The precursor of apatite: Octacalcium phosphate (OCP) in the earth and environmental sciences - A review

Octacalcium phosphate (OCP) is a solid phase that is well known in the biomedical field because it is widely used and tested as a precursor to bioapatite to treat various diseases affecting bones and teeth. In contrast, the knowledge of OCP in the earth sciences and its actual and possible applications in the environmental field are much less well known. With this review, we aim to fill this gap by showing that OCP is a much more widespread phase than is thought in various geological systems, and its properties allow its effective use in the environmental field, especially when it is used in the removal of various contaminants from wastewater and drinking water. This review not only lists the occurrences of OCP in nature or its uses in the environmental field but also proposes a critical analysis of the literature. In particular, a thorough examination of the failure to recognize OCP in numerous studies has highlighted the importance of performing low-angle XRD measurements in order to achieve proper identification of calcium phosphate minerals. A useful analytical protocol for recognizing and distinguishing OCP from similar phases such as those in the apatite group is suggested. Finally, we propose that OCP be recognized as a natural mineral species by the International Mineralogical Association-Commission on New Minerals Nomenclature and Classification (IMA-CNMNC). Such recognition, in our opinion, would provide a renewed and stimulating impetus to studies on the geochemical and mineralogical cycle of phosphorus.