Thermodynamics, crystal chemistry and structural complexity of the Fe(SO4)(OH)(H2O) x phases: Fe(SO4)(OH), metahohmannite, butlerite, parabutlerite, amarantite, hohmannite, and fibroferrite

Abstract

Using combination of acid-solution, high-temperature oxide-melt, relaxation, and differential scanning calorimetry, we have determined the thermodynamic properties of all phases in the system Fe(SO₄4)(OH)–H₂O (estimated values in parentheses): x in Fe(SO₄)(OH)(H₂O)_x Δ_fH° (kj·mol^-1) S° Δ_fG° (j·mol^-1·K^-1 log K_sp (kj·mol^-1) Fe(SO₄)(OH) 0 -1160.2 ± 2.3 145.9 ± 1.2 -1013.7 ± 2.4 -2.862 Metahohmannite 1.5 -1608.2 ± 1.7 (200.3) -1373.6 ± 1.8 -3.594 Butlerite 2 -1758.2 ± 1.7 (214.0) -1492.7 ± 1.8 -3.685 Parabutlerite 2 -1758.6 ± 1.7 214.0 ± 1.4 -1493.3 ± 1.8 -3.799 Amarantite 3 -2056.5 ± 1.8 243.8 ± 1.9 -1730.5 ± 1.9 -3.801 Hohmannite 3.5 -2197.8 ± 1.8 (271.4) -1845.3 ± 1.9 -3.138 Fibroferrite 5 -2641.5 ± 1.8 332.2 ± 2.7 -2202.8 ± 2.0 -3.456 Using these data, phase diagrams for low-temperature (25 °C) systems in contact with aqueous solutions predict that these phases should precipitate from extremely concentrated, low-pH solutions. In a relative humidity–temperature space, only Fe(SO₄)(OH), parabutlerite, and amarantite have stability fields; the higher hydrates would require unrealistically high air humidities to form as stable phases. High-temperature high-pressure phase diagrams produce reasonable topologies, although the details of the phase relations between Fe(SO₄)(OH), Fe₂(SO₄)₃, and hydronium jarosite deserve more work. We also present a new structural model for amarantite, including the positions of the H atoms, and an analysis of the hydrogen bonding network in this mineral. Using the concept of information density in minerals, the Fe(SO₄)(OH)(H₂O)_x phases were analyzed. This analysis lends some validity to the premise that more complex structures are also the more stable ones, but other systems should be investigated to test these trends.