“Envelope crystals” collected during The Great Barrier Reef Expedition in May 1929 have been studied using low-temperature synchrotron single-crystal X-ray diffraction. The crystals are shown to be weddellite, with the largest content of zeolitic water reported to date. A new H2O site has been located within the crystal structure. Study of the crystals show that the end-member formula for weddellite should be reported as CaC2O4·(2.5 – x)H2O, where 0 ≤ x ≤ 0.25, instead of CaC2O4·2H2O or CaC2O4·(2 + x)H2O. This is also the first report of weddellite occurring in a coral reef.