PHASE TRANSITIONS IN LAWSONITE: A CALORIMETRIC STUDY

Abstract

The specific heat of lawsonite, CaAl2Si2O7(OH)(2).H2O, has been measured in the temperature range 125 K - 325 K. An anomaly is seen at 273 K, which is related to the Cmcm - Pmcn phase transition. The magnitude of the total excess entropy associated with this transition is not reproducible, varying in the range 5.93 - 6.24 J K-1 mol(-1). On heating, the specific heat anomaly is consistent with a tricritical phase transition. However, on cooling, significant hysteresis is observed, and the form of the C-p anomaly is quite different. In all measurements extensive pre-transitional effects are observed above T-C. Analysis of existing specific heat data in the temperature range 75 K - 175 K shows an anomaly associated with the Pmcn - P2(1)cn phase transition. The excess entropy associated with this transition is 6 (1) J K-1 mol(-1). These data are interpreted as showing that both transitions are caused by the interaction of proton ordering and displacive changes in the aluminosilicate framework. The standard entropy of lawsonite at 298 K is recalculated, incorporating the effects of the two transitions. Two methods are used for this recalculation, giving values of S-298(0) = 233.27 and 234.96 JK(-1)mol(-1) respectively.