Ca²⁺-keggin anion colloidal particles as templates for the growth of star-shaped calcite crystal assemblies

The formation of star-shaped CaCO₃ crystals by reaction of an aqueous solution of phosphotungstic acid and Ca²⁺ ions with CO ₂ is described. The phosphotungstate ions [(PW₁₂O ₄₀)^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.