The question of whether clay minerals can be biogenically transformed as a result of lichen activity at the lichen-rock interface remains unresolved. We applied several microscopical and analytical techniques--scanning electron microscopy-back-scattered electron (SEM-BSE), energy dispersive spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM)--in an attempt to address this issue. Unaffected granitic biotite and bioweathered material from the granitic biotite and Parmelia conspersa lichen thalli interface were examined using HRTEM after ultrathin sectioning. The n-alkylammonium treatment of ultrathin sections was carried out in order to study the biogenous mineralogical transformation of the biotite. Microsamples proceeding from unaffected biotite zones demonstrated homogenous 10-Aa d(001)-value biotite phase. HRTEM images of lattice fringes of samples taken from the lichen-biotite contact zone reveal large areas of both unexpanded (10-Aa) and randomly and R = 3 distributed expanded (from 14- to 30-Aa) layers of phyllosilicates identified as interstratified biotite-vermiculite. Results of artificial biotite weathering (replacement of K by Ca ion) also revealed the biotite-vermiculite phase formation, indicating that K release in biotite is one of the mechanisms responsible for interstratified mineral phase formation. Two parallel processes, physical exfoliation of biotite and interlayer ionic exchange of K and subsequent vermiculite formation, are the mechanisms for biotite bio-weathering induced by lichens.