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phosphorite A sedimentary rock rich in phosphate, usually in the form of carbonate hydroxyl fluorapatite (Ca10(PO4CO3)6F2–3). 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.