Corundum-quartz metastability: the role of silicon diffusion in corundum

The synthesis of the Al₂SiO₅ polymorphs kyanite, sillimanite and andalusite in a pure Al₂O₃–SiO₂–H₂O (ASH) system has long been known to be impeded. In order to decipher individual aspects of the reaction: corundum + SiO₂aq, which repeatedly fails to produce thermodynamically stable Al₂SiO₅, we conducted experiments within the stability fields of kyanite and sillimanite (500–800 ℃; 0.2–1 GPa) with the aim of forming reaction coronas on corundum. Results showed that metastable corundum + quartz assemblages form persistently in pure ASH, even in Al₂SiO₅ seeded experiments, despite the presence of catalyzing fluid and evidence of fast reaction kinetics. Coronas on corundum spontaneously formed when additional components (Na, K, N, and Mg) were added to the experiment. In a similar experiment with baddeleyite (ZrO₂) instead of corundum in silica saturated water, a zircon corona formed readily. This implies that nucleation and growth of Al₂SiO₅ is obstructed under conditions of Al and Si saturation in aqueous fluid, while both corundum and quartz saturated aqueous fluid are willing participants in other reactions towards stable corona formation. Instead of Al₂SiO₅ precipitation, an unexpected fluid-aided silica diffusion process into corundum was documented. The latter included the formation of nanometer wide hydrous silicate layers along the basal plane of the corundum host, which enhanced the silica diffusion rate drastically, leading to silica supersaturation in the host mineral, and ultimately to precipitation of quartz inside corundum. We conclude that the natural metastable assemblage of quartz and corundum is not necessarily the result of dry or fluid absent conditions, given that the aqueous fluid in experiments does not promote Al₂SiO₅ formation, but rather seems to support the formation and preservation of a metastable assemblage.