AbstractRare layers of an aluminous, muscovite‐rich rock from the Lewisian Complex at Stoer, North‐West Scotland, display evidence which suggests that the rock has undergone local partial melting to form quartz‐bearing veins and a corundum‐bearing restite. The assemblages observed in these rocks match those predicted by modelling in the system KAlO2‐NaAlO2‐Al2O3‐SiO2‐H2O (KNASH) where certain bulk compositions melt peritectically to give corundum‐bearing restites and quartz‐normative melts. Study of the model system shows that the observed parageneses could have formed from a range of bulk compositions with a variety of possible values ofaH2O which could have been internally or externally buffered. The KNASH petrogenetic grid, together with another in the system CaO‐Na2O‐FeO‐Al2O3‐SiO2‐H2O (CNFASH), allows theP–Tpath of the rocks to be delineated and an estimate to be made of the conditions at the peak of metamorphism as>11 kbar and 900‐925°C. This estimate is in agreement withP–Testimates using thermobarometric methods on adjacent lithologies: The activity of H2O in the system throughout metamor