Abstract Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2OMgOAl2O3SiO2H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 2030kbar and 700800C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeitetalcquartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nybite (Nyb), AlNa-cummingtonite (AlNa-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite+4quartz+4H2O=7talc, nybite+3quartz=3jadeite+talc, 3AlNa-cummingtonite + 11quartz + 2H2O=6jadeite + 5talc, and 3 sodium anthophyllite+13quartz+4H2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of AlIV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [NaAAlIV]=[ASi] (edenite) toward nybite, and [Na(M4)AlVI]=[Mg(M4)MgVI] toward cummingtonite. The extent of nybite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.