Martín Olalla, José MaríaHayward, Stuart A.Meyer, Hinrich-WilhemRamos Vicente, Saturiodel Cerro Gonzalez, JaimeCarpenter, Michael A.2024-06-132024-06-132001Martín Olalla, J.M., Hayward, S.A., Meyer, H., Ramos Vicente, S., del Cerro Gonzalez, J. y Carpenter, M.A. (2001). Phase transitions in lawsonite: a calorimetric study. European Journal of Minearology, 13 (1), 5-14. https://doi.org/10.1127/0935-1221/01/0013-0005.0935-12211617-4011https://hdl.handle.net/11441/160451The specific heat of lawsonite, \ce{CaAl2Si2O7(OH)2.H2O}, has been measured in the temperature range $[\SI{125}{\kelvin},\SI{325}{\kelvin}]$. An anomaly is seen at \SI{273}{\kelvin}, which is related to the $\mathrm{Cmcm} – \mathrm{Pmcn}$ phase transition. The magnitude of the total excess entropy associated with this transition is not reproducible, varying in the range $[\SI{5.93}{\joule\per\kelvin\per\mole},\SI{6.24}{\joule\per\kelvin\per\mole}]$. 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 $[\SI{75}{\kelvin},\SI{175}{\kelvin}]$ shows an anomaly associated with the $\mathrm{Pmcn} – \mathrm{P2_1cn}$ phase transition. The excess entropy associated with this transition is \SI{6(1)}{\joule\per\kelvin\per\mole}. 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 $S^0_{298} = \SI{233.27}{\joule\per\kelvin\per\mole}$ and and $S^0_{298} = \SI{234.96}{\joule\per\kelvin\per\mole}$ respectively.application/pdf10 p.englawsonitephase transitionsH orderingcalorimetryinfo:eu-repo/semantics/articleinfo:eu-repo/semantics/openAccesshttps://doi.org/10.1127/0935-1221/01/0013-0005