Crystal-chemistry of sulfates from Apuan Alps (Tuscany, Italy). I. Crystal structure and hydrogen bond system of melanterite, Fe(H2O)6(SO4)•H2O

Abstract

Melanterite, ideally Fe(H₂O)₆SO₄·H₂O, from the pyrite + iron oxide ore deposit of Fornovolasco (Apuan Alps, Tuscany, Italy) has been fully characterized through electron microprobe analysis, micro-Raman spectroscopy, and X-ray diffraction. Melanterite occurs as cm-sized greenish fibrous efflorescences on pyrite or rare pseudo-octahedral colorless crystals, up to 5 mm in size. Electron microprobe analysis (in wt% - average of ten spot analyses normalized to 100 wt% without H₂O) gave: SO₃ 52.98, FeO 45.53, MgO 1.49, sum 100.00. Assuming the occurrence of 7 H₂O groups per formula unit, the chemical formula can be written as (Fe_0.95Mg_0.06)_Σ1.01(SO₄)·7H₂O. The Raman spectrum of melanterite is characterized by bending and stretching modes of (SO₄) and H₂O groups. Melanterite crystallizes in the space group P2₁/c, with unit-cell parameters a = 14.0751(8), b = 6.5014(4), c = 11.0426(6) Å, β = 105.632(3)°, V = 973.11(10) ų, Z = 4. The crystal structure of melanterite has been refined down to R₁ = 0.024 on the basis of 3457 reflections with F_o > 4σ(F_o) and 179 refined parameters. It can be described as formed by undulating layers showing the alternation, along a, of SO₄ groups and Fe-centered octahedra coordinated by H₂O groups. The occurrence of a complex hydrogen bond system plays a fundamental role in the crystal structure of melanterite.