Experimental studies of sodic microperthites from the Loch Ailsh syenite

Solvus curves under both ‘dry’ and ‘hydrothermal’ (P_H2O = 1 kbar) conditions have been determined for two microperthitic alkali feldspar starting materials from the Loch Ailsh syenite complex, Assynt, Scotland. One sample (Ab₇₄ Or₂₄ An₂) possessed a K-phase showing dominant monoclinic symmetry to both powder and single-crystal X-ray methods, the other (Ab₈₈Or₃₀An₂) had maximum microcline as K-phase. These materials gave essentially identical solvi for both types of experiment, interpreted as indicating similar degrees of local order in the K-phase, but with differing domain size, leading to the different X-ray properties. The ‘dry’ solvus curves confirm the determination of the microcline-low albite solvus of Bachinski & Müller. The hydrothermal curves are used to estimate P_H1O <3.5 kbars, T > 755 °C for emplacement of the Loch Ailsh syenites.