The phase transitions that we discussed in Section 6.6 are all discontinuous phase transitions. They are step-wise changes in the structure of matter, for instance, the destruction of the crystalline structure during melting or sublimation, or the breakdown of molecular bonds in a liquid during boiling. These are microscopic changes that are accompanied by a macroscopic exchange of heat with the environment, what we call the enthalpy of transition (melting, vaporization, etc.), or also “latent heat”. There is another type of phase transition, which takes place without there being a discontinuity either in the microscopic structure of a substance or in its macroscopic properties, and during which there is no energy exchange with the environment. Such phase transitions are called continuous or critical phase transitions, and play important roles in many planetary processes. For example, they underlie exsolution phenomena such as are observed in feldspars, pyroxenes, oxides and meteoritic metal, hydrogen–helium unmixing in fluid planets and liquid immiscibility phenomena in magmatic systems. They also explain order–disorder transformations in crystalline substances. Critical phase transitions also play an important role in the study of fluids (Chapter 9).

An intuitive approach to critical phase transitions

Consider a binary solution between components A and B with unit site multiplicity.