Phase Diagrams Explained
Phase diagrams are admittedly not necessarily that intuitive, but they are the easiest graphical way to illustrate what happens in the crystallization of a melt to yield specific products. The example we're using here is the simplest case scenario imaginable: a melt that, when it crystallizes fully, will produce ONLY two minerals. Most real melts, of course, will typically crystallize out four, five or more minerals – depending on the chemistry of the melt and the cooling rate.
The diagram above is one for a melt compositional range that would have the potential to crystallize into Mineral A and Mineral B. (You can think of these as augite and a calcium-rich plagioclase feldspar, if having real mineral names makes it easier to understand.) Notice that the melting temperatures for the pure minerals are approximately 985°C for Mineral A, and 945°C for mineral B.
The vertical scale on this 2-component (as it's called) phase diagram represents temperatures - and I've put some numbers on that to try to make it clearer as well.
The horizontal scale is composition, showing all possible mixes of Mineral A and Mineral B. At the far left margin, a melt composition here could only crystallize A (augite), but on the other side, the composition represents 100% B (plagioclase feldspar). The closer you are to the end-member, the higher the percentage of that material in the melt, and the less of the other component.
So, if you're 3/4 of the way from B to A, the melt composition represented is 75% A and 25% B. In the cartoon above, a melt with a composition of approximately 48% A and 52% B will have the lowest melting (=crystallization) temperature (~845°C) of any possible mix of these two minerals.
ANY melt with a composition that would place it to the left of this point in the diagram will crystallize out phenocrysts of mineral A as it cools; a melt to the right would crystallize out phenocrysts of B. However, as the melt cools and these minerals are removed, the bulk chemistry of the remaining melt moves towards the other end – away from A or B, respectively. However, when it gets to the point where the bulk composition is 52% B and 48% A, ALL the remaining melt will crystallize out into these two minerals, in these proportions, at a temperature of approximately 845°C. Thus, the groundmass will have the same composition no matter what the phenocrysts happen to be. The farther the initial melt is from this composition, the more phenocrysts there will be. The slower it cools, the larger those phenocrysts will be.
Experimental Petrology
Mineralogists and petrologists use many types of phase diagrams. All are designed to show variation in mineral or mineral assemblage stability as physical or chemical conditions change. The most common kind of mineralogical phase diagram, referred to as a P-T diagram, has pressure and temperature as its two axes. Normally, pressure corresponds to the vertical axis, and temperature to the horizontal axis. P-T diagrams show the stability fields for specific minerals or mineral assemblages in pressure-temperature space. Figure 8 is a P-T diagram for the chemical system a system that includes quartz and its polymorphs. Pressure increases upward and temperature increases to the right. Some petrologists flip the vertical axis and draw P-T diagrams with pressure increasing downward, because the pressure within the Earth increases with depth.
Geologists generally derive phase diagrams by conducting experiments in the laboratory. They combine minerals or chemicals, allow them to react under different conditions, and then examine the results. We call this area of research experimental petrology. After many careful experiments, experimental petrologists can construct phase diagrams. Like most scientific results, phase diagrams are refined or modified when experimental petrologists gain more information or conduct further experiments. Experimental petrology is expensive and painstaking work. Consequently, only a few laboratories are producing most of the best results today.
Which of the polymorphs are stable under different pressure-temperature conditions. To determine the stable polymorph at any P-T, draw a horizontal line at the pressure of interest and a vertical line at the temperature of interest. They intersect within a specific stability field, labeled with the stable mineral name. Although the temperature scale does not go below , at room temperature and pressure ( and1 atm) low quartz is stable. At the highest pressures, stishovite is stable because, as we shall see, it is the densest of the minerals. At the highest temperatures, melts.
Although P-T diagrams such as in Figure are often useful and informative, they only apply to specific chemical systems. Sometimes elemental substitutions in minerals cause significant changes in size and location of stability fields. Furthermore, phase diagrams only apply to mineral systems that stay in equilibrium, and many geological systems do not. For example, cristobalite, a mineral normally stable only at high temperature, sometimes crystallizes as chalcedony in low temperature sedimentary rocks.