Oolitic iron ores are made up of silicate, oxide, and carbonate minerals that are part of solid solution series. The compositions of the minerals can depart from pure end members and show variation among deposits. This variability can be quantified using electron microprobe techniques. Iron silicates are represented by chamositic clays, both berthierine and chamosite. Glauconite was not seen in any of these samples. Berthierine predominates in younger deposits, whereas the Paleozoic ores studied contained only chamosite. Compositions of the chamositic clays, however, were independent of age. Iron oxides are mostly goethite in younger deposits or hematite in older deposits. Both minerals lack appreciable Al in solid solution. Most analyses show small amounts of Al but usually balanced by Si in the same proportion as in chamosite, indicating a slight admixture of clay. Iron carbonates are siderite with appreciable substitution of Ca and Mg but not of Mn. Apatite is common, but no iron phosphates were found. The compositions of the silicates and carbonates are close to those reported for early diagenetic minerals from other types of sedimentary rocks, indicating that ironstone formation does not require unusual chemical conditions other than a large supply of iron.The analytical results can be used to argue against several hypotheses of ironstone genesis. The lack of Al in the goethitic and hematitic ooliths is inconsistent with an origin either by redeposition of soil ooliths or by oxidation and leaching of earlier chamositic ooliths. The presence of high Al contents in the chamositic ooliths shows that they cannot have formed from earlier aragonitic ooliths, and the presence of small amounts of chamositic clay and apatite as discrete laminae in goethitic and hematitic ooliths suggests that this type of oolith could also not have formed by replacement of aragonite.Times of ironstone deposition show a correlation with climate and sea-level stands that suggests a mechanism for generating locally high iron concentrations in seawater. A combination of a warm, humid climate and low average continental relief would intensify lateritic weathering and the formation of iron-enriched detritus. Coincidence of this increased detrital iron supply with intensification and shallowing of the oxygen minimum layer in the ocean could lead to mobilization of iron from offshore shelf clastics, to be redeposited near the shoreline in intertidal to shallow subtidal environments.