The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin. The texture and the K-Ar age (95.0 ± 1.8 Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+ 0.20–0.74Na+ 0–0.10Ca2 + 0–0.05)0.28–0.75 (Fe3 + 0.63–1.20Fe2 + 0.08–0.24Al3 + 0.19–0.97 Mg2 + 0.29–0.52)2.01–2.12 [Al3 + 0.09–0.35Si4 + 3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2 + and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2 + and Fe2 + to form glauconite and aqueous Na+, Ca2 + and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0 ± 1.6 Ma) and its compositional variability, (K+ 0.29–0.45Na+ 0–0.10Ca2 + 0–0.06)0.30–0.55 (Fe3 + 0.16–0.29Fe2 + 0–0.10Al3 + 1.37–1.68Mg2 + 0.18–0.43)2.00–2.12 [Al3 + 0.17–0.39Si4 + 3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2 + concentrations. 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.