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 (Fe-2(SO4)(3)center dot 7H(2)O) and pentahydrated ferric sulfate (Fe-2(SO4)(3)center dot 5H(2)O) was experimentally determined using the humidity-buffer technique together with gravimetric measurements and Raman spectroscopy at 0.1 MPa in the 36-56 degrees 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. (C) 2011 Elsevier B.V. All rights reserved.