Models of the bone salt and its synthetic analogues have been strenuously, and sometimes emotionally debated since the late nineteenth century. The main protagonist in the drama is the ubiquitous CO3=ion whose role has never been clearly understood. Initially regarded as an essential part of the calcium phosphate crystal complex, it came to be dubiously designated as a separate phase CaCO3, as an adsorbed ion, or even as a mere contaminant. More recent studies provide evidence that the original impression may be more nearly correct. Of particular interest in defining the role of CO3= in bone are the reactions involved in the formation of CO3-apatite under conditions approximating the physiological. These observations suggest that the synthesis of bone mineral involves hydrolysis of an initial acidic calcium phosphate precipitate to octacalcium phosphate, which is then converted to octacalcium phosphate carbonate (OCPC) by virtue of the replacement of PO4 identical to (HPO4=) by CO3=. OCPC satisfies many criteria for a satisfactory definition of the nature of the bone mineral. It can explain its solubility behavior and the intrinsic relationship between PO4 identical to (HPO4=) and CO3=, the normal variations in bone composition, the sequence of events in bone mineral maturation, and the loss of CO3= under normal and pathological conditions.