X-ray Rietveld and [sup.57]Fe Mossbauer studies of epidote and piemontite on the join [Ca.sub.2][Al.sub.2][Fe.sup.3+][Si.sub.3][O.sub.12](OH)-[Ca.sub.2] [Al.sub.2][Mn.sup.3+][Si.sub.3][O.sub.12](OH) formed by hydrothermal synthesis

[Fe.sup.3+] and [Mn.sup.3+] distributions on octahedral M1, M2, and M3 sites in synthetic epidote/piemontite from [Ca.sub.2][Al.sub.2][Fe.sup.3+.sub.l-q][Si.sub.3][O.sub.12.5] starting material and their effects on the crystal structure were investigated using X-ray Rietveld and [sup.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.sub.fluid] of 200-400 MPa and a temperature of 500[degrees]C, using standard cold-seal pressure vessels. The [Mn.sub.2][O.sub.3]-Mn[O.sub.2] buffer was used to produce fog adequate to maintain [Fe.sup.3+] and [Mn.sup.3+]. The Rietveld refinements converged to goodness-of-fit ranges from 1.21 to 1.60. At this temperature, site preferences of [summation]([Fe.sup.3+] + [Mn.sup.3+]) for octahedral sites are M3>MI(>>M2). [K.sub.D] values of [summation]([Fe.sup.3+] + [Mn.sup.3+]), where [K.sub.D] = [([Fe.sup.3+] + [[Mn.sup.3+] )/Al].sup.M1] /[([Fe.sup.3+] + [[Mn.sup.3+])/Al].sup.M3], (0.05-0.13) are similar to those of individual [Mn.sup.3+] and [Fe.sup.3+] vs. [Al.sup.3+], respectively. However, the [K.sub.D] values of [Fe.sup.3+] and [Mn.sub.3+] for M1 and M3, where [K.sub.D] = [([Fe.sup.3+]/[Mn.sup.3+]).sup.M1]/ [([Fe.sup.3+]/[Mn.sup.3+]).sup.M3], vary with [Fe.sup.3+.sub.Total]: [Mn.sup.3+.sub.Total] ratios. In epidote with [Fe.sup.3+] content larger than 0.4 atoms per formula unit (apfu) and [Mn.sup.3+] < 0.6 apfu, [Fe.sup.3+] has a stronger preference for M1 than [Mn.sup.3+]. In piemontite with 0.12 [Fe.sup.3+] and 0.73-0.78 [Mn.sup.3+] apfu, the preference of [Mn.sup.3+] for M1 is greater than that of [Fe.sup.3+]. The site occupancies of individual [Mn.sup.3+] and [Fe.sup.3+] are governed by the individual [K.sub.D] values and the [Mn.sup.3+] and [Fe.sup.3+] concentrations in corresponding epidote and piemontite. Variations of the unit-cell parameters indicate the combined result of linear variation due to Al [left and right arrow] [Fe.sup.3+] substitution and nonlinear variation due to Al [left and right arrow] [Mn.sup.3+] substitution. Keywords: Epidote, piemontite, synthesis, Rietveld refinement, M6ssbauer spectroscopy DOI: 10.2138/am.2010.3418