Synthesis and Stability Relations of Epidote, Ca2Al2FeSi3O12 (OH) |
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| Authors: | LIOU J G |
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| Institution: | Department of Geology, Stanford University Stanford, California 94305 |
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| Abstract: | Hydrothermal investigation of the bulk composition 2CaO·Al2O3·l/2Fe2O2·3SiO2+excessH2O (Ps 33 +excess H2O) has been conducted using conventionalapparatus and solid oxygen buffer techniques. Coarse-grainedepidotes (over 150 microns in some cases) were readily synthesizedfrom oxide mixtures with a 98 per cent yield as well as fromtheir high temperature equivalents at 600–700 °C and5 kb Pfluid and over a range of oxygen fugacities. Electronmicroprobe analyses show that maximum Fe+3 content of syntheticepidotes varies as a function of fo2. Epidote is most iron-rich(Ps 33 ± 2) at high (HM and CCO) oxygen buffers and becomesprogressively more aluminous (Ps 25 ± 3) with decreasingfo2 values and temperatures. Such variation is consistent withthe change of refractive indices and cell dimensions. The meanrefractive indices and cell dimensions for synthetic epidote(Ps 33) are N = 1.745 ± 0.005, N = l.786±0.005,a = 8.920±0.005 Å, b = 5.645±0.004 Å,c = 10.190 ű0.006 Å, and ß = 115°31'±4' and for epidote (Ps 25) are N = 1.735±0.005,N = 1.775±0.005, a = 8.891±0.005 Å, b =5.625±0.004 Å, c = 10.177±0.006 Å,and ß = 115° 30'±3'. Mössbauer spectraindicate synthetic epidotes are relatively disordered. Garnets of intermediate composition in the grossular-andraditeseries were synthesized and the cell dimensions and refractiveindices vary linearly with composition. With successive decreasein fo2, garnet synthesized on the Ps 33 bulk composition movestoward the grossular end member with simultaneously increasingalmandine component; concomitantly the hercynite component ofthe coexistent magnetite increases. The fo2-T-Pfluid relations were determined by employing mineralmixtures of synthetic epidote and its high temperature equivalentin subequal proportions. Equilibrium was demonstrated for thereactions (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 stableup to 748 °C, 5 kb, 678 °C, 3 kb, and 635 °C, 2kb Pfluid. With fo2 defined by the NNO buffer, the epidote (Ps25) high temperature stability limit is reduced about 100 °Cat 5kb Pfluid. At slightly lower fo2, than defined by the QFMbuffer, epidote is not stable at any temperatures; the assemblagehedenbergite+anorthite+garnet38+fluid replaces epidote in thepresence of excess quartz. Combined with previously determined equilibria for prehnite,andradite, and hedenbergite, isobaric fo,-T relations were furtherinvestigated by chemographic analysis interrelating the phasesprehnite, epidote, grandite, hedenbergite, wollastonite, anorthite,and magnetite in the system CaO-Fe2O3-Al2O3-SiO2-H20. Such analysisallowed the construction of a semi-quantitative petrogeneticgrid applicable to natural parageneses in low µCO2 environments,and the delineation of the low temperature stability limit ofepidote as a function of fo2. Enlargement of the epidote stabilityrange toward both high and low temperatures with increasingfo2, is consistent with widespread occurrences of epidote inlow- and mediumgrade metamorphic rocks. |
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