The mineral mirabilite (di-sodium sulfate decahydrate) has been extensively studied by X-ray and neutron powder and single-crystal diffraction methods. The structure contains an interesting square ring of water molecules that is fully disordered at high temperatures, but which becomes partially ordered on cooling. One of the factors that appears to control this ordering process is a thermally-induced libration of the sulfate ions to which the square rings are hydrogen-bonded. In an effort to understand this relationship further, I propose to study the ordering process - both on slow cooling and rapid quenching - in selenium- and chromium-substituted analogues of mirabilite. High-resolution neutron powder diffraction is essential to characterise both the H-atom disorder and to measure the very subtle structural distortions known to be associated with the change in partial order.