Anelastic behaviour of leucite KAlSi₂O₆

An examination of the anelastic response of leucite, KAlSi₂O₆, to frequency modulated stress is presented. Leucite is a naturally occurring open framework aluminosilicate closely related to analcime (a zeolite compound) and is found in the earth's crust. It shows two phase transitions at (T_c1) 665 °C and (T_c2) 645 °C, from cubic Ia3d through I4₁/acd to I4₁/a. Consequently the low temperature I4₁/a phase contains both lamellar and merohedral twins; the intermediate I4₁/acd phase contains lamellar twins only. Leucites typically show a high density of twin walls. Recent studies have shown that lamellar microstructures can give rise to anelastic softening via twin wall motion [R.J. Harrison, S.A.T. Redfern, J. Appl. Phys. 95 (2004) 1706-1717; R.J. Harrison, S.A.T. Redfern, E.K.H. Salje, Phys. Rev. B 69 (2004) 144101-1-144101-10; R.J. Harrison, S.A.T. Redfern, J. Street, Am. Mineral. 88 (2003) 574-582]. This could be a mechanism for seismic attenuation in crustal and mantle rocks. We propose that this process also occurs in leucite on the basis of results of dynamical mechanical analysis. A frequency modulated stress (0.4-16 Hz) was applied in three point bend geometry to single crystals of leucite containing twins. The dynamic modulus and tan δ functions have been measured, though these are partially obscured by the presence of the higher temperature phase transition. The apparent activation energies for twin wall motion (600 kJ mol^-1 or more) are unusually high, suggesting possible dependence on silicon diffusion within the aluminosilicate framework. Softening of the modulus in the high-temperature cubic phase, on cooling towards the Ia3d to I4₁/acd transition, is consistent with coupling to optic modes which show strong dispersion with three phonon branches softening equally in all directions.