Synthesis and Stability Relations of Epidote, Ca2Al2FeSi3O12 (OH)

Hydrothermal investigation of the bulk composition 2CaO·Al2O3·l/2Fe2O2·3SiO2+excess H2O (Ps 33 +excess H2O) has been conducted using conventional apparatus and solid oxygen buffer techniques. Coarse-grained epidotes (over 150 microns in some cases) were readily synthesized from oxide mixtures with a 98 per cent yield as well as from their high temperature equivalents at 600–700 °C and 5 kb Pfluid and over a range of oxygen fugacities. Electron microprobe analyses show that maximum Fe+3 content of synthetic epidotes varies as a function of fo2. Epidote is most iron-rich (Ps 33 ± 2) at high (HM and CCO) oxygen buffers and becomes progressively more aluminous (Ps 25 ± 3) with decreasing fo2 values and temperatures. Such variation is consistent with the change of refractive indices and cell dimensions. The mean refractive indices and cell dimensions for synthetic epidote (Ps 33) are Nα = 1.745 ± 0.005, Nγ = l.786±0.005, a = 8.920±0.005 Å, b = 5.645±0.004 Å, c = 10.190 ű0.006 Å, and β = 115° 31′±4′ and for epidote (Ps 25) are Nα = 1.735±0.005, Nγ = 1.775±0.005, a = 8.891±0.005 Å, b = 5.625±0.004 Å, c = 10.177±0.006 Å, and β = 115° 30′±3′. Mössbauer spectra indicate synthetic epidotes are relatively disordered. Garnets of intermediate composition in the grossular-andradite series were synthesized and the cell dimensions and refractive indices vary linearly with composition. With successive decrease in fo2, garnet synthesized on the Ps 33 bulk composition moves toward the grossular end member with simultaneously increasing almandine component; concomitantly the hercynite component of the coexistent magnetite increases. The fo2-T-Pfluid relations were determined by employing mineral mixtures of synthetic epidote and its high temperature equivalent in subequal proportions. Equilibrium was demonstrated for the reactions (1) epidote (Ps 33) = anorthite+grandite+FeOx+quartz + fluid, and (2) epidote (Ps 25) (+quartz) = garnet38+anorthite+magnetitc+fluid. With fo2 defined by the HM buffer, epidote (Ps 33) is stable up to 748 °C, 5 kb, 678 °C, 3 kb, and 635 °C, 2 kb Pfluid. With fo2 defined by the NNO buffer, the epidote (Ps 25) high temperature stability limit is reduced about 100 °C at 5kb Pfluid. At slightly lower fo2, than defined by the QFM buffer, epidote is not stable at any temperatures; the assemblage hedenbergite+anorthite+garnet38+fluid replaces epidote in the presence of excess quartz. Combined with previously determined equilibria for prehnite, andradite, and hedenbergite, isobaric fo,-T relations were further investigated by chemographic analysis interrelating the phases prehnite, epidote, grandite, hedenbergite, wollastonite, anorthite, and magnetite in the system CaO-Fe2O3-Al2O3-SiO2-H20. Such analysis allowed the construction of a semi-quantitative petrogenetic grid applicable to natural parageneses in low μCO2 environments, and the delineation of the low temperature stability limit of epidote as a function of fo2. Enlargement of the epidote stability range toward both high and low temperatures with increasing fo2, is consistent with widespread occurrences of epidote in low- and mediumgrade metamorphic rocks.