Correctly formulated thermodynamics provides the only reliable means of applying the results of mineral equilibria experiments to the calculation of the temperature and pressure of formation of mineral assemblages. The problems of formulating geothermometers and geobarometers are considered with particular reference to calculating the conditions of formation of garnet lherzolites in kimberlites. Incorrect expressions for the thermodynamics of minerals will provide unreliable methods for calculating conditions of formation. The reactions on which these methods are based must be chosen so that composition parameters which are uncertain in a mineral analysis (for example, the amounts of ferrous iron on each octahedral site in a pyroxene) do not have a central position in a method. For this reason it is suggested that the use of alumina solubility in enstatite in equilibrium with garnet is not suitable as a geobarometer for garnet lherzolites. The orthopyroxene-Ca-rich clinopyroxene miscibility gap geothermometer is shown to be very sensitive to the mineral compositions when more correctly formulated, treating the pyroxenes as non-ideal solutions. Two new methods are presented which are based on Ca-Mg exchange reactions involving orthopyroxene-Ca-rich clinopyroxene and orthopyroxene-garnet. These methods allow the calculation of both temperature and pressure for garnet lherzolites. It is observed that the sheared lherzolites and the granular lherzolites formed at lower pressures than has been suggested previously, implying a steeper geotherm at the time the minerals attained their present compositions.