I do not see any problems with the synthesis of crystals FeCO3. See review DOI: 10.1179/174327808X286437 . For nanoparticles synthesis method can be used with surfactants.
There are some papers you can follow them to prepare FeCO3. One of those paper is mine: http://pubs.rsc.org/en/content/articlelanding/2016/ra/c5ra26112k#!divAbstract if you can not get this paper please email me (my email: [email protected])