The role of Fe on the formation and diagenesis of interstratified glauconite-smectite and illite-smectite: A case study of Upper Cretaceous shallow-water carbonates

We propose that the substitution of Al3 + for Fe3 +, Fe2 + and Mg2 + in the octahedral sheet shifts the stability field of the kaolinite-Fe-Al-Mg-smectite-Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2 + concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.