Calcium carbonate was precipitated from saturated solutions of calcium bicarbonate in the presence of magnesium and organic additives in order to investigate the effect of Mg-incorporation on calcite morphologies. A range of concentrations of Mg and organic additives were investigated, and the structure, composition and morphologies of the crystals were determined using X-ray diffraction and electron microscopy. The action of the organic additives in Mg-free solutions was highly specific, producing elongated single calcite crystals. A much wider range of calcite morphologies was observed in the presence of both Mg and organic additives, and a transition from single crystal to aggregates occurred on increasing the Mg concentration. The MgCO3 content of the crystals increased with the solution Mg concentration, but showed little correlation with the organic additive concentration. Single crystals of magnesian calcite containing up to approximately 10% MgCO3 were prepared, and typically exhibited rounded faces and edges. The polycrystalline aggregates had morphologies ranging from dumbbells to spheres and occluded up to about 22% MgCO3. The experiments suggest that Mg2+ ions act in combination with organic additives to affect calcite morphologies by mechanisms such as adsorption on specific crystal faces which inhibits growth, and by altering the calcite nucleation and growth processes.