Hydrothermal syntheses have been made with starting materials of compositions Di70Jd30, Di60Jd4o and Di50Jd50 at temperatures between 450°C and 650°C and at pressures between 1000 and 5000 atm. The phases that appear in the system are clinopyroxenes, plagioclase and a sodium-rich silicate phase. There is a continuous increase of the jadeite content in the clinopyroxenes when the temperature is lowered and the pressure increased. The slope of the curves combining runs with the same jadeite content is roughly 20 ± 6 bar/°C. The critical atomic position in the omphacite structure is regarded to be Al in six-coordination. At the intermediate stages in the P-T diagram where the Tschermak's silicate is going in as a component in the omphacite, sodium and silica are released, allowing the formation of a sodium-rich phase and plagioclase and preventing crystallisation of nepheline.