In our high-pressure studies of candidate outer solar system minerals using PEARL, we have found that MgSO4.7H2O (epsomite) decomposes to a lower hydrate mixed with water ice. Due to resolution limitations and uncertainty concerning the true composition of the hydrate, we were unable to index the neutron powder data. Using a new STFC grant, we have constructed a piston-cylinder apparatus and liquid-nitrogen recovery system, synthesised the high-pressure mixture and recovered it to ambient pressure. Using X-ray powder diffraction, we have been able to determine that the hydrate is a new polymorph of MgSO4.5H2O, with orthorhombic symmetry rather than the triclinic symmetry of the known room-pressure phase.Our goal is to complete the structure of the new pentahydrate, identifying the sites of all H-atoms, and to precisely quantify the thermal expansion.