Intercrystalline exchange of Al and Fe,+ between grossular-andradite and clinozoisite-epidote solid solutions
Heuss-Aßbichler, Soraya; Fehr, Karl Thomas
published: Sep 10, 1997
ArtNo. ESP154017201005, Price: 29.00 €
The intercrystalline distribution of Al and Fe3+ between grossular-andradite and clinozoisite-epidote solid solution series were determined experimentally within the temperature range of 500-650°C at 0.3 GPa, HM-buffer. The Al-Fe exchange reaction exhibits a remarkable dependence on composition and temperature due to the existence of two intermediate solvi along the join clinozoisite-epidote. The immiscibility is produced by ordering phenomena, resulting in different local environments for the octahedral (M3) sites of the different epidote phases. The exsolved phases can be detected only by methods sensitive to short-range order phenomena. In the presence of grandite and abundant fluid phase these domains act as seeds and macroscopic phases crystallize. The solvi project in the range of xFe epi = 0. 54-0.76 and XFe epi = 0.24-0.50 at 500 °C/0.3 GPa, HM buffer, respectively. The miscibility gaps are separated by a narrow range of single-phase epidote at about xFe epi = 0. 52. The solvi exhibit steep flanks, resulting in critical temperatures of 740 °C and 790 °C. The closing temperatures are metastable with respect to the upper thermal stability of intermediate epidotes. As a consequence 5 sets of intercrystalline distribution coefficients for AI-Fe3+ between grandite and epidote occur. At Fe-rich and AI-rich epidote compositions the intercrystalline K. J's vary significantly with temperature, at Fe-rich compositions K. J's increase with temperature and at Al-rich compositions K. J's decrease with temperature. The intercrystalline partition data exhibit an azeotropic behaviour due to the nonideal thermodynamic mixing properties of grandites and epidotes and a reversal in preference of Al and Fe3+ occur at xFegrt = O4.l1XFe/pi = 0.90. According to Engi & Wersin (1987) and Fehr (1992) the mixing properties of grandites are asymmetric and have to be described by the means of a 2-parameter or a 3-parameter Redlich-Kister equation, respectively. The thermodynamic mixing model for stable ordered epidotes by Bird & Helgeson (1980) predicts degrees of disorder which are too high and has to be revised. The application of the distribution curves on granditelepidote pairs of a skarn deposit (Kitamura 1975) demonstrates that the intercrystalline Al-Fe exchange reaction between grandite and epidote can be used as a geothermometer with regard to the composition of the equilibrated epidote phases.