The importance of mineralogical thermodynamics grew with increasing knowledge of chemical and physical properties of rocks during the last four decades. This book attempts to introduce beginners to a thermodynamic treatment of equilibria between minerals and fluids, offering practical aspects by worked examples. This subject is developed in a step-by-step manner, commencing with the simple case of equilibria between end-member minerals plus or minus a pure fluid, ending up with the solid-solid and solid-fluid equilibria in multicomponent systems. The emphasis throughout is on practical computation of equilibria. In each case, the algorithm required for the calculations is given, along with some explicit worked examples. Most of these are suitable for class work or for self-study.