Weathering of glauconite in alkaline soils of temperate climate: A case study from Gorniki, eastern Poland

The aim of the present study was to investigate the process of glauconite weathering in an alkaline soil under temperate climatic conditions. Calcaric Cambisol developed on Miocene sand in eastern Poland, and the sand containing parent glauconite, were sampled and analyzed using optical microscopy, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIRS), Mossbauer spectroscopy, and scanning electron microscopy (SEM) with energy dispersive spectroscopy. The bulk soil material contained quartz, calcite, feldspars, glauconite, smectite, and traces of kaolinite. Both physical disintegration and chemical dissolution of the parent glauconite were evidenced by morphology of the pellets observed by SEM. Textural pedofeatures of the soil observed under the optical microscope indicated that no intense translocation of the finest material occurred within the profile. The green pellets separated from the sample representing the parent material contained almost pure glauconite with only traces of glauconite-smectite mixed-layered minerals. The pellets separated from middle and upper soil horizons contained mixed-layered smectite-rich glauconite-smectite, kaolinite, and minor amounts of goethite, as well as glauconite. Separated clay fractions contained glauconite, smectite-rich glauconite-smectite mixed-layered minerals, kaolinite, and traces of goethite. Observed increases, upward in the profile, in the proportions of glauconite-smectite mixed-layered minerals and kaolinite in the pellets and in the clay fraction suggest that both smectite and kaolinite are products of the glauconite weathering. The aluminous character of the smectitic phases, indicated by the position of the 060 XRD reflection and the FTIRS analyses, which show a gradual decrease in the content of Fe3+ in fine clays upward in the profile, suggest that leaching of iron from the glauconite took place during the weathering. The iron most likely precipitated in the form of goethite and XRD-amorphous iron compounds. Since Fe2+ constituted only 4% of the total iron present in the parent glauconite and accounted for only 0.05 of total layer charge, the oxidation of iron should not be considered the main mechanism of the glauconite smectitization.