| Defining the stability field of glaucophane is critical to our understanding of blueschist facies metamorphic terrains. Reversals in the Na2O-MgO-Al 2O3-SiO2-H2O system along the glaucophane + 2 quartz = 2 albite + talc were done using synthetic glaucophane, talc, and albite, natural quartz was used. The reaction has been bracketed at 11.0-12.5 kbars at 600°C; 14.5-16.0 kbars at 700°C; and 16.5-17.0 kbars at 750°C. The initial (nearly pure) glaucophane composition changed slightly to the average composition: Na0.18(Na1.78Mg0.22)(Mg 3.23Al1.77)(Al0.17Si7.83)O22(OH) 2.; Calculated activities using a regular solution theory with WNa-□ = 9.7, WMg-Na= 20, and WMg-Al = 21.4 kJ for glaucophane and talc gave Ka values 1.10, 1.04, and 1.02 at 600, 700, 750°C, respectively. Using these Ka values and applying them to this study's experimental reaction boundary allowed us to refine the DeltaHf and So values of end-member glaucophane to -1 1969.804 +/- 10 kJ and 0.532 +/- 0.020 kJ/K, respectively. |
