The single-crystal X-ray technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P21/c pigeonite. Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo6En76Fs18) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308-2 (Wo10En47Fs43) from the BTS308 Paranà rhyodacite, were annealed at temperatures ranging from 600 to 1000 °C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2. The two geothermometric equations calculated by linear regression of ln KD* vs. 1/T are:

\[ln\ \mathit{K}_{D}{\ast}\ =\ {-}3291({\pm}\ 269)/\mathit{T}(K)\ +\ 0.971({\pm}0.253);\ (\mathit{r}^{2}\ =\ 0.974)\]
\[ln\ \mathit{K}_{D}{\ast}\ =\ {-}2816({\pm}\ 83)/\mathit{T}(K)\ +\ 0.542({\pm}0.083);\ (\mathit{r}^{2}\ =\ 0.995)\]

respectively for PCA82506-3 (XFe* = 0.20) and BTS308-2 (XFe* = 0.49) [XFe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo10.

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