IRON AND MANGANESE DISTRIBUTION IN NODULES FROM A YOUNG TEXAS VERTISOL

Iron and Mn distributions in the black nodules from the BCk3 horizon o f the microlow position of a Burleson soil, a fine, montmorillonitic, thermic, Udic Haplustert, from a terrace of the Brazos River were mapp ed with scanning electron microscopy/energy dispersive spectroscopy (S EM/EDS) x-ray analysis. Spherical nodules were concentrically banded, and the irregularly shaped nodules lacked well-developed banding of Fe and Mn. More and better expressed banding was found by decreasing pix el size used for mapping and by increasing the count time per pixel fo r banded nodules. Bands of higher or lower Fe and Mn concentration wer e not coincident. The Mn/Fe weight ratios of individual nodules analyz ed by EDS were between 0.71 to 2.85. Banded nodules had a narrower ran ge of Mn and Fe concentrations and a narrower range in Mn to Fe ratios than unbanded nodules. Barium concentration in the nodules was unusua lly high, averaging 30 g kg(-1), and correlated strongly with Mn conce ntration. Some interelement correlations (e.g., Fe vs. Mn, r = -0.56; Fe vs. Al, r = -0.58) reflected partitioning into the oxide or silicat e minerals of the soil. Fractured nodules revealed the interior to hav e only a few very small pores. The 30 nodules examined are well develo ped solid bodies that are expected to persist in the soil environment. They appear to contain unique and important information about the wea thering history of the Burleson soil and the landscape in which it occ urs. The order and expression of the banding is a relict of the redox history of the nodules, which can be used to induce the redox history of the soil.