Calcite precipitation occurs when trimethylamino‐N‐oxide and medium are added to surface waters collected above Lodestone and Keeper Reefs of the Great Barrier Reef. The calcium content of the water was initially 430 ppm. After 30 days incubation at 28 °C, the pH had risen and only 90 ppm of calcium remained in solution. The precipitate was analyzed by X‐ray diffraction and was found to be calcite.