Abstract

The kinetics of the Fe-Mg intracrystalline exchange reaction in P21/c pigeonite (Wo10En47Fs43) free of exsolved augite, from the Paraná rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe2+. The linear regression of ln kD* vs. 1/T yielded the following equation:

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