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.
