ALUMINUM SUBSTITUTION IN GOETHITE FROM THE LATE ORDOVICIAN NEDA FORMATION: IMPLICATIONS FOR ENVIRONMENT OF FORMATION AND POST-BURIAL WEATHERING

Mole percent substitution of Al for Fe in goethite was determined for samples from Late Ordovician Neda Formation oolitic ironstone paleosols using the X-ray diffraction (XRD) method of Schulze (1984). Goethite (111) and (110) XRD reflection shapes and positions were corrected with halite as an internal standard. Halite was previously calibrated to National Institute of Standards and Testing, XRD Standard Reference Material 640b. Samples were collected along depth profiles in Neda Formation paleosols from Kankakee River State Park, IL; Neda, WI; Katell's Falls WI; and Hendrick's Ravine, IA. Preliminary results indicate mol percent Al substitution of ~4% for the type locality samples, which is low relative to the range of 0 to 36% observed in soil formed goethites found in nature.

Al substitution variations in goethite may be an indication of post-burial alteration (e.g., near-surface weathering) if the goethite was originally formed in an environment of low Al activity and re-precipitated in a high Al activity environment. Weathering events during the Late Ordovician and/or Early Silurian (the time in which the soil horizons developed) likely occurred in the absence of immediate contact with Al sources (e.g., clay minerals). Post burial alteration of the Neda Formation during the Permian (Kean, 1981) also appears to not to have changed the Al composition of the goethite. The absence of an immediate contact between the Neda Formation and an Al source does not necessarily prevent recrystallization of goethite during its post burial and recent surface exposure. Tests that can provide evidence for recrystallization of goethite (i.e., the formation of secondary goethite) would be useful for constraining the timing of events such as those involving the inclusion of carbon into soil hydroxide minerals.

Kean, W.F. (1981) Paleomagnetism of the Late Ordovician Neda iron ore from Wisconsin, Iowa, and Illinois. Geophysical Research Letters, v. 8, p. 880-882.

D. G. Schulze (1984) The influence of aluminum on iron oxides; VII, Unit-cell dimensions of Al-substituted goethites and estimation of Al from them. Clays and Clay Minerals, v. 32, no. 1, p. 36-44.