Ikaite (CaCO₃·6H₂O) transforms to calcite, aragonaite and/or vaterite, which are the polymorphs of calcium carbonate, at normal temperature and pressure. The presence of water promotes the transformation of natural ikaite to calcite in a room but the presence of magnesium ion in a solution reduces the transformation rate. The formation of calcium carbonate polymorphs through the transformation is influenced by temperature and magnesium ion concentration in a solution which contacts with ikaite, being analogous to those on the formation of calcium carbonate polymorphs from calcium bicarbonate solution. Rapid transformation of ikaite is favorable to the formation of vaterite, a metastable phase.