Hydrogen bonding, thermal expansion, and phase transition of cranswickite (MgSO4 · 4H2O)

Cranswickite, the rarer of the two natural polymorphs of MgSO4·4H2O, has until now been known only from its type locality and characterized solely by X-ray powder methods, leaving its hydrogen-atom positions undetermined. The crystal structure of perdeuterated cranswickite, MgSO4·4D2O, prepared by controlled-humidity dehydration of the heptahydrate, has been refined by Rietveld analysis of time-of-flight neutron powder diffraction data at 10 K and 295 K, with all deuteron positions and displacement parameters refined without restraints, yielding the first complete hydrogen-bond geometry for this phase. Variable-temperature measurements (10–340 K) reveal strongly anisotropic thermal expansion: one principal axis of the expansion tensor is negative over the entire temperature range, reaching α1 = −3.7(3) × 10-5 K-1 at 335 K, which is balanced by positive expansion of similar magnitude within the ac plane, such that the principal expansivities [α1:α2:α3 = −1.04(4):2.90(4):1.00(5) at 295 K] yield zero area expansion of the ac plane. At 330 K cranswickite’s transformation to starkeyite accompanied by a volume reduction of 4.72% was observed, which is among the largest known density increases associated with a phase transition. The relevance of these findings to MgSO4 hydrates as candidate rock-forming minerals on Mars and the icy Galilean satellites is discussed.