The study of 20 samples of glauconites from the Bohemian Upper Cretaceous has confirmed the general (Burst /1958/ and Hower /1961/) thesis that the chemical variability of glauconites is due to their mineralogical heterogeneity. From this point of view the dependence between the percentage of expandable layers and the content of all main chemical components in the structure of the glauconites, has been examined. In glauconites with decreasing percentage of 14 Å layers, the content of K, AlIV and Fe3+ rise linearly, while the Si, AlVI and Mg content decrease linearly.

From the quantitative relations between the main chemical components and the proportion of expandable layers in natural (10 Å/14 Å) glauconites, the following crystalo-chemical formula has been derived by extrapolation for a homogeneous 10 Å glauconite:

K0.78(NaCa)(Fe3+1.03Al0.45Mg0.34Fe2+0.13) (Si3.69Al0.31)O10(OH)2.

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