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An experimental investigation of the reaction: glaucophane + 2 quartz = 2 albite + talc

Department of Geological Sciences and Environmental Studies, Binghamton University, PO Box 6000, Binghamton, NY 13902-6000, USA

^* Corresponding author, e-mail: jcorona1{at}binghamton.edu

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 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: 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 unit-cell volume. The average composition of talc was Na_0.26Mg_2.68Al_0.25Si_3.91O₁₀(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 131 2 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 H⁰_f,298 and S⁰ ₂₉₈ values of end-member glaucophane of –11,972.60 ± 16.5 kJ and 0.530 ± 0.017 kJ/K, respectively.

Key-words: blueschist facies, experimental petrology, high-pressure studies, glaucophane, metamorphic petrology.

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