The geochemical constraints that determine the environments of formation of calcium and aluminum phosphates are assessed in relation to the stabilities of these compounds. In neutral to acidic environments, the weathering of calcareous phosphorites may result in the formation of crandallite and wavellite (low phosphate concentration) or montgomeryite (high phosphate concentration). Phosphatization of argillaceous sediments will invariably lead to the formation of millisite and taranakite, and may involve palmerite as an intermediate phase. Variscite is unstable relative to the other aluminum hydroxyphosphates and is unlikely to be a major constituent in phosphorite beds. The phase relations show the characteristic association of quartz – apatite – clay minerals to be thermodynamically stable in near neutral environments. The predicted reaction paths during progressive desilication of phosphatized clays and decalcification of apatite are clays→ (palmerites)→ millisite→ wavellite and apatite→ crandallite→ wavellite→ augelite; these match the paragenetic sequence observed in many localities. Within the framework of phosphate – clay mineral reaction, calcitization and silicification of apatite may be effected even in acidific environments.The effect of Eh on the formation of phosphatic beds is indirect and mediated by the iron cycle. The association of iron minerals with phosphorites is considered and the suggestion made that iron oxide accumulates may be the source material for some phosphorite formations.