Experimental investigation of zoisite–clinozoisite phase equilibria in the system CaO–Fe₂O₃–Al₂O₃–SiO₂–H₂O

Abstract.

The system Ca₂Al₃Si₃O₁₁(O/OH)–Ca₂Al₂FeSi₃O₁₁(O/OH), with emphasis on the Al-rich portion, was investigated by synthesis experiments at 0.5 and 2.0 GPa, 500–800 °C, using the technique of producing overgrowths on natural seed crystals. Electron microprobe analyses of overgrowths up to >100 µm wide have located the phase transition from clinozoisite to zoisite as a function of P–T–X_ps and a miscibility gap in the clinozoisite solid solution. The experiments confirm a narrow, steep zoisite–clinozoisite two-phase loop in T–X_ps section. Maximum and minimum iron contents in coexisting zoisite and clinozoisite are given by ${\rm X}_{{\rm ps}}^{{\rm zo}} {\rm (max) = 1}{\rm .9*10}^{ - 4} T{\rm + 3}{\rm .1*10}^{ - 2} P - {\rm 5}{\rm .36*10}^{ - 2}$ and ${\rm X}_{{\rm ps}}^{{\rm czo}} {\rm (min)} = {\rm (4}{\rm .6} * {\rm 10}^{ - {\rm 4}} - 4 * {\rm 10}^{ - {\rm 5}} P{\rm )}T + {\rm 3}{\rm .82} * {\rm 10}^{ - {\rm 2}} P - {\rm 8}{\rm .76} * {\rm 10}^{ - {\rm 2}}$ (P in GPa, T in °C). The iron-free end member reaction clinozoisite = zoisite has equilibrium temperatures of 185±50 °C at 0.5  GPa and 0±50 °C at 2.0  GPa, with ΔH _r ⁰=2.8±1.3 kJ/mol and ΔS _r ⁰=4.5±1.4 J/mol×K. At 0.5 GPa, two clinozoisite modifications exist, which have compositions of clinozoisite I ~0.15 to 0.25 X_ps and clinozoisite II >0.55 X_ps. The upper thermal stability of clinozoisite I at 0.5 GPa lies slightly above 600 °C, whereas Fe-rich clinozoisite II is stable at 650 °C. The schematic phase relations between epidote minerals, grossular-andradite solid solutions and other phases in the system CaO–Al₂O₃–Fe₂O₃–SiO₂–H₂O are shown.