Abstract <p>Study of the internal structure of the recently described new species of sponge <i>Gzhelistella cornigera</i> (Davydov et al., 2023) and fusulinids from the Gzhelian stage (Moscow region) revealed numerous polymineral biomorphoses composed of mixed-layer clay minerals, goethite, chalcedony, and sanidine. A detailed structural and crystal-chemical study of clay minerals from biomorphoses and the host rock revealed a structural heterogeneity of the mixed-layer minerals. Using the X-ray diffraction pattern modeling method, it has been established that the clay material in the studied samples is represented by two authigenic mixed-layer (glauconite-nontronite) types with a contrasting relationship between the content of different layers and/or their alternation order, but identical structural and crystal-chemical characteristics of the crystallites. It is shown that such formations should be considered a single heterogeneous mixed-layer structure, with the relationship and/or alternation order of different layer types in crystals varying within certain limits. It has been established that the structural and crystal-chemical characteristics of mixed-layer minerals from brown biomorphoses and host rocks are almost indistinguishable, whereas these parameters are significantly different for green biomorphoses. It is assumed that the newly formed clay minerals from host rocks, like their brown analogs, were formed under similar physicochemical conditions. It is also obvious that during the formation of mixed-layer phases, brown biomorphoses were already deprived of the biogenic organic matter, while green biomorphoses retained it in sufficient quantity to change the environmental conditions within themselves in some places.</p>