Abstract: Mesoperthite feldspars from hypersolvus granites, with granulite-facies mineralogy from the Scourian complex, N.W. Scotland have an average bulk composition of (Ab_0.58Or_0.42)₈₈An₁₂. The mesoperthites have an Ab/Or ratio close to the critical composition of the alkali feldspar solvus for anorthite-free compositions. The critical temperature of the ternary feldspar solvus is estimated at high anorthite contents and used to indicate a temperature in excess of 1000°C for the crystallization of the granite sheets prior to granulite-facies metamorphism and permits a maximum water content in the melt of 1%.