EXPERIMENTAL STUDY ALONG THE MAGNESIO-HORNBLENDE--GLAUCOPHANE JOIN

Amphiboles have had a leading role in metamorphic petrology. They help define the metamorphic facies and their compositions can provide geothermobarometry if their thermodynamic mixing properties are known. In this study the mixing properties of amphibole solid solutions between Mg-hornblende [(Ca_1.85Mg_0.15)(Mg_4.10Al_0.90)(Si_7.1Al_0.9)O₂₂(OH)₂ = Hb] and glaucophane [(Na₂Mg₃ Al₂ Si₈O₂₂(OH)₂ = Gl] were investigated. Amphiboles were made from mixtures of reagent oxides and carbonates at 10 mol% increments between Hb and Gl. They were treated in the piston-cylinder press at 720-780°C and 15 - 25 kbars for about 72 hours. The products were examined by powder X-ray diffraction (XRD) and appeared to have good yields (92% -100%) of amphibole, though minor quartz, talc, smectite and amorphous material were often present initially, but could often be reacted out with retreatment. Also a positive deviation from ideality is present in the volume - composition plot, supporting the presence of a miscibility-gap along this join. Mid-infrared (MIR) spectra were obtained in the range of 350-4000 cm^-1 from samples pressed in KBr disks. The MIR spectra were analyzed by autocorrelation to derive ΔCorr values, indicating positive deviations in the ΔH^mix and an asymmetric miscibility gap. Asymmetric parameters W_Hb and W_Gl were fitted to the ΔCorr values assuming that Hb and Gl mix by coupled substitution only on the two M4 sites.

The end-member Hb and Gl amphiboles were mixed in a molar ratio of 1:1 and treated at 800°C (3 days), 700°C (5 days), and 600°C (one week) at 20 kbar to see if these amphiboles could re-equilibrate to intermediate compositions and provide the maximum width (half brackets) of the miscibility gap. Results acquired so far show good convergence at 800°C to a single-phase amphibole, partial convergence at 700°C, and poor convergence at 600°C which indicates the critical temperature of the miscibility-gap is between 700°C and 800°C. Estimating the critical temperature as 750°C, and using a ratio of W_Hb/W_Gl of 1.65 (from autocorrelation analysis), one can derive absolute values of W_Hb (17.19) and W_Gl (10.43) in kJ. This produces an asymmetric miscibility gap with a critical composition at a Gl mole fraction of about 0.64.