Detective image
Detective image

Equilibrium Assemblages

4.1 Overview

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.

4.2 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.
H2O phase diagram

Figure 4.01. H2O phase diagram. Click on the image to see a larger version.

Figure 4.01 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
The reactions are simple to write and simple to balance: 1 (H2O)ice = 1 (H2O)water. Equilibrium conditions for these reactions are well known because of many experiments, including some by geologists (Burnham and others, 1969a,b ). It is also known from these experiments that it may take time to achieve equilibrium. Water can be cooled below the ice-water reaction curve ("super cooled") without immediately reacting to form ice (see Discover Magazine video) or heated above the water-vapor reaction curve ("super heated") without immediately reacting to form vapor (see video by Discovery UK). Kinetics such as nucleation will need to be considered when using equilibrium tools to understand relations among phases.

How do the stabilities of H2O phases help us understand igneous and metamorphic rocks?