With the background in thermodynamics we are now ready to approach some phase diagrams to appreciate conditions under which minerals form. Most important are crystallization from a melt, phase transitions (relevant to the deep Earth), and solid solutions, including exsolution.
Introduction
Mineral (phase) equilibrium diagrams show the limits of stable existence of minerals at different conditions. They are plotted either on the basis of thermodynamic calculations and the phase rule, as illustrated in the previous chapter, or as a graphic representation of experimental results.
Phase diagrams, as a rule, are plotted as a function of two variables such as:
• temperature T versus total pressure P (Figure 19.2);
• temperature T versus partial pressure p (Figure 19.3);
• oxidation–reduction potential Eh versus pH (Figure 19.5);
• temperature T versus composition of system X.
Sometimes three or four variables are used, especially for complex compositions. Be aware that most phase diagrams of geological systems assume idealized situations.
The principles of interpretation of phase diagrams are straightforward. They allow us to determine, for example, which phase is in equilibrium at a certain temperature and pressure. Temperature–composition phase diagrams, however, which describe crystallization of a magma and subsolidus exsolution processes in minerals, deserve further elaboration.
Diagrams for crystallization from a melt
The binary system diopside (CaMgSi2O6)–anorthite (CaAl2Si2O8) (Figure 20.1) is a classic example and has direct application to our understanding of crystallization processes in basaltic magmas. There are two components (CaMgSi2O6 and CaAl2Si2O8) and one free parameter (temperature, m = 1) in this system. The solid lines correspond to the univariant equilibrium between two phases (p = c + 1 − f = 2 + 1 − 1 = 2 for equation 19.39), which are a mineral (anorthite or diopside) and a melt.
At point E, where the two univariant lines meet, the number of degrees of freedom is zero, while the number of coexisting phases is three (p = c + 1 − f = 2 + 1 − 0 = 3): diopside, anorthite, and melt. E is called the eutectic point and corresponds to the lowest temperature at which the melt and solid phases can coexist.