In the Clinton oolitic ironstone (Silurian) of Pennsylvania, ooids that were initially composed of chamositic clay minerals were diagenetically altered to ferric oxide. Evidence for this alteration includes the preservation of intermediate stages in a variety of partly altered ooids, finely laminated chamositic ooids in marginal zones of ferric oxide storm beds, bed-parallel transitions of ooid compositions in bioturbated oolite, and spastolithic shapes of ferric oxide ooids. Electron microprobe analysis demonstrates that this oxic alteration did not leave significant abundances of residual silicon and aluminum. Based on textural characteristics and on a Permian age of remagnetization, the most likely time for the alteration was during early phases of the Alleghany Orogeny. Recognition that the ooids were initially chamositic clay minerals resolves at least part of the "oxidation-reduction paradox" (Maynard 1986), for ferrous phyllosilicate could presumably accumulate in the low-oxygen conditions associated with the medial Silurian oceanic anoxic event.