Acoustic attenuation due to transformation twins in CaCl2: Analogue behaviour for stishovite

CaCl2 undergoes a tetragonal (P42/mnm) to orthorhombic (Pnnm) transition as a function of temperature which is essentially the same as occurs in stishovite at high pressures. It can therefore be used as a convenient analogue material for experimental studies. In order to investigate variations in elastic properties associated with the transition and possible anelastic loss behaviour related to the mobility of ferroelastic twin walls in the orthorhombic phase, the transition in polycrystalline CaCl2 has been examined using resonant ultrasound spectroscopy (RUS) at high frequencies (0.1–1.5 MHz) in the temperature interval 7–626 K, and dynamic mechanical analysis (DMA) at low frequencies (0.1–50 Hz) in the temperature interval 378–771 K. RUS data show steep softening of the shear modulus as the transition temperature is approached from above and substantial acoustic dissipation in the stability field of the orthorhombic structure. DMA data show softening of the storage modulus, which continues through to a minimum ∼20 K below the transition point and is followed by stiffening with further lowering of temperature. There is no obvious acoustic dissipation associated with the transition, as measured by tan δ, however. The elastic softening and stiffening matches the pattern expected for a pseudoproper ferroelastic transition as predicted elsewhere. Acoustic loss behaviour at high frequencies fits with the pattern of behaviour expected for a twin wall loss mechanism but with relaxation times in the vicinity of ∼10−6 s. With such short relaxation times, the shear modulus of CaCl2 at frequencies corresponding to seismic frequencies would include relaxations of the twin walls and is therefore likely to be significantly lower than the intrinsic shear modulus. If these characteristics apply also to twin wall mobility in stishovite, the seismic signature of the orthorhombic phase should be an unusually soft shear modulus but with no increase in attenuation.

► Tetragonal (P42/mnm) to orthorhombic (Pnnm) transition in CaCl2.
► Elastic anomalies and acoustic dissipation by RUS at ∼1 MHz and DMA at ∼1 Hz.
► Softening of modulus: at both high and low frequencies.
► Acoustic dissipation: only at high frequencies, due to twin wall movement.
► Analogue behaviour for stishovite in the interior of the Earth.