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 Na2O-MgO-Al2O3-SiO2-H2O 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 H2O 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 + H2O, 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: Na0.18(Na1.78Mg0.22)(Mg3.23Al1.77)(Al0.17Si7.83)O22(OH)2; this change in chemical composition also correlates with an increase in unit-cell volume. The average composition of talc was Na0.26Mg2.68Al0.25Si3.91O10(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, WMg-Na=20, and WMg-Al= 21.4 kJ for glaucophane, and published values of W_−Na, WAl-Mg, andWAl-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 H0f,298 and S0298 values of end-member glaucophane of −11,972.60 ± 16.5 kJ and 0.530 ± 0.017 kJ/K, respectively.

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