Ordering behaviour of the mineral lawsonite

Lawsonite crystals have been investigated by dilatometry and dynamic mechanical analysis as well as by measurements of the optical birefringence and the dielectric constant in the temperature range 90 K less than or equal to T less than or equal to 633 K. Two successive phase transitions have been found at low temperatures. The analysis of the spontaneous strain and the excess birefringence data reveals that the transition around 273 K (T-1) is tricritical. The other transition around 120 K (T-2) is of second order and proper ferroelectric. The mineral shows remarkable pretransitional effects in a temperature range up to 200 K above T-1. The data below T-2 are interpreted in terms of a mutual interference of the two different ordering processes associated with T-1 and T-2, respectively.