The Great Barrier Reef Expedition 1928-29: The crystal structure and occurrence of weddellite, ideally CaC₂O₄·2.5H₂O, from the Low Isles, Queensland

"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.