Magnesian calcites were precipitated from 10 mM Ca(HCO3)2 (Ca 2+, 400 mg/1) solutions contain ing 35 or 53.5 mM Mg ions (Mg2+, 850 or 1300 mg/1) in the presence of fluoride and phosphate ions in various concentrations. Magnesian calcite formation from aqueous solution was favored by the presence of fluoride ions (0.5 to 2.5 mM) or phosphate ions (0.025 to 0.5 mM) in a parent solution. Aragonite and/or monohydrocalcite are formed below or above the critical concentrations of these ions. The presence of phosphate or fluoride ions in a solution inhibits adsorption of Mg2+ ions on calcite surface, which may cause the magnesian calcite formation.