Thermodynamic and crystallographic discussion of the formation and dissociation of zircon

Abstract

The mechanism for the formation of zircon from a system of Si(OC2H5)4-ZrOCl2 · 8H2O-C2H5OH solution was constituted from the following steps; the formation of sol composed of an equimolar Zr4+ located in the neighbourhood of a linear siloxane polymer, deposition of metastable fine (tetragonal) T-ZrO2 by heating, the tetragonal to monoclinic (T → M) transformation of the fine T-ZrO2 aggregates separated from the strain energy of grain boundary between two oxide components, the initiation of the interface reaction between T-ZrO2 and amorphous SiO2, and the acceleration of the formation by the Hedvall effect caused by the crystallization of SiO2. The key technology for the symhesix of zircon from the oxide components ZrO2 and SiO2 was the mechanochemina! activation of the surface layer by milling, the formation of transition phase between the two componems by adding the guest element, and the presentation of metamorphic surface state. Zircon dissociated into M-ZrO2 and SiO2 glass, and the dissociation temperature was expected to be above 1700° C, which required the correction of the phase diagram for ZrO2-SiO2 system used previously.