The undecahydrate of magnesium sulfate (meridianiite) is likely to occur a major rock-forming mineral in the interiors of large icy satellites such as Ganymede, Callisto and Titan, to be a major water reservoir in the martian regolith, and is now known to be occur in terrestrial peroglacial environments. We have previously characterised the thermal expansion tensor of this triclinic crystal at ambient pressure using powder diffraction, and have made structure refinements as a function of T and P (up to 5 kbar) using single-crystal methods. We now propose to make high precision measurements of the elastic strain tensor (and its temperature dependence) up to 5 kbar as a complement to the recent high-pressure structural study on SXD. The data will allow us to determine an important thermo-elastic cross term, the Anderson-Gruneisen parameter.