Do exsolutions influence the kinetics of the order-disorder reaction in orthopyroxene?

A series of isothermal annealing experiments at 750 degrees C has been carr ied out on an orthopyroxene single crystal from Johnstown diogenite contain ing (100) exsolved augite lamellae and GP zones. After each annealing exper iment the Fe-Mg ordering degree has been measured by X-ray single-crystal d iffraction. The disordering rate constant, calculated using Mueller's equat ion (Mineralogical Society American Special Paper, 2, 83-93, 1969.), result ed to be K+ = 0.00116 min(-1). This value is much lower than that expected on the basis of the relation derived by Ganguly and Tazzoli (American Miner alogist, 79, 930-937, 1994.) using ail the previous data available in liter ature. This suggests an influence of exsolutions on the kinetics of the Fe- Mg exchange reaction in orthopyroxene.