Single-crystal X-ray techniques were used to study isothermal Fe2+-Mg disordering in two orthopyroxene samples, Opx 4 (Fs61) and Opx 5 (Fs85), at temperatures of 625, 675, 725 °C and 525, 575, and 625 °C, respectively. Heating was continued until cation distributions reached steady state, which was considered equilibrium. The structure data obtained correlate very well with all other work on orthopyroxene, including natural (heated and unheated) crystals and synthetic end-members. These crystallographic variations also provide an internal constraint on the disordenng process.

Instability of the orthopyroxene crystal structure was noted for some crystals where heating was continued after cation distribution equilibrium was attained. This was indicated by a decrease in Fe2+ in the M2 site, with no corresponding increase in Fe2+ in the M1 site. In addition, there was a small decrease in the size of the M1 site, which is thought to be caused by vacancy introduction in the M2 site, with Fe2+-Fe3+ substitution in the M1 site and the Fe sublimating during heating.

The Mueller chemical rate law was used to calculate rate constants of 4.70 × 109 and (1.97 × 109)/min and activation energies of 47.1 and 41.3 kcal/mol for Opx 4 and Opx 5, respectively. From these results, together with values in the literature, we have shown that a distinct step is found at Fs50–Fs60 in the activation energies, with higher activation energies of approximately 60 kcal/mol for Mg-rich samples and lower activation energies of approximately 50 kcal/mol for the Fe-rich samples. The rate constants of Opx 4 and Opx 5 are comparable with previously determined disordering rates.

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