Geologists are detectives. They look for clues to past events and use their knowledge and reasoning to interpret those clues. They argue for their conclusions to a jury of peers by presenting the evidence they have assembled during their investigations. Like detectives Sherlock Holmes, Nancy Drew, and Precious Ramotswe, geologists have experience and analytical tools that help them see beyond the raw data and solve difficult cases.
Because igneous and metamorphic rocks form at high temperatures, commonly over long periods of time, they are more likely to have approached chemical equilibrium during their history than sedimentary rocks. The possibility of chemical equilibrium adds the laws of thermodynamics to the detective toolkit of petrologists and gives them permission to use the results of laboratory experiments as evidence when interpreting the history of rocks. In this chapter, some easy-to-use principles from that chemical toolkit are explained, including equilibrium mineral assemblages, their graphical representation, and reaction possibilities.
Phases
Chemical equilibrium is defined in terms of phases, physically homogeneous substances, such as minerals, magmatic liquids, hydrous liquids, and gases. To understand some of the important features of phases and chemical equilibrium, it is useful to start by thinking about water, a familiar phase. Figure 01 shows the temperature (T) - pressure (P) stability (at equilibrium) of three phases of H2O: water, ice, and vapor (steam). Click on the diagram to see a larger version and then click the "Show 1 atm" button to highlight the freezing and boiling temperatures at 1 atmosphere pressure. Observe that only one of the three phases is stable at most T-P values. If a random T-P point is selected (throw a dart at the diagram), it is likely that just one H2O phase will be stable. Only along the black curves separating the one-phase regions are two phases stable. The black curves mark the equilibrium conditions for the reactions
- (H2O)ice = (H2O)water
- (H2O)ice = (H2O)vapor
- (H2O)water = (H2O)vapor
How do the stabilities of H2O phases help us understand igneous and metamorphic rocks?