EXPERIMENTAL INVESTIGATION OF THE REACTION: GLAUCOPHANE + 2 QUARTZ = 2 ALBITE + TALC

Defining the stability field of glaucophane is critical to our understanding of the blueschist facies metamorphic terrains. The lower pressure stability of glaucophane in the presence of quartz can be attributed to the following reaction: Glaucophane + 2 Quartz = 2 Albite + Talc. Reversals in the Na₂O-MgO-Al₂O₃-SiO₂-H₂O system were obtained using a 0.5 inch diameter piston-cylider press, using chromel-alumel thermocouples to an accuracy of +/- 0.5 kbars and +/- 10 °C. Synthetic talc, albite, glaucophane and natural quartz were used for the starting mixtures. Reversals were made with low H₂O contents to avoid the growth of vermiculite. The addition of water was done by placing the samples in a humidity chamber for five – ten minutes. The reaction has been bracketed at 12.5-13.5 kbars/600°C; 14.5-16.0 kbars/700 °C; and 16.5-17.5 kbars/750°C. At 800 °C a different reaction occurs: Glaucophane + Talc = 3 Enstatite + 2 Albite + H₂O, and at temperatures below 600°C vermiculite appeared. Run products were analyzed using the electron microprobe and x-ray powder diffraction. Reaction direction was judged by changes in peak heights on XRD patterns. X-ray powder diffraction also provided information about changes in cell dimensions for talc and glaucophane which could be compared to changes in mineral composition from electron microprobe analysis. The initial (nearly pure) glaucophane composition changed slightly to the average composition: Na_0.18(Na_1.78Mg_0.22)(Mg_3.23Al_1.77)(Al_0.17Si_7.83)O₂₂(OH)₂, this change in chemical composition also correlates with an increase in cell volume. The average composition of talc was Na_0.26Mg_2.68Al_0.25Si_3.91O₁₀(OH)₂ which also correlates with an slight increase in cell volume, this is likely due to the addition of Na. Changes in order/disorder in albite were also analyzed using XRD patterns, values of Δ131 2Θ were greater than 1.7 and not exceeding 2.0 indicating albite remains essentially disordered. Activities using regular solution theory with W_Na-Vac = 9.7, W_Mg-Na = 20, and W_Mg-Al = 21.4 kJ for glaucophane and talc gave Ka values 1.052, 1.057, and 1.034 at 600, 700, 750 °C, respectively. Using these Ka values and applying it to this study's experimental boundary we have refined the ΔHf and S° values of glaucophane to -11961.72 kJ and 0.5406 kJ/K respectively.