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

Use the pressure correction to a mineral reaction for the solids, and the gas thermodynamic activity $f/P_0$ for water. These are separate contributions: do not also add a gas-volume pressure correction after including $RT\ln (f/P_0)$. For this reaction,At $7000\mathrm{bar}$ with pure solids and an ideal gas, $f=P$ and the solid-activity ratio is one. ThereforeandThe pressure unit in the solid term is bar, matching the supplied $\mathrm{J}\mathrm{ba}{\mathrm{r}}^{-1}$. The gas logarithm is dimensionless because $P/P_0=7000$. High water chemical fugacity opposes dehydration and raises the equilibrium temperature substantially.