American Mineralogist, Volume 95, pages 1237–1246, 2010 0003-004X/10/0809–1237$05.00/DOI: 10.2138/am.2010.3418 1237 X-ray Rietveld and 57 Fe Mössbauer studies of epidote and piemontite on the join Ca 2 Al 2 Fe 3+ Si 3 O 12 (OH)–Ca 2 Al 2 Mn 3+ Si 3 O 12 (OH) formed by hydrothermal synthesis Mariko NagashiMa 1,2, * aNd Masahide akasaka 1 1 Department of Geoscience, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan 2 Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland 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 investi- gated using X-ray Rietveld and 57 Fe 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 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 Σ(Fe 3+ +Mn 3+ ) for octahedral sites are M3>M1(>>M2). K D values of Σ(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 3+ Total :Mn 3+ Total ratios. In epidote with Fe 3+ content larger than 0.4 atoms per formula unit (apfu) and Mn 3+ < 0.6 apfu, Fe 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 gov- erned 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. Keywords: Epidote, piemontite, synthesis, Rietveld refinement, Mössbauer spectroscopy iNtroductioN Epidote-group minerals are important rock-forming phases oc- curring in a variety of geological conditions. The general formula of epidote-group minerals is A1A2M1M2M3Z 3 O 12 (OH). The main components of most epidote and piemontite are Ca 2 Al 3 Si 3 O 12 (OH), Ca 2 Al 2 Fe 3+ Si 3 O 12 (OH), and Ca 2 Al 2 Mn 3+ Si 3 O 12 (OH). Ca 2+ occu- pies 9-coordinated A1 and 10-coordinated A2 sites; Al, Mn 3+ , and Fe 3+ are distributed over octahedral M1, M2, and M3 sites, and Si cations occur at the tetrahedral Z site. The octahedral sites form two types of chains of edge-sharing octahedra: a single chain of M2 octahedra and a multiple chain with central M1 and peripheral M3 octahedra (Ito et al. 1954; Dollase 1968, 1969, 1971). The M3 octahedron is larger and more distorted than M1 and M2. The volumes of the three crystallographically independent octahedra vary as M3>M1>M2, where M2 is the smallest and least distorted octahedron. The distribution of Mn 3+ and Fe 3+ among the three types of octahedral sites and the structural changes caused by Mn 3+ and/ or Fe 3+ substitution for Al in epidote-group minerals have been investigated by single-crystal structure refinements (Dollase 1969, 1971; Kvick et al. 1988; Ferraris et al. 1989; Bonazzi et al. 1990, 1992; Bonazzi and Menchetti 1994, 1995; Giuli et al. 1999; Langer et al. 2002; and others) and X-ray Rietveld refine- ment (Nagashima and Akasaka 2004). The crystal chemistry of natural epidote-group minerals has also been investigated using a variety of spectroscopic methods (Burns and Strens 1967; Dollase 1973; Paesano et al. 1983; Fehr and Heuss-Assbichler 1997; Taran and Langer 2000; Langer et al. 2002). However, it is a challenge to investigate the specific influence of unique Fe 3+ and Mn 3+ substitution for Al on structural changes because natural epidote-group minerals commonly contain not only Fe 3+ but also Mn 3+ . Although Fe 3+ and Mn 3+ have similar ionic radii, their electronic configurations and induced distortions are different. Moreover, Sr and REE often occur at the A sites. High Sr and/or Ba contents at A2 are known to promote the incorporation of transition elements rather than Al at the octa- hedral sites (Armbruster et al. 2002; Nagashima and Akasaka 2004; Fukushima et al. 2005). Therefore, the influence of Fe 3+ or Mn 3+ substitution for Al was investigated in synthetic pure Al-Fe 3+ and Al-Mn 3+ binary series, respectively, by Anastasiou and Langer (1977), Giuli et al. (1999), Langer et al. (2002), and Nagashima and Akasaka (2004). However, the intracrystalline partitioning of Fe 3+ and Mn 3+ in pure Al-Fe 3+ -Mn 3+ ternary series has not yet been examined. In this study, we investigate the distribution of Fe 3+ and Mn 3+ among octahedral sites and the variations of structural changes caused by Fe 3+ and Mn 3+ substitution for Al. Unit-cell parameters and interatomic distances and angles are measured on a synthetic Al-Fe 3+ binary series and a Al-Fe 3+ -Mn 3+ ternary series of epidote- group minerals from Ca 2 Al 2 Fe 3+ q Mn 3+ 1–q Si 3 O 12.5 +H 2 O for starting materials with q = 1.0, 0.75, 0.5, and 0.25. * Present address: Department of Earth Science, Graduate School of Science and Engineering, Yamaguchi University, 753-8512, Japan. E-mail: nagashim@yamaguchi-u.ac.jp