Fe3+ and Mn3+ distributions on octahedral M1, M2, and M3 sites in synthetic epidote/piemontite from Ca2Al2Fe3+qMn3+1−qSi3O12.5 starting material and their effects on the crystal structure were investigated using X-ray Rietveld and 57Fe Mössbauer methods. Epidote and piemontite were crystallized as almost single phases from q = 1.0, 0.75, 0.5, and 0.25 starting materials at Pfluid of 200–400 MPa and a temperature of 500 °C, using standard cold-seal pressure vessels. The Mn2O3-MnO2 buffer was used to produce fO<inf>2</inf> adequate to maintain Fe3+ and Mn3+. The Rietveld refinements converged to goodness-of-fit ranges from 1.21 to 1.60.

At this temperature, site preferences of ∑(Fe3++Mn3+) for octahedral sites are M3>M1(>>M2). KD values of ∑(Fe3++Mn3+), where KD = [(Fe3++Mn3+)/Al]M1/[(Fe3++Mn3+)/Al]M3, (0.05–0.13) are similar to those of individual Mn3+ and Fe3+ vs. Al3+, respectively. However, the KD values of Fe3+ and Mn3+ for M1 and M3, where KD = (Fe3+/Mn3+)M1/(Fe3+/Mn3+)M3, vary with FeTotal3+:MnTotal3+ ratios. In epidote with Fe3+ content larger than 0.4 atoms per formula unit (apfu) and Mn3+ < 0.6 apfu, Fe3+ has a stronger preference for M1 than Mn3+. In piemontite with 0.12 Fe3+ and 0.73–0.78 Mn3+ apfu, the preference of Mn3+ for M1 is greater than that of Fe3+. The site occupancies of individual Mn3+ and Fe3+ are governed by the individual KD values and the Mn3+ and Fe3+ concentrations in corresponding epidote and piemontite. Variations of the unit-cell parameters indicate the combined result of linear variation due to Al ↔ Fe3+ substitution and nonlinear variation due to Al ↔ Mn3+ substitution.

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