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. [ABSTRACT FROM AUTHOR]