The melting and breakdown reactions of anorthite at high pressures and temperatures

The reaction 3 CaAl₂Si₂O₈⇆Ca₃Al₂Si₃O₁₂ + 2 Al₂SiO₅ + SiO₂, important to geobarometry, has been re-investigated in solid-media pressure apparatus, using very small quantities of oxalic acid dihydrate as a flux and improved pressure calibration based on runs with NaCl pressure medium at 1100° and 1150°C. The reaction was reversed at 7 temperatures between 1100° and 1400°C. The equation for the curve is P = −2.1 + 0.0232 T (P = kbar, T = °C), and the slope is 23.2 bars/°C. The curve is in good agreement with calculations using recent thermochemical data.

The (anhydrous) melting of anorthite was also investigated, and is congruent to 9 kbar where a maximum temperature of 1570°C is reached and corundum appears with the melt. With increasing pressure corundum continues to develop and the melting curve assumes an increasingly negative slope, reaching 1450°C at 29 kbar. Corundum is also produced at pressures >9 kbar by exsolution from solid anorthite by a process that is independent of the in-congruent melting, and that takes place at temperatures well below melting; stoichiometric anorthite is unstable at high pressures. The presence of corundum both below and above the melting curve complicates the phase relations in the vicinity of the (virtual) intersection with the breakdown curve, and requires that, for the composition CaAl₂Si₂O₈, a small amount of the reaction anorthite (Al-deficient) + corundum ⇄ grossular + kyanite take place before the principal breakdown reaction.