Phase diagrams map the stability fields of minerals and mineral assemblages as functions of temperature, pressure, and composition. Each region of the diagram represents conditions where a specific mineral or assemblage has the lowest Gibbs free energy and is therefore thermodynamically stable. Boundaries between fields represent conditions where two assemblages coexist in equilibrium (univariant reactions). The Clausius-Clapeyron equation (dP/dT = delta-S/delta-V) governs the slope of these boundaries. Phase diagrams are the primary tool for interpreting metamorphic grade, predicting magmatic crystallization sequences, and understanding the mineralogical structure of Earth's interior.
Phase diagrams are to geochemists what circuit diagrams are to electrical engineers -- they encode the fundamental constraints governing the system's behavior into a visual map. Reading a phase diagram means understanding what minerals exist under what conditions and what happens when conditions change.
The thermodynamic foundation is simple: at any given T-P-X condition, the stable assemblage is the one with the lowest total Gibbs free energy. Phase boundaries are the T-P loci where two assemblages have equal free energy. The Gibbs phase rule (F = C - P + 2) determines how many intensive variables (T, P, composition) can be independently varied while maintaining the observed assemblage. A divariant field (F=2) allows both T and P to change freely. A univariant line (F=1) constrains one variable once the other is fixed. An invariant point (F=0) fixes both T and P -- these are the diagnostic triple points or reaction intersections that calibrate geothermometers and geobarometers.
P-T diagrams are most commonly used for metamorphic petrology. A rock's mineral assemblage records the P-T conditions where it last equilibrated, and the sequence of mineral reactions (preserved as inclusion textures, reaction rims, and pseudomorphs) traces the rock's P-T path through time. Clockwise P-T paths (burial, heating, then exhumation) characterize collision zones; counterclockwise paths characterize contact metamorphism. These P-T paths, reconstructed from phase diagrams, constrain tectonic models.
T-X (temperature-composition) diagrams govern igneous crystallization. Binary and ternary phase diagrams predict the sequence of minerals that crystallize from a cooling magma: which mineral appears first, how compositions evolve with cooling (fractional crystallization), and what happens at eutectic and peritectic points. The lever rule quantifies the proportions of solid and liquid at any temperature. These diagrams explain why basalts and granites have characteristic mineral assemblages and why fractional crystallization produces compositional diversity in igneous suites.