AL, FE MINERALOGY AND GEOCHEMISTRY OF DUNAL SOILS IN OREGON, USA

Weathering in the Oregon dunes is forming authigenic aluminum minerals, such as allophane, imogolite and gibbsite, as well as authigenic iron oxides, which all effect groundwater chemistry and slope stability in the dunes. Allophane and imogolite, both amorphous Al-Silicates are replacing roots, while gibbsite (Al hydroxide) precipitates in the pore spaces of coarser sands and fills cavities. The amorphous Al silicates and the gibbsite do not occur together. The imogolite has a platy morphology while the allophane occurs as “strings or rootlets” within the imogolite matrix. These roots can be 10s of cm long and up to 4 cm in diameter. We differentiated the two by their Al to Si ratio, XRD and TGA. The imogolite has a Al to Si ratio of about 1.8 and the allophane has a Al to Si ratio of about 1.4. based on Microprobe and EDX from SEM. The iron oxides occur within the finer sand grains, as nodules and as hardpans. The information of the Al, Fe mineralogy and geochemistry is used to interpret aquifer geochemistry, age of paleosols to date dune emplacement, and potential preservation of artifacts. The Al minerals indicate a high mobility of Al and a low pH in the groundwater, while the Fe minerals indicate a high redox potential in the dunal soils. However the timeframe over which these processes occur and the fate of the silica are still in question.