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

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