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Chemical components of most ooidal ironstones and phosphorites were initially derived from deeply weathered uplands and deposited in shallow waters of epeiric seas and continental margins that received very little clastic sediments. They were then commonly transformed during diagenesis largely by upwelling seawater and redeposiled in winnowed layers.

Ironstones and phosphorites differ, however. Ironstones were first laid down as ferric oxide and kaolinite ooids and peloids, whereas phosphorites were formed first as phosphatic carbonate granules, crusts, and hardground. Ironstones were then transformed to berthierine (or rarely nontronite) during diagenesis, whereas phosphorites were transformed to carbonate fluorapatite (CFA).

In terms of their distribution in the geologic record ironstones and phosphorites are also dissimilar. A few ooidal ironstones were deposited in Early Proterozoic time. Most accumulated during greenhouse phases of the Phanerozoic (Ordovician–Devonian and Jurassic–early Cenozoic). With the exception of the late Cretaceous–early Cenozoic deposits, major phosphorites are notable for their absence in rocks of those ages. In fact, phosphorites were most abundant in the Late Proterozoic and early Cambrian times.

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