  Potassium sulfate single crystals that are grown from aqueous solutions lose
upon the first heating up to 1% of mass that is assumed to be water. This mass
loss occurs in the vicinity of the PT from orthorhombic to hexagonal K2SO4.
Only in the first heating run of K2SO4 that has not yet released water,
pretransitional thermal effects can be observed in the DTA curve. If K2SO4
crystals are grown from solutions containing 4 wt.% Cd, Cu, or Fe, only Cu or
Fe can be incorporated significantly with concentrations of several 0.1%. The
phase transformation temperature measured for such solid solutions depends on
the heating rate. For pure K2SO4, the phase transformation temperature is
independent on heating rate 581.3 deg. C and the enthalpy of transformation is
(5.8+/-0.2) kJ/mol.
