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

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.

Keywords

• melanterite
• Fornovolasco
• Apuan Alps
• thermal analysis
• <i>in situ</i> HT-XRPD
• <i>in situ</i> HT-FTIR