The physical state of bone carbonate. A comparative infra-red study in several mineralized tissues.

A precise understanding of the crystal properties of bone and other calcifications is essential in defining pathological and physiological mineralization. Bone mineral is predominately calcium and orthophosphate, but contains about 6 per cent carbonate. The physical state of carbonate in bone has been controversial but has attracted considerable interest since carbonate probably influences bone solubility and crystallinity and provides a reservoir of CO2 in metabolic and respiratory diseases. In 1953, Neuman and Neuman1 proposed a unifying concept of the nature of the bone salt for which the chemical and physical properties of "impure," microcrystalline-OH-apatite provided the working model. In contradistinction to those who suggested that CO2 was an essential part of the bone crystal complex" Neuman regarded CO2 as an impurity. The view that CO2 together with magnesium, sodium, citrate, and other elements present in small quantities are contaminants absorbed or trapped in the OH-apatite crystal surfaces persists as the predominant working hypothesis. Recent investigations in our laboratory indicate that bone carbonate may exist in two forms that are physicochemically and physiologically distinct. One form may be lost completely in long-standing uremia and acidosis. The other form is intimately related to the crystal, possibly as a double salt, and is not lost even in the most severe uremia. These observations provide a concept that is somewhat different from the conventional OH-apatite model, and its implications with respect to the state of CO3.' Direct evidence for the physical state of C03 in bone and some other mineralized tissues is wanting. The poor crystallinity of many carbonate