Leaf adpression fossils vary in their organic content, relief, and quality of preservation. Some of the most enigmatic adpressions, known as leaf molds, retain fine morphological and anatomical details despite being found in coarse sandstones—a widespread phenomenon attributed to the presence of fine-grained minerals on the fossil surface. Previous taphonomic studies have demonstrated the importance of microbial biofilms in promoting mineralization and argued that authigenic iron oxides can serve as the preserving medium. Here, we propose that this role is played more commonly by biologically precipitated aluminosilicate phases (clays). To test this hypothesis, we conducted energy dispersive X-ray spectroscopy (EDS) analysis of thin sections through fossil leaves from five localities differing in age and depositional environment. Point spectra taken directly from the leaf-sediment interface revealed that cation-rich clays separate the leaf fossils from the matrix. Additional EDS analyses of biofilms on a fossil leaf and on modern oak leaves decaying in freshwater also revealed aluminosilicates, for which we infer a biofilm-mediated, authigenic origin. These results are the basis of a novel ‘Biofilm-Clay Template' taphonomic model, whereby microbially mediated clay authigenesis is commonly the first step in leaf adpression preservation.