METASTABLE REVERSAL OF SILLIMANITE-CORUNDUM-QUARTZ IN THE KYANITE STABILITY FIELD: COMPETITION BETWEEN STABLE AND METASTABLE REACTIONS

The equilibrium Sil = Cnd + Qtz has been metastably reversed, approximately 700 to 800 MPa inside the kyanite stability field, at 600 °C and 1625 MPa; 700 °C and 1725 MPa; 800 °C and 1825 MPa; and 900 °C and 1925 MPa using a NaCl setup with a graphite furnace in a non-endload piston cylinder press with thermal gradients over the flat-lying Pt capsule minimized to < 5 °C. Charges consisted of gem-quality sillimanite, corundum, and quartz plus H₂O as a flux, all placed in the Pt capsule which was arc-welded shut and folded. The Sil-Cnd-Qtz equilibrium corresponds to an enthalpy of formation from the elements for sillimanite of -2586.05 kJ/mole and an entropy of 95.39 J/K, both at 298.15 K and 1 bar. The positive slope of the Sil-Cnd-Qtz equilibrium is found to approximately parallel the positive slopes of both the Cnd-Qtz-Ky and Sil-Ky equilibria (Harlov and Milke 2002). As a consequence, for geologic pressures and temperatures inherent in the crust and upper mantle of the Earth, the Sil-Cnd-Qtz equilibrium never intersects the Cnd-Qtz-Ky equilibrium. This implies that in these regions sillimanite is always metastable relative to corundum and quartz.

Experimentally, the metastability of the assemblage Sil + Cnd + Qtz implies that Sil ↔ Cnd + Qtz is always in competition with Cnd + Qtz → Ky and Sil → Ky. Early on during the experiment it may be assumed that a steady state between dissolution and growth rates is established. However, due to the sluggish nucleation of kyanite there is an induction period of P-T dependent duration during which Cnd + Qtz → Sil. Once kyanite does appear, the reaction proceeds very fast to kyanite via reactions Cnd + Qtz → Ky and Sil → Ky. The kyanite surface area is probably a major factor in controlling the overall reaction rates. Under constant P and T, the system evolves from metastable sillimanite formation to sillimanite consumption, which is only dependent on the kyanite surface area. Similar competition between stable and metastable reactions could occur during contact metamorphism where metastable mineral growth is observed. It could also help explain the presence of metastable corundum + quartz +/- Al₂SiO₅ assemblages or 2 to 3 co-existing Al₂SiO₅ phases in high-grade aluminous metamorphic rocks.

Harlov DE and Milke R (2002) Am Mineral, 87, 424–432