Commercial samples of four different MgSO 4 hydrates were investigated (as listed in Table S1 in SI). Powder X-ray diffraction measurements were made on a Panalytical XPert Pro diffractometer in Bragg-Brentano geometry using non-monochromated CuKα radiation (λ ave = 1.5418 Å). Samples were prepared on glass flat-plate sample holders, and data were measured over the range 2θ = 5 For terahertz measurements, the crystalline samples were ground gently in an agate mortar with a pestle, and the polycrystalline samples were then mixed with polyethylene (Induchem, Volketwil, Switzerland) to a defined concentration that varied for different hydrates, as detailed in Table in the SI.