FeCO3 is insoluble in water. Most ionic salts containing CO3 are insoluble in water with the exception of those involving ammonium or group one metallic elements.
The two greatest factors that determine the solubility of an ionic compound in water are the attraction of the individual ions with the compound to each other and the attraction of the ions to H2O molecules. Generally, the higher the attraction of the ions to each other, the lower the solubility of the overall compound. Conversely, the greater the attraction of the ions to individual water molecules in the solution, the greater the solubility of the compound.