Experimental determination of the phase boundary between kornelite and pentahydrated ferric sulfate at 0.1 MPa

Recent findings of various ferric sulfates on Mars emphasize the importance of understanding the fundamental properties of ferric sulfates at temperatures relevant to that of Martian surface. In this study, the phase boundary between kornelite (Fe2(SO4)3·7H2O) and pentahydrated ferric sulfate (Fe2(SO4)3·5H2O) was experimentally determined using the humidity-buffer technique together with gravimetric measurements and Raman spectroscopy at 0.1 MPa in the 36–56 °C temperature range. Through the thermodynamic analysis of our experimental data, the enthalpy change (− 290.8 ± 0.3 kJ/mol) and the Gibbs free energy change (− 238.82 ± 0.02 kJ/mol) for each water molecule of crystallization in the rehydration of pentahydrated ferric sulfate to kornelite were obtained.

Research Highlights
► The boundary between Kornelite and pentahydrated ferric sulfate was determined.
► It was experimentally determined at 0.1 MPa in the 36–56 °C temperature range.
► The phase boundary was expressed in a temperature–% relative humidity space.
► The derived thermodynamic data are consistent with previous works.
► Our derived thermodynamic parameters have much smaller uncertainties than others.