Spontaneous strain below the {Mathematical expression} transition in anorthite at pressure

The phase transition between the {Mathematical expression} and {Mathematical expression} phases of anorthite has been studied at elevated pressure by single-crystal X-ray diffraction in a diamond-anvil cell. The phase transition is shown to be first-order in character for both end-member anorthite (CaAl₂Si₂O₈) and for an anorthite with a small amount of albite component (NaAlSi₃O₈) in solid solution. Reversals of the transition across the phase boundary at three other compositions show that the transition pressure (P_Tr) increases with increasing albite content. This behaviour is compared with that observed at elevated temperatures, and is analysed in terms of Landau theory.