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(H 2 O) 6 SO 4 ·H 2 O, 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 H 2 O) gave: SO 3 52.98, FeO 45.53, MgO 1.49, sum 100.00. Assuming the occurrence of 7 H 2 O groups per formula unit, the chemical formula can be written as (Fe 0.95 Mg 0.06 ) Σ1.01 (SO 4 )·7H 2 O. The Raman spectrum of melanterite is characterized by bending and stretching modes of (SO 4 ) and H 2 O groups. Melanterite crystallizes in the space group P 2 1 / c , with unit-cell parameters a = 14.0751(8), b = 6.5014(4), c = 11.0426(6) A, β = 105.632(3)°, V = 973.11(10) A 3 , Z = 4. The crystal structure of melanterite has been refined down to R 1 = 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 SO 4 groups and Fe-centered octahedra coordinated by H 2 O groups. The occurrence of a complex hydrogen bond system plays a fundamental role in the crystal structure of melanterite.