Weathering of glauconite in an alkaline environment : a case study from Krakow area, Poland

Abstract The purpose of the study was to investigate glauconite weathering in regolith profile developed on Santonian marls in Zabierzow (South Poland) most likely during Paleogene. Because of the high carbonates content the studied regolith appears to constitute a unique case where glauconite underwent weathering under alkaline conditions. Unweathered glauconite–bearing marls and regolith profile were sampled. Samples pH was determined for 1:1 rock–water solution. Mineral composition of bulk rock, clay fraction ( 2+ /Fe 3+ in unweathered glauconite. Scanning electron microscope with energy dispersive X-ray spectroscopy was used for observation of morphology and chemical analysis of the separated pellets. Samples were characterized by alkaline reaction (pH between 7.66 and 7.98). XRD analysis of unweathered marls showed presence of calcite, glauconite, quartz, smectite and traces of kaolinite, apatite and pyrite. The weathered material was depleted in glauconite and pyrite while enriched in goethite. Mossbauer spectroscopy showed that oxidation of part of octahedral Fe 2+ located within the glauconite took place. FTIR spectroscopy showed that in regolith profile intensities of OH–stretching band assigned to glauconite decreased upward while goethite bands intensities increased, relatively to unweathered marls. In the studied profile glauconite underwent dissolution. Si and Mg were leached out with weathering solution while due to alkaline pH Fe 3+ crystallized in situ in form of goethite having likely substitutions of Al. Neoformation of smectite–rich mixed layered mica–smectite minerals from the solution was also likely to take place.