Chemical substitution is a valuable technique for probing the behaviour of materials since this exerts a stress comparable in many respects to changes in pressure or temperature. We have used this method to study the properties of a common earth and planetary mineral known as mirabilite - sodium sulfate decahydrate. It has long been known that analogous decahydrates can also be prepared from sodium selenate, sodium chromate, sodium molybdate, sodium tungstate, sodium manganate and sodium monofluorophosphate. Until now, the crystal structures of the molybdate, tungstate and manganate were not known. However, we have recently been able to determine their heavy-atom arrangements from X-ray powder diffraction data. We propose to use neutron single-crystal diffraction to complete the structural characterisation (including H-atoms) with the highest accuracy using SXD.