The coarsening kinetics of exsolution lamellae have been determined for an initial composition of 0.541 CaMgSi206-0.459 MgzSizO 6 (Di54.1) at 1300 ~ 1200 ~ and 1100 ~ C. Portions of this material were annealed for varying lengths of time, and the average wavelength (2) was obtained from measurements made by utilizing transmission electron microscopy (TEM). All experiments were conducted within the (001) coherent spinodal for this system (McCallister and Yund, 1977), and the lamellae are coherent up to and including the largest wavelength observed, 1054A.
At all three temperatures mentioned previously the data are consistent with the following relationship:
2(A)=)~o(A)+kt 1/3
The activation energy for the process, as determined from the respective kinetic constants (k), is 99 + 2 Kcal/mol. This value is similar to that found for Ca self diffusion in pseudo-wollastonite and wollastonite, and suggests the possibility that the diffusion of Ca may be controlling the rate of coarsening.
A comparison of exsolution microstructures in the synthetic samples with those observed in clinopyroxenes found in kimberlites and with similar Ca/ Ca+Mg indicates that the latter cooled rapidly to the effective quench temperature.