A sedimentary rock rich in phosphate, usually in the form of carbonate hydroxyl fluorapatite (Ca₁₀(PO₄CO₃)₆F_2–3). Phosphorites occur as nodules and crusts formed in oceanic areas where sedimentation rates are very low, by the early, near-surface diagenetic alteration (see diagenesis) of ooids, pellets, and bioclasts, or as accumulations of bones and fish scales in fluviatile to shallow marine environments. See also guano.