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Ordering and elasticity associated with low-temperature phase transitions in lawsonite
-
P. Sondergeld
,
W. Schranz
Veröffentlicht/Copyright:
28. März 2015
Abstract
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.
Received: 2003-1-10
Accepted: 2004-7-23
Published Online: 2015-3-28
Published in Print: 2005-2-1
© 2015 by Walter de Gruyter Berlin/Boston
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- Igneous thermometers and barometers based on plagioclase + liquid equilibria: Tests of some existing models and new calibrations
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- Experimental data on the Tschermak substitution in Fe-chlorite
- Nanometer-sized, divalent-Mn, hydrous silicate domains in geothermal brine precipitates
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