Mechanism of hydrothermal synthesis of zircon in a fluoride medium

The hydrothermal synthesis of zircon in a fluoride medium leads principally to two kinds of zircons, differentiated by their morphology, chemical composition, thermal stability and solubility. Differences in the thermodynamic parameters of solution (pH, temperature, [F-]) could explain the formation of these two kinds of zircons. The main hypothesis involves two crystallization mechanisms. Both include a step of gel dissolution with formation of soluble species that are responsible for the crystallization. In each case, the silicon species seem to be monomeric without fluorine. However, the nature of soluble zirconium species depends on the thermodynamic parameters of the solution. At acid pH and with fluoride ions, the reactive species for the crystallization seem to be fluoride rich zirconium complexes. In this case, the zircon obtained contains a large amount of fluoride presumably located in silicon sites (Zircon A). At basic pH, and whatever the concentration of fluoride, the reactive species for crystallization seems to be linear polymeric entities. In this case, the zircon obtained shows a typical morphology in the form of layered agglomerates and a high specific surface area (Zircon B).