An investigation of the deliquescence of ferricopiapite and the onset of deliquescence of epsomite crystal faces of differing Miller indices

Deliquescence is the process whereby a solid interacts with a humid atmosphere and begins to dissolve when the partial pressure of H 2O in the atmosphere exceeds the partial pressure of H2O at which the solid is in equilibrium at a given temperature. Deliquescence of sulfate minerals poses a risk to both plant and animal life as metals and acid that are stored in the mineral structure are released. Detailed solid-solution transition boundaries for sulfate minerals aid in the understanding of acid mine waste behavior in the event of environmental changes. The deliquescence behaviors of ferricopiapite, melanterite, halite and epsomite have been studied as a function of temperature and relative humidity. A device has been developed to determine the onset of deliquescence as the temperature and/or relative humidity around a sample of sulfate mineral is varied. Experiments were conducted on several synthetic analogues of minerals. Ferricopiapite was studied as it is the most likely mineral to exhibit this behavior in common acid mine drainage sites where sulfates are present. Because no experimental data exist for the deliquescence of ferricopiapite, in order to properly test the device experiments in which the relative humidity was raised above the deliquescence point and then reversed were conducted using synthetic melanterite and halite for which solid-vapor equilibrium constants for temperature and relative humidity are known. Results of the reversal experiments for melanterite and halite show that the surface resistivity probe designed for this study accurately determines the onset of deliquescence as predicted by the known solid-liquid-vapor phase diagrams. The deliquescence of copiapite is not a reversible process as it does not re-precipitate from the liquid created by deliquescence as the relative humidity is lowered. Epsomite was studied to determine if there was any crystallographic control of the onset of deliquescence between different crystal faces. A significant and consistent difference in the onset of deliquescence was observed between the {111} and {100} faces with the {111} face always being the first to deliquesce as a function of increased relative humidity.