‘Nature is adept at controlling mineralisation processes to produce single crystals and polycrystalline and amorphous structures with remarkable morphologies and mechanical properties’ (F. Meldrum, Int. Mater. Rev., 2003, 48, 187; F. Meldrum and H Cölfen, Chem. Rev., 2008, 108, 4332). Calcium carbonate is one of the most common biominerals. Summarize the chemistry involved in the biomimetic engineering of CaCO₃.