MINERALOGICAL TRANSFORMATION OF BIOWEATHERED GRANITIC BIOTITE, STUDIED BY HRTEM - EVIDENCE FOR A NEW PATHWAY IN LICHEN ACTIVITY

The question of whether clay minerals can be biogenically transformed as a result of lichen activity at the lichen-rock interface remains un resolved. We applied several microscopical and analytical techniques-s canning electron microscopy-back-scattered electron (SEM-BSE), energy dispersive spectroscopy (EDS) and high-resolution transmission electro n microscopy (HRTEM)-in an attempt to address this issue. Unaffected g ranitic biotite and bioweathered material from the granitic biotite an d Parmelia conspersa lichen thalli interface were examined using HRTEM after ultrathin sectioning. The n-alkylammonium treatment of ultrathi n sections was carried out in order to study the biogenous mineralogic al transformation of the biotite. Microsamples proceeding from unaffec ted biotite zones demonstrated homogenous 10-Angstrom d(001)-value bio tite phase. HRTEM images of lattice fringes of samples taken from the lichen-biotite contact zone reveal large areas of both unexpanded (10- Angstrom) and randomly and R = 3 distributed expanded (from 14- to 30- Angstrom) layers of phyllosilicates identified as interstratified biot ite-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 respon sible for interstratified mineral phase formation. Two parallel proces ses, physical exfoliation of biotite and interlayer ionic exchange of K and subsequent vermiculite formation, are the mechanisms for biotite bio-weathering induced by lichens.