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Ordering and elasticity associated with low-temperature phase transitions in lawsonite

¹ Institut für Experimentalphysik, Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria
² Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
³ Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria
⁴ Institute for Computer Science, Faculty of Electrical Engineering, Technical University of Czestochowa, Armii Krajowej 17, 42-200 Czestochowa, Poland
⁵ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.

Correspondence: ^* E-mail: Peter.Sondergeld{at}esc.cam.ac.uk

The two low-temperature phase transitions of lawsonite have been studied using single-crystal X-ray diffraction from 86 to 318 K and a single-crystal high-frequency continuous-wave resonance technique from 323 to 102 K. While recently published data of the variations of strains, birefringence, and IR line widths are consistent with the (271 K) Cmcm-Pmcn transition being simply tricritical, our investigation of critical X-ray reflections and the six diagonal elastic constants of lawsonite reveals, consistently, a more complex crossover pattern in the temperature range of 205–225 K. Below 205 K the overall pattern is again in good agreement with a tricritical solution of the Cmcm-Pmcn transition and a second-order behavior of the (120 K) Pmcn-P2₁cn transition. The structure determination from single-crystal X-ray data at 215 K reveals a possible orientational disorder of some of the hydroxyl groups in the Pmcn phase. From this and a recent strain analysis of deuterated and hydrogenated lawsonite we conclude that down to 205 K the Cmcm-Pmcn transition is driven by a displacive component, as observed in strain and birefringence data, plus an order/disorder component or dynamical effects associated with proton ordering. Below 205 K only the displacive component plays a role, and the (120 K) Pmcn-P2₁cn transition is driven by a single order parameter. The remarkable elastic softening of C₆₆ ahead of the Cmcm-Pmcn transition indicates another orthorhombic-monoclinic transition, which is suppressed on cooling through the low-temperature phase sequence Cmcm-Pmcn-P2₁cn, but can be observed on applying pressure to the mineral.

This article has been cited by other articles:

				R. E.A. McKnight, M. A. Carpenter, T. W. Darling, A. Buckley, and P. A. Taylor Acoustic dissipation associated with phase transitions in lawsonite, CaAl2Si2O7(OH)2{middle dot}H2O American Mineralogist, October 1, 2007; 92(10): 1665 - 1672. [Abstract] [Full Text] [PDF]

				M. A. Carpenter Elastic properties of minerals and the influence of phase transitions American Mineralogist, February 1, 2006; 91(2-3): 229 - 246. [Abstract] [Full Text] [PDF]