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The differentiation of Fe and Mn within the nodules and its relation to the fabric is a well-known phenomenon. However, the relationship of this differentiation to the nodules mineralogy was not studied yet. This study aimed to fill this gap through the micro-mineralogical and geochemical investigation of nodules from soils with different hydromorphic conditions by electron probe microanalysis and micro-X-Ray diffractometry. Different conditions of hydromorphism resulted in the same nodule types and their similar vertical distribution in soils. In soils with shallow groundwater table, nodules exhibited more developed fabric, and crystalline hydrous Fe oxides (goethite) prevailed in them. Contrarily, nodules showed smaller size, lower Fe enrichment, and a higher frequency of non-crystalline hydrous Fe oxides in soils with regular flooding and stagnant surface water. The difference could be related to the lower intensity of water oscillation and the shorter redox periods in the latter soils. The formation of the nodules’ fabric may have been followed by the slow crystallization of Fe-oxyhydroxides when favorable redox oscillation conditions occur allowing subsequent recrystallization. The association of hydrous Fe oxides to clay minerals may have also affected the crystallization process within the nodules. Although the observation of assumed differentiation of hydrous Fe oxides failed within the nodules, soils with different conditions of hydromorphism could be characterized by different Fe-oxyhydroxide mineralogy, at least in the progression of their crystallization process. |
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