The magnesium sulfate - water binary system
In the magnesium sulfate - water system, one eutectic point and four peritectic points are found. The Eutectic point and three of the peritectic points are shown in the figure to the right.
At temperatures above 73°C, the solubility of magnesium sulfate decreases with increasing temperature. This solubility behavior up to 140°C is described very well by the Extended UNIQUAC model as shown in the figure to the right.
In the phase diagram, the solid phase at temperatures between the eutectic point and 2°C is marked as MgSO4·12H2O. New research shows that this solid is actually Meridianiite, MgSO4·11H2O. This mineral was found recently on a frozen pond in British Colombia, Canada by Peterson et al. (R.C. Peterson, W. Nelson, B. Madu, and H.F. Shurvell, Meridianiite: A new mineral species observed on Earth and predicted to exist on Mars, American Mineralogist; October 2007; v. 92; no. 10; p. 1756-1759; DOI: 10.2138/am.2007.2668).
The other solid phases marked in the phase diagram are:
- Epsomite, MgSO4·7H2O - stable solid phase in binary aqueous solution at temperatures between 2 and 52°C
- Hexahydrite, MgSO4·6H2O - stable solid phase in binary aqueous solution at temperatures between 52 and 73°C
- Kieserite, MgSO4·H2O - stable solid phase in binary aqueous solution at temperatures to at least 194°C