Formation of iron oxyhydroxides as a result of glauconite weathering in soils of temperate climate

Abstract

The aim of this study was to investigate glauconite weathering in soils of temperate climate, with special attention paid to formation of iron oxyhydroxides at the expense of the glauconite. Five soil profiles developed on parent material in which glauconite was the main (if not the only) phyllosilicate and source of Fe were selected for the study. Basic physical and chemical properties of bulk (<2 mm) soil samples (color, texture, pH, and carbonate and soil organic carbon content) were determined. Clay fractions (<2 µm and <0.2 µm) and green pellets were separated from the studied soils and analyzed using X-ray diffractometry, Fourier-transform infrared spectroscopy, Mössbauer spectroscopy, and inductively coupled plasma - optical emission spectrometry. Thin sections prepared from selected undisturbed soil materials were observed under optical microscope. Three of the soils were acidic with pH ranging from 4.3 to 5.5 and the remaining two were alkaline with pH from 6.8 to 8.6. Parent glauconites from each profile showed quite similar chemical composition, except that in one case Fe(II) was much more abundant than in the others. In both acidic and alkaline soils, glauconite weathering products included Al-smectite-rich glauconite-smectite mixed-layered minerals (G-S), kaolinite, and Fe oxyhydroxides. G-S were likely formed by transformation of glauconite, whereas kaolinite and iron oxyhydroxides (i.e. goethite, lepidocrocite, and perhaps ferrihydrite) crystallized in the studied soils from substances dissolved from parent glauconites. Leaching of Fe and Mg from the glauconite structure and loss of K were indicated. In acidic soils the weathering products concentrated in the clay fractions, whereas in alkaline soils they occurred both in clays and in the glaucony pellets. Mössbauer spectroscopy indicated that fine crystalline and especially nanocrystalline iron oxyhydroxides are substantial products of glauconite weathering in all the studied clays. Formation of the Fe oxyhydroxides was controlled by hydrological conditions (and related redox activity) and by soil pH. Alkaline conditions seemed to favor goethite crystallization, whereas redox activity led to formation of lepidocrocite and likely ferrihydrite. Soil color was mostly controlled by the presence of glauconite. Only the relatively significant lepidocrocite content in a few of the studied soil horizons was marked by brown or rusty colors indicating high redox activity.