Resonant ultrasound spectra of a single crystal and a polycrystalline sample of lawsonite [CaAl2Si2O7(OH)2·H2O] have been measured at room temperature and at low temperatures in the region 20–300 K. The influence of known phase transitions at 125 and 270 K is seen in the frequency variations of the resonance peaks, which are indicative of elastic stiffening, and in values for the quality factor QQF, which are indicative of dissipation. Two dissipation peaks, at ~250 and ~210 K, are interpreted as being due to the proton order-disorder processes associated with the two species of hydrogen atoms in the structure: one in hydroxyl OH groups and one in the H2O molecules. These occur below the CmcmPmcn transition point but coincide with changes in the shear elastic constants and in features of IR spectra reported elsewhere. A third, much smaller, dissipation peak occurs immediately below the PmcnP21cn transition point. The combination of these anomalies in acoustic dissipation and in elastic constants is consistent with the view that the CmcmPmcn transition is driven both by displacive and proton ordering effects. For the PmcnP21cn transition, dissipation and the transition are more closely related, consistent with the view that the transition is driven essentially by proton ordering.

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