Debate about the origin of widespread oolitic chamosite-rich Phanerozoic ironstones focuses mainly on the mode of formation of ferriferous ooids themselves. Among the several leading hypotheses, two relate the origin of the ooids to the formation of bauxite pisoids: 1) ooids formed diagenetically in situ in colloidal clay-iron hydroxide rich sediments by dissolution and reprecipitation by a process similar to that forming pisolitic bauxite, and 2) oolitic ironstones formed by placer concentration of laterite ooids. Inasmuch as the process in the two explanations is the same as that forming pisoids in bauxites and laterites, ironstone ooids and bauxite/laterite pisoids should show similar internal fabric of the constituent crystalline particles. Scanning electron microscopic examination of the internal fabric of the ironstone ooids and bauxite/laterite pisoids reveals that the former is characterized by tangential-concentric, and the latter by radial-concentric orientation of the constitutent mineral particles. Accordingly, a common genetic process cannot have produced both ironstone ooids and bauxite/laterite pisoids. Formation of radial fabric is inherent in crystallization-precipitation of inequidimensional crystals from solution on nucleating surfaces. In contrast, tangential fabric is probably produced by mechanical accretion of already existing particulate matter on rolling nuclei (snow balling). Widespread occurrence of both kinds of fabric in Recent carbonate ooids suggests both these processes, whereas ironstone ooids, showing mostly tangential fabric, were probably formed by mechanical accretion of detrital clay (kaolinite) and hydrated iron oxide minerals. Subsequent early diagenetic reactions transformed the kaolinite into chamosite by isomorphous substitution. Sedimentary facies relations and a genetic facies model for the oolitic ironstones are most compatible with a mechanical accretion mode of ironstone ooid formation, and also strongly discount replacement of carbonate oolite for the origin of oolitic ironstones.

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