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Phase transitions in lawsonite

¹ Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, Apartado 1065, E-41080 Sevilla, Spain
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom

^* email: olalla{at}cica.es

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 — P2₁cn 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 S⁰₂₉₈ = 233.27 and 234.96 JK^–1mol^–1 respectively.

Key-words: lawsonite, phase transitions, H ordering, calorimetry.

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