Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2023 / Paper 1 / 7 / a / Solution

For pure phases at the reference pressure $P_0=1\mathrm{bar}$, the Gibbs energy of reaction is ${\Delta }_{r}G=\Delta H^{\circ }-T\Delta S^{\circ }$. Chemical equilibrium requires ${\Delta }_{r}G=0$, soThe reaction is muscovite dehydration to sanidine, corundum and water; its positive standard entropy of reaction makes the product assemblage favored at high temperature under this constant-property approximation.