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Back-scattered electron images of Proterozoic ironstones. (A) Composite hematitic ooid with nuclei composed of ooid fragments in the ca. 1,880 Ma Gibralter Formation, indicating ooid reworking during ironstone genesis. (B) Hematitic ooid and hematite-cemented quartz sand in the ca. 750 Ma Galeros Formation. (Scale bars, 100 μm.)
