X-ray Rietveld and 57Fe Mossbauer studies of epidote and piemontite on the join Ca2Al2Fe3+Si3O12(OH)-Ca2Al2Mn3+Si3O12(OH) formed by hydrothermal synthesis

Abstract

Fe 3+ and Mn 3+ distributions on octahedral M1, M2, and M3 sites in synthetic epidote/piemontite from Ca 2 Al 2 Fe 3+ q Mn 3+ 1− q Si 3 O 12.5 starting material and their effects on the crystal structure were investigated using X-ray Rietveld and 57 Fe Mossbauer methods. Epidote and piemontite were crystallized as almost single phases from q = 1.0, 0.75, 0.5, and 0.25 starting materials at P fluid of 200–400 MPa and a temperature of 500 °C, using standard cold-seal pressure vessels. The Mn 2 O 3 -MnO 2 buffer was used to produce f O 2 adequate to maintain Fe 3+ and Mn 3+ . The Rietveld refinements converged to goodness-of-fit ranges from 1.21 to 1.60. At this temperature, site preferences of ∑(Fe 3+ +Mn 3+ ) for octahedral sites are M3>M1(>>M2). K D values of ∑(Fe 3+ +Mn 3+ ), where K D = [(Fe 3+ +Mn 3+ )/Al] M1 /[(Fe 3+ +Mn 3+ )/Al] M3 , (0.05–0.13) are similar to those of individual Mn 3+ and Fe 3+ vs. Al 3+ , respectively. However, the K D values of Fe 3+ and Mn 3+ for M1 and M3, where K D = (Fe 3+ /Mn 3+ ) M1 /(Fe 3+ /Mn 3+ ) M3 , vary with Fe Total 3+ :Mn Total 3+ ratios. In epidote with Fe 3+ content larger than 0.4 atoms per formula unit (apfu) and Mn 3+ 3+ has a stronger preference for M1 than Mn 3+ . In piemontite with 0.12 Fe 3+ and 0.73–0.78 Mn 3+ apfu, the preference of Mn 3+ for M1 is greater than that of Fe 3+ . The site occupancies of individual Mn 3+ and Fe 3+ are governed by the individual K D values and the Mn 3+ and Fe 3+ concentrations in corresponding epidote and piemontite. Variations of the unit-cell parameters indicate the combined result of linear variation due to Al ↔ Fe 3+ substitution and nonlinear variation due to Al ↔ Mn 3+ substitution.