CLAY MINERALOGY OF ORTSTEIN HORIZONS FROM LAKE SUPERIOR BEACH RIDGES: IMPLICATIONS FOR THE HYDROLOGY OF RIDGE AND SWALE ENVIRONMENTS
Ortstein samples were studied using powder X-ray diffraction and transmission electron microscopy (TEM) techniques. Powder X-ray diffraction data from the < 2 micrometer size fraction indicate that the clay material is dominantly X-ray amorphous. TEM investigation shows that the clay material is composed of amorphous Fe-Al oxides of variable chemical composition. Four types of textures are recognized: capsule-shaped particles, sponge-like particles, platy particles and spherical particles. Capsule-shaped particles are interpreted as being the result of accumulation of oxides on bacterial surfaces. Other particles appear to be inorganic in origin. Examples of transformations between amorphous and more crystalline textures were observed. Crystalline oxides and clay-sized tectosilicates were observed in some samples. The wide range in size and shape of amorphous Fe-Al oxide particles within and between samples may explain why ortsteins can develop low vertical permeability even when they are relatively young. This study shows that ortstein formation involves complex mineralogic and biologic processes with profound implications for local hydrology.