With this experiment at ID02 we will explore the crystallization and transformations of newly discovered amorphous and metastable calcium orthophosphates (CaPs) yielding hydroxyapatite (HAP). As the main constituent of bones and teeth, HAP is among the most thoroughly investigated minerals, however, the precise mechanisms underlying its formation do remain debated. Especially the role of hydrogen phosphate-based intermediates at near-neutral pH, which have only been reported recently, remains unclear. At ID02, WAXS/SAXS measurements will resolve the transformation pathways of these and other metastable phases under controlled pH and temperature conditions. Special attention will thereby be paid to changes in crystal structures, shape and polydispersity of the particles during their transformation into HAP. The results will clarify the role of intermediates in HAP formation, and advance the understanding of CaP transformation pathways at conditions relevant for the formation of bone.