Quenching of Na2SO4 - CaSO4 melts down to room temperature results in diffe
rent forms of metastable solid solutions dependent on the CaSO4 content. Th
e transformation of these solid solutions into stable phases were investiga
ted after various times and temperatures by X-ray powder diffraction and th
ermal analyses. An equilibrium phase diagram is derived, which is in accord
ance with all experimental facts.