The lower pressure stability of glaucophane in the presence of quartz can be attributed to the reaction: glaucophane + 2 quartz = 2 albite + talc. Reaction reversals in the Na 2 O-MgO-Al 2 O 3 -SiO 2 -H 2 O system were obtained using a ½ inch diameter piston-cylinder press, synthetic talc, albite, glaucophane and natural quartz for the starting mixtures. Reversals were made with low H 2 O contents to avoid the growth of smectite. The reaction has been bracketed at 11.0–12.5 kbar at 600 °C; 14.5–16.0 kbar at 700 °C; and 16.0–17.50 kbar at 750 °C. At 800 °C a different reaction occurs: glaucophane + talc = 3 enstatite + 2 albite + H 2 O, and at temperatures below 600 °C smectite appeared. Run products were analysed using the electron microprobe and X-ray powder diffraction. The initial (nearly pure) glaucophane composition changed slightly to the average composition: Na 0.18 (Na 1.78 Mg 0.22 )(Mg 3.23 Al 1.77 )(Al 0.17 Si 7.83 )O 22 (OH) 2 ; this change in chemical composition also correlates with an increase in unit-cell volume. The average composition of talc was Na 0.26 Mg 2.68 Al 0.25 Si 3.91 O 10 (OH) 2 which also correlates with a slight increase in unit-cell volume, due to the incorporation of Na into the structure. Changes in order/disorder in albite were also analysed using XRD patterns. Values of the Δ 131 2 Θ parameter were within the range 1.8–2.0 indicating albite remains essentially disordered. Activities using regular solution theory withW_ −Na = 9.7, W Mg-Na =20, and W Mg-Al = 21.4 kJ for glaucophane, and published values of W_ −Na , W Al-Mg , andW Al-Si for mica applied to talc gave equilibrium constants ( K ) of 0.966, 0.955, and 0.949 at 600, 700, 750 °C, respectively. Using these K values with this study’s experimental boundary yields Δ H 0 f,298 and S 0 298 values of end-member glaucophane of −11,972.60 ± 16.5 kJ and 0.530 ± 0.017 kJ/K, respectively.