COMPLETE SOLID SOLUTION
Solid solution is defined as a mineral which exhibits a variable composition represented by the substitution of one chemical species for another, e.g.:
- Mg for Fe in olivine
- NaSi for CaAl in plagioclase.
These two systems are the most applicable to geological examination of solid solution crystallization and melting.
The appearance of this system at first glance is totally different from any other binary system we have examined. The reason for this is due to the solid solution between the two end member components - albite and anorthite.
The system contains a Liquidus and a Solidus. Above the liquidus only liquid exists, as in the other systems. Below the solidus only solids exist. In this phase field instead of having two distinct solids of fixed composition there exists a single solid of variable composition - plagioclase.
Applying the phase rule to points 1, 2 and 3 shown on the above diagram, shows that:
- Point 1
Lies above the liquidus within the liquid field.
- P = 1 - Liquid
- C = 2 - two components Ab and An
- F = 2 - two degrees of freedom
- To maintain equilibrium we can change temperature, increase or decrease, and change composition, more or less An, and still have only one phase (liquid) present.
- Point 2
Lies in the field between the liquidus and solidus.
- P = 2 - Plagioclase solid solution and Liquid
- C = 2 - two components Ab and An
- F = 1 - one degree of freedom
- To maintain equilibrium we can change only temperature or composition, but not both, to stay within this field. If we fix the solid as plagioclase of a specific composition then the liquid which is in equilibrium with this plagioclase becomes fixed.
- Point 3
Lies below the solidus in the field of PlagSS.
- P = 1 - plagioclase solid solution
- C = 2 - two components Ab and An
- F = 2 - two degrees of freedom
- As with Point 1, we can change temperature and composition and still have only one phase (plagioclase) present.
