Recrystallization of almost fully amorphous zircon under hydrothermal conditions: An infrared spectroscopic study

Hydrothermal experiments were carried out with powder from an almost fully amorphous, natural zircon under various P–T–t conditions mainly in a 0.1 N HCl solution. Powder infrared spectroscopic measurements on the experimental products reveal that first structural changes occurred at a fluid temperature as low as 75 °C. Significant recrystallization started at 200 °C, as indicated by an increase in the absorption intensity of the zircon fundamental IR bands and the formation of sharp OH stretching bands at 3385 and 3420 cm−1. Although the powder has fully reacted at 400 °C, the zircon fundamental absorption bands are not fully recovered, indicating the occurrence of significant amounts of amorphous remnants. The experimental results in neutral to acidic solutions are consistent with the idea that water (H+ and possibly H2O) diffuses into the amorphous network where it ‘catalyses’ solid state recrystallization. During this process, Zr and Si were leached from the amorphous network.