EXSOLUTION AND COARSENING IN IRON-FREE CLINOPYROXENE DURING ISOTHERMAL ANNEALING

Abstract

Exsolution and coarsening in Fe-free clinopyroxene of composition En45.1Di54.9 was studied at temperatures of 1300, 1200, and 1100°C, and annealing times between 10 min and 4320 h. Based on the wavelength of the exsolution lamellae, the exsolution process can be divided into exsolution sensu stricto and coarsening. During exsolution sensu stricto, the average wavelength of the “001” and “100” lamellae remains constant in contrast to the subsequent coarsening process. A progressive development of the microstructure is observed before coarsening which includes formation of island-like regions of dark contrast without phase separation, occurrence of pigeonite and diopside domains predominantly elongated along (100), formation of “100” exsolution lamellae, dissolution of the “100” lamellae, and predominant occurrence of “001” exsolution lamellae. All observations are in accordance with the exsolution mechanism of spinodal decomposition. After termination of the exsolution process, coarsening of the exsolution lamellae is observed. The “001” lamellae coarsen according to the rate law with λt the average wavelength at time t, λ0 the average wavelength at time t0, ΔH an activation energy, R the gas constant, T the temperature [K], and n and k empirical constants.From a multiple regression analysis the following values of the constants (± standard error) are obtained: n = 2.86 ± 0.15, k = 1.29*1014 ± 3.41*108 [nm2.86/h], and ΔH = 80.34 ± 1.76 [kcal/mol]. The initial wavelength λ0 was found to increase with increasing temperature (11.0 nm at 1100°C; 17.5 nm at 1200°C; 20.7 nm at 1300°C). The time t0 to complete exsolution decreases with increasing temperature (60 h at 1100°C; 18 h at 1200°C; 8 h at 1300°C). The exponent n is within the error limits equal to 3, indicating volume diffusion as rate-controlling process. Coarsening of the exsolution lamellae was found to be substantially faster than reported in previous studies.