MINERALOGY AND REPLACEMENT IN FERRIFEROUS OOIDS FROM DIVERSE PALEOZOIC AND MESOZOIC LOCALITIES; EVIDENCE FROM ANALYSES BY SEM WITH EDS

Ferrous iron phyllosilicates like berthierine (7Ǻ) and chamosite (14 Ǻ) that form the cortex of ferriferous ooids are thought to form by the combination of iron (oxyhydr)oxide with kaolinitic clay, probably as a byproduct of the metabolism of consortia of iron reducing and oxidizing bacteria. When plotted on a ternary diagram with (FeO + MgO), SiO₂ and Al₂O₃ at the vertices analyses plot close to a mixing line between hematite and kaolinite with an Al₂O₃/SiO₂ ratio of ~ 0.85. However, this ratio varies rather consistently depending on locality e.g. Jurassic localities: Cleveland seams (1.03), Marlstone (0.80), Northants ironstone (0.80-0.73); Ordovician locality: Hooker seam (0.83). Average (FeO + MgO) values for the clay mineral range between 45-55%. Residual clays in hematitic ooids from the Silurian Red Mountain Fm. (and to a lesser extent in the Northants ironstone) are anomalous. Percentages of (FeO + MgO) are very low with an Al₂O₃/SiO₂ ratio ~ 0.67 and abnormal percentages of K₂O (~7%). Ooids from the Hickory Nut seam are typical with a range in (FeO + MgO) from 11- 90% depending on relative percentages of clay mineral and hematite. Clay mineral and hematite alternate in the ooidal cortex with oxidation and iron content increasing towards the periphery. Adjacent hematite and clay mineral laminae show inverse percentages of (FeO + MgO) suggesting that iron was leached from the clay mineral to form iron oxide (? dissimilatory reduction followed by oxidation). The low (FeO + MgO) values, presence of K₂O and low Al₂O₃/SiO₂ ratio is atypical of chamosite and suggests the presence of illite or a mixed layer illite/chlorite.