let’s watch this happen for a small part of the crystal.
Concentrate on the area outlined in blue. To begin with it has half green cations and half pink cations.
Now the area outlined in blue has 25% pink and 75% green cations
This would happen in all regions of the crystal. For simplicity, only one area was shown in this example.
This is what it would look like for all regions of the crystal. The dark gray anions go with the mineral that has 75% green cations; the light gray anions go with the mineral that has 75% pink cations.
There are two different minerals here, but they exist together, intermingled with each other.
The original mineral exsolved into two different minerals. The original mineral was a solid solution, and the two new mineral regions are also both solid solutions. The major difference is the proportions of cations in each, although the arrangement of the anions is usually shifted slightly as well.
This phenomenon of exsolution happens a lot with the alkali feldspars. “Perthite” is what we call an alkali feldspar that has exsolved into little areas of sodium-rich alkali feldspar within a potassium-rich alkali feldspar host. Exsolution happens in other minerals, too. A type of pyroxene called pigeonite can exsolve into little layers (lamellae) of augite within an orthopyroxene host if it cools slowly enough.
