A composite consisting of a polymer and calcium carbonate has been synthesized by allowing carbon dioxide to diffuse into a poly(ethylene oxide) film containing a source of calcium ions. The X-ray pattern of the synthetic composite material shows that the least thermodynamically stable polymorph of calcium carbonate, vaterite is the mineral phase which is formed, rather than the most thermodynamically stable calcite phase. This effect has been seen in other reports of crystallization of inorganic phases which have been mediated by a polymer matrix and/or other organic moieties. In this paper, some possible explanations are presented which may help explain this phenomenon.