Isothermal annealing and continuous cooling experiments have been performed on synthetic orthopyroxene crystals with compositions En81Fs19 and En49Fs51. Their intracrystalline Fe2+, Mg-partitioning was determined by X-ray structure analysis. En81Fs19 was equilibrated at 550°C, 675°C and 850°C, En49Fs51 at 550°C, 650°C, 675°C, 800°C, and 850°C. The Fe2+, Mg distribution coefficients vary with temperature according to  
\[\mathrm{In(K_{D})}\ =\ 0.633(31)\ {-}\ 2625(31)/\mathrm{T[K]\ (En_{81}Fs_{19})}\]
\[\mathrm{In(K_{D})}\ =\ 0.594(52)\ {-}\ 2581(51)/\mathrm{T[K]\ (En_{49}Fs_{51})},\]
suggesting that in this compositional range In(KD) values do not significantly depend on the Fs content. Rate constants were determined in ordering runs at 550°C:  
\[\mathrm{k}_{\mathrm{dis}}\ =\ 0.00165(37)\ \mathrm{[h^{{-}1}]\ (En_{81}Fs_{19})}\]
\[\mathrm{k}_{\mathrm{dis}}\ =\ 0.0080(18)\ \mathrm{[h^{{-}1}]\ (En_{49}Fs_{51})}.\]
The results closely agree with predictions from Arrhenius equations given by Ganguly & Tazzoli (1994) and Kroll et al. (1997).

Non-linear continuous cooling experiments were performed on both orthopyroxenes to test the ability of the Mueller rate equation (Mueller, 1967, 1969) to correctly predict the evolution of ordering. The crystals were cooled from 850°C to 250°C at an average rate of 13°C/day. The frozen site occupancies could be fully reproduced by the calculated ordering paths when the Mueller equation was run with the temperature dependencies of KD given above and Arrhenius parameters for kdis, taken from Kroll et al. (1997) and slightly adjusted within their error limits.

Site refinements were also performed under the assumption that an error of ±1% total Fe had occurred in the microanalysis. This resulted in In (KD) lines which almost coincide for En49Fs51, but are clearly different for En81Fs19. Consequently, the error of calculated cooling rates is negligible for En49Fs51, but becomes significant for En81Fs19.

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