We request 4 days of beamtime on SXD in order to locate the protons and quantify their ordering within the structure of hopeite Zn3(PO4)2.4H2O. Hopeite is an important phase in the formation of corrosion-resistant coatings on steels and alloys and is a significant metal-remediation mineral that forms during the application of in situ phosphate remediation of metal contaminated land. Despite these applications there is considerable confusion associated with the nature, stability and crystallographic characterisation of hopeite. Our ongoing studies into this system have led us to the conclusion that one of the protons within the hopeite structure is disordered across two sites. To test this hypothesis we propose a variable temperature study on SXD to precisely locate the protons in a natural and a synthetic hopeite and to quantify the proton ordering as a function of temperature.