In the aureole of the late Caledonian Omey granite corundum develops in the Dalradian country rocks in a zone up to 250 m from the granite contact. The distribution of andalusite and K-feldspar in pelites and calcite+wollastonite+grossularite in marbles is consistent with inner-aureole metamorphic conditions of 615±25°C at 2.5±0.25 kbar, and XH2Ofluid⋍0.85. Corundum develops from the reaction muscovite → corundum + K-feldspar+H₂O and first appears over 100 m further from the granite than the assemblage wollastonite + grossularite + anorthite. Experimentally determined equilibria can be satisfied only if XH2Ofluid for the corundum-producing reaction was less than 0.6 and perhaps as low as 0.4. Corundum always grows within large muscovite crystals;f_H2O within the crystal lattice is unrelated to that in the grain-boundary fluid of the surrounding rock.

Although whole-rock oxidation ratios are irregularly distributed within the aureole they are uniformly low in corundum-bearing rocks. Reducing conditions probably resulted from localized flow of H₂O-rich fluid away from the granite in a diffuse channelway that contains most of the corundum localities and also a distinctive skarn. Although corundum growth within muscovite is sealed off from the external water vapour conditions, it is suggested that movement of H₂O down a thermal gradient (and hence down an f_H2O gradient at constant pressure) promotes the escape of (OH)⁻ from the muscovite lattice and so allows the corundum reaction to proceed.