| The location of blueschist facies metamorphism can be attributed to the stability of the amphibole glaucophane. The breakdown of glaucophane in the system Na2O-MgO-Al2O3-SiO2-H 2O in terms of pressure can be broken down to two reaction: (1) glaucophane = talc + 2 jadeite (high pressure) and (2) glaucophane + 2 quartz = 2 albite + talc (low pressure). Reversal experiments were conducted along reaction (1) using synthetic talc, jadeite, and glaucophane. The reaction has been bracketed at 2.5-2.7 GPa at 600 °C 2.8-3.0 GPa at 650 °C and 3.0-3.2 GPa at 700 °C. Run products were analyzed using the electron microprobe and XRD. The initial glaucophane composition changed slightly as so did the talc. Activities using regular solution theory gave equilibrium constants (K) of 1.176, 1.197, and 1.218 at 600, 650, 700 °C, respectively. Using these K values with this study's experimental boundary yields enthalpy of formation and third law entropy values of end-member glaucophane of -11,939.6 +/- 9.6 kJ and 0.56 +/- 0.01 kJ/K, respectively.Synthesis experiments were conducted along the talc-Fe talc join to see at what point Fe saturation in talc occurred. Results using XRD, FTIR spectroscopy, and Mossbauer spectroscopy indicate that saturation occurs at about ~ XFe = 0.4. An increase in unit-cell volume was observed with increasing Fe content. An extrapolation to the volume of theoretical end-member Fe talc was done, indicating a volume of 466 A3.Experiments were conducted along reaction (1 & 2) with variable Fe content to see how the stability of glaucophane would alter with the incorporation of Fe. Synthetic glaucophane with any Fe content was never successfully synthesized, so a natural glaucophane from Guatemala with XFe = 0.17 was used. Synthetic talc (XFe = 0.17), jadeite, and albite, and natural quartz were used as starting materials. Experiments conducted along reaction (2) were not successful as the appearance of cummingtonite always occurred. A half-bracket was observed at 650 °C along reaction (1) where talc and jadeite formed at the expense of glaucophane, not vice versa since a layer silicate (vermiculite/smectite) always appeared. |
