This invention relates to a process of treating a zirconium containing product such as zircon. The process comprises providing an alkali fusion decomposed zircon product (AFDZ) formed from reading zircon with a source of alkali metal at elevated temperatures, and treating the AFDZ to form a solid containing hydrated zirconium oxide and/or hydrated zirconium basic carbonate (hereinafter referred to as the hydrated zirconium product). The process further comprises treating the solid hydrated zirconium product to obtain in situ formation of basic zirconium sulphate as a solid thereon. The invention also relates to such a process for producing zircon derived material suitable for pigments and to such a process to produce opacifier material. The invention also relates to products of such processes.