The synthesis and stability of zirconium minerals was studied in aqueous solutions at a temperature of 400 degrees C and at a pressure of 900 atmospheres. The compositional fields of stability of the minerals zircon and baddeleyite are outlined approximately. Zircon has a wide field of stability in acid solutions but its field of stability in alkaline solutions is restricted by limits close to the neutral point. Baddeleyite has a restricted field in acid solutions and a very wide one in alkaline solutions. At least 4 zircono-silicates were obtained as products; three of these have not been described in the literature. Solubility tests showed that zirconium compounds are insoluble in alkaline solutions but are very soluble in dilute HCl solutions. From these tests and from the other experimental data some geological conclusions are drawn regarding the occurrence of zirconium minerals and the nature of the solutions which transported the element.