Mineral assemblages of grandite (grossular-andradite) and epidote (clinozoisite-pistazite) are widespread in calcsilicate rocks, rodingites, skarns and metabasites. Investigations on the intercrystalline Al-Fe3+ exchange according to the reaction
Ca3Fe2Si3O12 + 2 Ca2Al3Si3O12OH Ca3Al2Si3O12 + 2 Ca2Fe3Si3O12OH
Andradite Clinozoisite ¨ Æ Grossular Pistacite
were conducted in the temperature range of 500C to 650C, 3 kbar and oxygen fugacities of the MH buffer. Synthetic garnets and natural epidotes were used as starting materials; the epidotes were tempered first in order to achieve intracrystalline equilibrium.
Powder XRD failed to show detectable effects in compositional change. Electron microprobe analyses of final epidotes exhibit continuous variation from unreacted cores to equilibrated rims. In some experiments it was difficult to determine the final composition of the grandites due to spontaneous nucleation processes: a large number of idiomorphic garnets with grain sizes < 2 mm, distributed on the surface of grandite seed crystals, can be observed by SEM.
The results of our study indicate that the partition coefficient of Al-Fe3+ exchange between epidote and garnet is controlled both by the bulk composition and by temperature. These results are not consistent with the experimental data of Holdaway (1972) at 600C, 3 kbar, particularly for Al-rich epidotes and clinozoisites.
Grandites in equilibrium with epidotes of fixed Fe-content ([Al2Fe] > 0.7) provide a pronouced shift to grossular-rich compositions with decreasing temperature.
Intermediate epidotes ([Al2Fe] ~ 0.6) exhibit a compositional change in both directions, being more Al-rich and more Fe-rich than the starting material. This observation corresponds with the Mößbauer study on the intracrystalline equilibrium, which indicates a short-range order due to unmixing phenomena. The measured values plot around the miscibility gap suggested by some petrological studies (Raith, 1976). The equilibrated garnet, which coexists with these two epidotes, is grossular-rich, independent of the bulk composition. In order to maintain reaction balance, excess Fe is exsolved as hematite and/or magnetite.
Thermodynamic analysis of our data implies that the mixing model for clinozoisite-pistazite, as proposed by Bird and Helgeson (1980) does not describe the mixing behaviour adequately. With decreasing temperature the deviation from ideal behavior becomes more pronounced.
Bird, D.K. & Helgeson, H.C., Amer. J. Sci. 280, 907-941 (1980).
Holdaway, M.J., Contrib. Mineral. Petrol. 36, 307-340 (1972).
Raith, M., Contrib. Mineral. Petrol. 57, 99-117 (1976).