Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 8 / iii / Solution

For the reaction direction in part ii, pure solids and unit water activity give $K_c=1/a_{{\mathrm{H}}_4\mathrm{Si}{\mathrm{O}}_4^0}^2$. ThereforePlot a vertical line through the common intersection, independent of $y$. Increasing dissolved silica favors albite, consistently with its larger Si/Al ratio. The vertical line’s stable and metastable portions are selected by comparison with corundum, not by its own mass-action equation alone.