For many salts, there are solubility curves as a function of temperature that are smooth (don't have any kinks). Sodium sulfate ($\ce{NaSO4}$), however, has a kink in the solubility-T curve at 30 degrees as shown below:
What happens when the temperature is about 30 C?
"Sodium sulfate" is a mixture of hydrated phases in shifting equilibrium given temperature and concentration.
The phase diagram is also interesting here. Notice the convergence of three lines at about 30C and 30 wt% $\ce{NaSO4}$, right near where the kink in the solubility happens.
In case anyone needs it: the figure is from Angeli et al., 2010 [1], adapted from Hougen et al., 1954 [2].
[1] M. Angeli, R. Hébert, B. Menéndez, C. David, and J.-P. Bigas, ‘Influence of temperature and salt concentration on the salt weathering of a sedimentary stone with sodium sulphate’, Geological Society, London, Special Publications, vol. 333, no. 1, pp. 35–42, Jan. 2010.
[2] O. A. Hougen, K. W. Watson, and R. A. Ragatz, Chemical Process Principles. Part I. 2nd edn. Wiley, New York, 1954.
