The system MgCl2-MgSO4-H2O has been investigated experimentally and modeled
thermodynamically according to the Pitzer method at 50 and 75 degreesC. It
was found that, even when seemingly all requirements for reaching the stab
le thermodynamic equilibrium are fulfilled, the crystallization of higher h
ydrates as metastable phases is possible, and cannot be avoided in each cry
stallization field of a stable lower hydrate of magnesium sulfate. Crystall
ization of MgSO4.x H2O (x = 1, 4, 6) and MgCl2.6 H2O at 50 degreesC and of
MgSO4.H2O and MgCl2.6 H2O at 75 degreesC as stable phases has been observed
. Three metastable crystallization fields of MgSO4.x H2O (x = 4,6, 7) have
been detected at 50 degreesC and two of MgSO4.x H2O (x = 4, 6) at 75 degree
sC. The results obtained and the contradictions existing in the literature
with respect to the solubility and the crystallizing solid phases are discu
ssed in terms of the crystal structures.