The formation of star-shaped CaCO3 crystals by reaction of an aqueous solution of phosphotungstic acid and Ca2+ ions with CO2 is described. The phosphotungstate ions [(PW12O40)(3-) Keggin ions] complex with the Ca2+ ions in solution leading to the formation of highly uniform nanometer-sized colloidal particles of the salt, calcium phosphotungstate. Reaction of these nearly spherical colloidal particles with CO2 results in the growth of calcite with very regular, star-shaped morphology. Energy dispersive analysis of X-rays of the star-shaped calcite particles indicated the presence of the Keggin ions in the crystals suggesting that they play the role of nanoprecursors in the growth of calcite crystals with such unique morphology.