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Iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial. Our analysis of published data identifies end-member compositions and mixtures and allows us to recognize controls of formation of different mineral species. The compositional fields of glaucony, Fe-illite, Fe–Al smectites are determined in the M+/4Si vs. Fe/Sum of octahedral cations (M+ = interlayer charge). Solid solutions could exist between these phases. The Fe–Al and Fe-rich clay minerals form two distinct solid solutions. The earliest phases to be formed are Fe–Al smectites or berthierine depending on the sedimentation rate. Reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments. The Fe is incorporated first in pyrite and then in silicates after oxidation. Potassium ions diffuse from the sea-water–sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe-illite or glauconite or a mineral assemblage (berthierine–nontronite) according to the available Al ion amounts in the microsystem. Mixed-layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Despite the common belief of their value as palaeoenvironment indicators, these minerals can form in a variety of environments and over a period of millions of years during sediment burial.