Ordering behaviour of the mineral lawsonite

Lawsonite crystals have been investigated by dilatometry and dynamic mechan ical analysis as well as by measurements of the optical birefringence and t he 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 f ound at low temperatures. The analysis of the spontaneous strain and the ex cess birefringence data reveals that the transition around 273 K (T-1) is t ricritical. 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 interp reted in terms of a mutual interference of the two different ordering proce sses associated with T-1 and T-2, respectively.