In Situ High-Temperature X-ray Powder Diffraction and Infrared Spectroscopic Study of Melanterite, FeSO4·7H2O

Maria Lacalamita,Francesco Radica,Giancarlo Della Ventura,Gennaro Ventruti,Emanuela Schingaro,Daniela Mauro

doi:10.3390/min11040392

Abstract

The thermal behavior of melanterite from the Fornovolasco mine (Tuscany, Italy) has been investigated via differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature X-ray powder diffraction (XRPD) and Fourier-transform infrared spectroscopy (FTIR). The DTA curve showed endothermic peaks at 70, 100, 260, 500–560 and 660 °C whereas the TG curve evidenced a total mass decrease of ~68%, in keeping with the loss of all H2O and SO4 groups. Rietveld refinements were performed for all the collected patterns in the 25–775 °C range and converged at 1.57 ≤ R (%) ≤ 2.75 and 1.98 ≤ Rwp (%) ≤ 3.74. The decomposition steps FeSO4·7H2O → FeSO4·4H2O (25 ≤ T ≤ 50 °C) → FeSO4·H2O (50 < T ≤ 100 °C) → FeOHSO4 (75 < T ≤ 200 °C) → Fe2(SO4)3 (400 < T ≤ 500 °C) → Fe2O3 (500 < T ≤ 775 °C) were obtained. The high-temperature infrared analysis confirmed that melanterite undergoes a three-step dehydration in the 25–300 °C temperature range. The FeOHSO4 phase is stable over a wide range of temperature and transforms partially to Fe2(SO4)3 without the formation of Fe2O(SO4)2. The findings highlight a different behavior of the studied sample with respect to the synthetic salt.

Highlights

Melanterite, ideally Fe(H2O)6SO4·H2O, is a typical product of weathering of sulfide ore minerals, mine waste, or undisturbed low temperature geological settings
We investigate the thermal behavior of melanterite by combining in situ high temperature X-ray powder diffraction, differential thermal analysis, thermogravimetry and in situ high temperature
The studied specimen was collected in the Fornovolasco mine, Fabbriche di Vergemoli, Lucca (Tuscany, Italy) where melanterite occurs in sulfate piles as efflorescence and represents the first oxidation and hydration product of pyrite in the old tunnels of the mine [11,27,28]

Summary

Introduction

Melanterite, ideally Fe(H2O)6SO4·H2O, is a typical product of weathering of sulfide ore minerals, mine waste, or undisturbed low temperature geological settings. Ferrous iron-sulfate minerals may contain significant concentrations of other divalent metals in solid solution. Melanterite has been reported to contain a number of additional metals (e.g., Cu, Mg or Zn) in solid solution beyond ferrous iron [2] and the degree of substitution affects the dehydration pathway of the mineral [3]. For these reasons, ferrous iron-sulfate minerals are important markers of environmental conditions such as pH, relative humidity, sulfate activity, metal sequestration [4]. Synthetic FeSO4·7H2O is known as a waste or by-product which derives from industrial processes such as titanium dioxide production and steel pickling ([5] and references therein)

Methods

Results

Conclusion