Resonant ultrasound spectra of a single crystal and a polycrystalline sample of lawsonite [CaAl2Si2O7(OH)(2)center dot 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 Q(QF), which are indicative of dissipation. Two dissipation peaks, at similar to 250 and similar to 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 Cmcm <-> Pmcn 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 Pmcn <-> P2(1)cn transition point. The combination of these anomalies in acoustic dissipation and in elastic constants is consistent with the view that the Cmcm <-> Pmcn transition is driven both by displacive and proton ordering effects. For the Pmcn <-> P2(1)cn transition, dissipation and the transition are more closely related, consistent with the view that the transition is driven essentially by proton ordering.
