For the first time, non-linear continuous cooling experiments were performed in which the crystals were cooled from 850°C to 250°C at an average rate of 10°C/day. The Fe2+,Mg distributions were determined after the crystals had reached 650°C, 550°C, 450°C, 350°C, and 250°C, respectively. Using the Mueller rate equation (Ganguly, 1982) and employing the temperature dependencies of KD and kdis as given above, the experimentally delineated ordering path is closely reproduced by the calculated path.
However, due to the large activation energy, cooling rates calculated for the untreated crystals turn out to be physically unreasonable, i.e. some 10−5 K/My. By contrast, Arrhenius parameters determined in the literature on orthopyroxenes with compositions similar to the Johnstown crystals produce physically reasonable rates of some hundred K/My. TEM studies do not show a significant difference between the microtextures of untreated and annealed samples. All orthopyroxenes studied contain clinopyroxene exsolution lamellae and abundant Guinier-Preston zones.
At present, we can neither prove nor disprove the concept that the large activation energy of the Johnstown orthopyroxenes is related to their intricate exsolution microtexture.