A neutron diffraction study of hydrogen bonding in copper-substituted melanterites and their dehydration products.

摘要

Melanterite (FeSO₄·7H₂O) is an important by-product of sulfide mining, associated with the sulfuric acid solutions that develop where reactive sulfides such as pyrite are exposed and oxidized. These acidic solutions precipitate metal sulfate minerals such as melanterite ((Fe,Cu,Zn)SO₄·7H₂O) and seasonal changes in humidity and rainfall contribute to the dehydration of these minerals to other metal sulfates such as siderotil (FeSO₄·5H ₂O), rozenite (FeSO₄·4H₂O) and szomolnokite (FeSO₄·H₂O). The formation, dissolution and precipitation of these minerals is a mechanism for attenuation and release of Fe 2+, Cu2+, Zn2+ and SO₄ 2− which can have a detrimental impact on human health and the environment. Melanterite crystals of differing compositions were synthesized by precipitation from supersaturated solutions of commercial-grade ferrous and cupric sulfate and heavy sulfuric acid (D2SO4). Dehydration products were produced at 22°C in humidity-controlled chambers over concentrated sulfuric acid solutions. The Fe:Cu ratio was measured by atomic adsorption spectroscopy and the final D:H ratio was measured by infrared spectroscopy. Powder neutron diffraction data were obtained at Chalk River Laboratories (AECL, Canada). The atomic structures of melanterite and their dehydration products were refined using the Rietveld refinement program GSAS (Larson and VonDreele, 1994). Results of this study show structural distortions in these minerals with increasing Cu-content due to preferred site substitution. This work has contributed measured H-positions for minerals whose H-positions have previously been calculated or inferred and can show a weakening in H-bonding with Cu-substitution in distorted melanterite and its dehydration products.