The kinetics of the disordering process in Ca-rich orthopyroxenes was studied. The substitution of Ca for Mg and Fe in the orthopyroxene structure was investigated in order to evaluate the influence of this cation on the disordering process.
Geometrical parameters obtained by X-ray single-crystal diffraction on different Ca-rich orthopyroxenes showed that Ca enters the octahedral M2 site of orthopyroxene, causing an enlargement of this polyhedron. As a consequence, both tetrahedral chains extend to allow matching between the tetrahedral and octahedral layers.
The kinetic study was carried out at T = 730°C on an orthopyroxene from the volcanic rock sample L3. This orthopyroxene, with composition Wo4En60Fs36, contains thin exsolved augite lamellae but no Guinier-Preston zones. A series of isothermal annealing runs were performed on a single crystal in the presence of IW buffer followed by quenching. The ordering degree after each annealing run was determined by X-ray single-crystal diffraction. Analysis of the kinetic data using Mueller's (1967) theory yielded a disordering rate constant of 2.69(8)·10-2 min-1. This value is in perfect agreement with that expected for a “normal” kinetic behaviour. The presence of Ca and augite lamellae in the orthopyroxene matrix do not seem to be responsible for the drastic change in the activation energy of the Fe-Mg exchange process observed in the Ca-rich orthopyroxene from the Johnstown meteorite.