We have ombined NMR and XRD measurements to study the formation of sodium sulfate heptahydrate during cooling. The NMR results on the measured concentration are given in figure 1. Initially the experiment is started with a sample saturated at 35 oC  after which is sample is cooled down.

Figure 1: The measured sodium sulfate concentration as a function of the temperature.

As can be seen up on cooling the sample starts to supersaturate first with respect to mirabilite and later with respect to heptahydrate. As coon as the concentration reaches the spontaneous crystallization line, heptahydrate is formed and the concentration drops fast. In one experiment after reaching 0 othe temperature is increased again  and the concentration  is following the equilibrium heptahydrate line. Whereas in the other  experiment the sample was further cooled and upon reaching  -4 oC a transformation from heptahydrate into mirabilite is observed and upon heating up the equilibrium line of mirabilite is followed.

These experiments were also reproduced in an XRD experiment.  These experiments were conducted on the beamline 16.4 at the UK Synchrotron Radiation Source, Daresbury. The resulting XRD diffraction patterns are given in figure 2.

Figure 2. X-ray diffraction patterns as obtained during cooling.

As can be seen there again a very fast formation of heptahydrate is observed, whereas also the transformation into mirabilite can clearly be seen. Hence these independent measurements clearly confirm the formation of heptahydrate in a porous material.