57Fe Mössbauer spectroscopy, X-ray diffraction and local (WDX, EDX) and bulk chemical methods (ICP-AES complemented with Fe2 + titration) were used to study glauconites originating from three sandstone samples (NY1, NY3 and EWT) of the Upper Oligocene Eger Formation (Northern Hungary). The, during glauconitisation, increasing aggregate density and magnetic susceptibility offered a unique possibility to separate grains of different evolutionary stage from the same glauconite population. 57Fe Mössbauer spectra of both structurally and chemically well characterised glauconite fractions were decomposed into four doublets. Two-two of the doublets were assigned to octahedral Fe2 + and octahedral Fe3 + . Fe2 + /Fe values determined by Mössbauer measurements were used for the calculation of the chemical formula of glauconite samples [including octahedral occupancy (B O) and octahedral charge (X O) as well]. The gradual increase of Fe2 + /Fe was documented with the increase of aggregate density in correlation with the foregoing of glauconitisation. Our results give a quantitative demonstration that octahedral occupancy (B O) decrease and the substitution of octahedral Al by Fe2 + are the major processes allowing the interlayer K incorporation during glauconitisation. The systematic crystalchemical evolution as reflected in the systematic changes of the Mössbauer data are discussed in terms of the subsequent steps of glauconitisation.