Natural orthopyroxene crystals containing augite exsolution lamellae were heated at one bar and controlled oxygen fugacity to determine the temperature at which orthopyroxene + augite first react to form pigeonite ± melt. Pigeonite was detected by X-ray precession photography. The reaction to form pigeonite is initiated at the orthopyroxene–augite boundary, probably on the augite lamellae, which are structurally similar to pigeonite and where the orthopyroxene and augite are in chemical equilibrium. The upper temperature limit for the minimum stability temperature of pigeonite at one bar, determined from heating experiments on natural pyroxenes, varies from 1284°C at Mg/(Mg + ΣFe) = 0.92 (atomic), to 901°C and 0.04. Orthopyroxene which does not contain augite exsolution lamellae, and remains unsaturated with respect to augite component, transforms to pigeonite at the higher temperatures of the orthopyroxene–pigeonite stability field. Partial melting accompanies formation of pigeonite for orthopyroxene compositions more magnesian than Mg/(Fe+Mg) = 0.55 (atomic).

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