Exceptionally large crystals of posnjakite, Cu4SO4(OH)6(H2O), formed during corrosion of a Swagelock(tm) Snubber copper gasket within the MX1 beamline at the ANSTO-Melbourne, Australian Synchrotron. The crystal structure was solved using synchrotron radiation to R1 = 0.029 and revealed a structure based upon [Cu4(OH)6(H2O)O] sheets, which contain Jahn–Teller-distorted Cu octahedra. The sulfate tetrahedra are bonded to one side of the sheet via corner sharing and linked to successive sheets via extensive hydrogen bonds. The sulfate tetrahedra are split and rotated, which enables additional hydrogen bonds.