Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2025 / Paper 3 / 10 / d / Solution

The stability boundary in a mineral activity diagram requires the two coexisting minerals to be stable against the third. For the forward dissolution directions of (I) and (II),A negative value favours corundum over the corresponding silicate mineral. For (III),so low silica activity favours nepheline over albite, and high silica activity favours albite. Combining these comparisons gives the stable portions:

The opposite extension of each line is metastable because the third mineral has lower thermodynamic potential there. All three minerals can coexist at the intersection in this fixed-temperature, fixed-pressure activity model.