Coherent exsolution in Fe-free pyroxenes Available to Purchase

Mg-rich diopsides on the Mg₂Si₂O₆-CaMgSi₂O_e join were synthesized from glasses or gels and subsequently annealed at atmospheric pressure between 1000° and 1350°С for varying lengths of time. A marked change in the exsolution kinetics was observed at 1325°C for Di_54.1 and at 1275°C for Di_63.3. Below these temperatures the respective compositions exsolve rapidly (in one day or less), and the resulting microstructure, observed by transmission electron microscopy, consists of closely-spaced, irregular lamellae parallel to (001). Above 1325°C (for Di_54.1) and 1275°C (for Di_63.3) no exsolution was observed after equivalent or longer annealing times. In addition, no exsolution occurred for a composition of Di_71.5 annealed at temperatures as low as 1000°C for 144 hours.

A microstructure consisting of straight widely-spaced lamellae, also parallel to (001), was observed in long-term runs of Di_68.4 annealed at 1125°C for several hundred hours; however, the method of synthesis was different from that used for the other compositions and may have introduced chemical inhomogeneities into the starting material. In all cases the exsolved phase is perfectly coherent with the host, but inverts to low pigeonite during cooling. Because of the coherency between the phases, the exsolution must occur beneath the coherent solvus, and the sharp break in the kinetics is attributed to the (001) coherent spinodal.