HR: 1340h
AN: MR13C-1401    [Abstracts]
TI: Anelasticity Arising From Intra-crystalline Cation Exchange in Spinel
AU: * Walsh, J N
EM: jww31@cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences University of Cambridge Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Redfern, S A
EM: satr@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences University of Cambridge Downing Street, Cambridge, CB2 3EQ, United Kingdom
AB: The dynamic Young's modulus and anelastic properties of spinel have been measured by forced pendulum techniques as a function of frequency and temperature between room temperature and 1400 K and at frequencies between 10 Hz and 0.01 Hz. In this temperature range it is known that MgAl2O4 spinel displays variations in Mg-Al order disorder across the octahedral and tetrahedral sites of the structure, with increasing disorder towards an inverse spinel arrangement of cations at the highest temperatures of our experiments [1]. Our measurements of the mechanical properties across the same temperature interval of interest reveal a strong variation in the phase angle difference (?) between applied strain and stress, which corresponds to a Debye-like peak in the internal friction or anelastic attenuation, Q-1. This peak in the high temperature internal friction (IF) spectrum occurs at ~ 1080 K when the stress is cycled at 2 Hz. The peak appears to show no hysteresis between heating and cooling cycles and moves to higher temperatures with increasing frequency; consistent with a thermally activated process associated with point defect movements. Since the point defects must act as an elastic dipole, and the temperature the maximum in resonance occurs is at such an elevated temperature, this peak may be attributed to Mg-Al exchange between the tetrahedral and octahedral sites of the structure. Indeed, the temperature at which the resonance peak appears corresponds to the same temperature range that cation relaxation was observed in the kinetic experiments given in [1]. This is the first such observation of cation- exchange-related anelastic attenuation in a silicate, and points to the possibility of cation ordering significantly modifying the anelastic properties of minerals at high temperatures. [1] S A T Redfern, R J Harrison, H StC O'Neill, D R R Wood (1999) Thermodynamics and kinetics of cation ordering in MgAl2O4 spinel up to 1600 °C from in situ neutron diffraction. Am Mineral 84: 299–310
DE: 3904 Defects
DE: 3909 Elasticity and anelasticity
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting