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Mineralogical ClassificationCrystal structure and hydrogen bonding system of römerite

1st Jun 2019 14:28 UTCMarco E. Ciriotti Manager

Reference:

▪ Mauro, D., Biagioni, C., Pasero, M. Zaccarini, F. (2018): Crystal-chemistry of sulfates from Apuan Alps (Tuscany, Italy). II. Crystal structure and hydrogen bonding system of römerite, Fe2+Fe3+2(SO4)4(H2O)14. Atti della Società Toscana di Scienza Naturali, Memorie, Serie A, 125, 5-11.

Abstract:

Römerite, ideally Fe2+Fe3+2(SO4)4(H2O)14, from the Fornovolasco mine (Apuan Alps, Tuscany, Italy) has been characterized through electron microprobe analysis, micro-Raman spectroscopy, and X-ray diffraction. Römerite occurs as pinkish microcrystalline aggregates or as rare euhedral brownish crystals, up to some mm in size. Electron microprobe analysis (in wt% - average of ten spot analyses) gave: SO3 40.46(94), FeOtot 26.64(94), MgO 1.14(11), total 68.25(104). Assuming the occurrence of 14 H2O groups per formula unit and recalculating the Fe2+:Fe3+ atomic ratio according to the structure refinement, the chemical formula can be written as (Fe2+0.82Mg0.22)Σ1.04Fe3+2.08S3.95O16·14H2O. The Raman spectrum of römerite is characterized by the four fundamental modes of SO4 groups and by bending and stretching vibrations of H2O groups. Raman bands at wavenumbers lower than 400 cm-1 are related to Fe-O vibrational modes. Römerite crystallizes in the space group P1–, with unit-cell parameters a = 6.4512(6), b = 15.323(2), c = 6.3253(6) Å, α = 90.131(5), β = 100.900(4), γ = 85.966(4)°, V = 612.41(10) Å3, Z = 1. The crystal structure of römerite has been refined to R1 = 0.023 on the basis of 4346 reflections with Fo > 4σ(Fo) and 226 refined parameters. The crystal structure of römerite is formed by isolated {Fe2+(H2O)6} octahedra and [Fe3+(SO4)2(H2O)4] clusters, bonded through H-bonds.