M O R P H O L O G I C A L A N D CHEMICAL FEATURES OF BIOWEATHERED GRANITIC BIOTITE I N D U C E D B Y LICHEN ACTIVITY

to study the physico-chemical activity of lichens on micaceous components of granitic rocks, samples covered by thalli of Parmelia conspersa (Ehrht) Ach. and Aspicilia intermutans (Nyl.) Arn. were collected and examined with Scanning Electron Microscopy (SEM) equipped with a Back Scattered Electron (BSE) detector and an Energy Dispersive Spectroscopy (EDS) microanalytical system. The biophysical activity of both lichen species leads to a deep alteration of biotite, which results in detachment, separation and exfoliation of biotite plates. Chemically, the bioweathering process of biotite in the lichenmineral contact zone involves considerable depletion of potassium (K) from interlayer positions in biotite and removal of several elements, corresponding to a 9.7% loss in matter. The sequence of the loss of elements is: K § >> Fe wt > Ti 4+ ~ Mg 2§ There are also some gains in the order: Ca ~§ > Na § >> AP + > Si 4§ attributed to dissolution of co-existing Ca and Na rich minerals. Geochemical mass balance results suggest the transformation of K-rich biotite to scarcely altered biotite interstratified with a biotite-vermiculite intermediate phase in the lichen bioweathered contact zones. Key Words--Aspicilia interrnutans, Biotite, Bioweathering, Granite, Lichens, Parmelia conspersa. I N T R O D U C T I O N The role of physical and chemical factors in the weathering of micaceous minerals has been adequately documented. A c o m m o n biotite weathering route is the suppression o f K release due to oxidation during synthetic degradation processes (Barshad and Kishk 1968; Rich 1972; Gilkes 1973; Gilkes et al. 1973; Ross and Rich 1974; Goulding 1983; Scott and Amonet te 1988; Dreher and Niederbudde 1994). In those laboratory exper iments b iot i te al tered to ve rmicu l i t e and/or mixedlayer clay minerals. Natural weathering of biotite may also produce kaolinite, halloysite, illite, smectite, chlorite and sesquioxides (Stoch and Sikora 1976; Gilkes and Suddhiprakarn 1979; Craw et al. 1982; White et al. 1985; A1Dahan and Morad 1986; Morad 1990; Robertson and Eggle ton 1991). In contrast to the abundant data on synthetic and natural physico-chemical weathering of micaceous minerals, the involvement of biological alteration processes is not as wel l understood. Lichens commonly occur on rocks, both in their natural state and also when used as monumenta l and building stones. Thus, l ichen-encrusted rocks provide an ideal environment for studying biological aspects of mineral weathering. The first studies of l ichen weathering of rocks led to the conclusion that the effect might almost be entirely ascribed to physical causes. It is known today that the l ichen-rock interface is a place of considerable chemical activity, due to extracellular excret ion o f organic acids and organic compounds with chelating capacity (Jones and Wilson 1985). Biotite bioweather ing is o f great importance to the study of pedogenet ic processes. Biot i te bioweather ing is important, both for understanding pedogenet ic processes and biodeterioration o f granitic monumenta l and building stones. Few works have invest igated the morphologica l and chemical features affecting bioti te during biochemical weathering induced by lichens. Iskandar and Syers (1972), in laboratory experiments studying meta l -complex formation by l ichen acids with aqueous suspensions o f biotite, granite and basalt observed release of Fe, A1, Ca and Mg from silicates. Chelating properties of l ichen acids were investigated later by Ascaso et al. (1976) and Ascaso and Galvan (1976) who reacted mica and other minerals with suspended lichens. Those laboratory works demonstrated that l ichens could alter their mineral substrates and provided strong circumstantial evidence that this alteration was mediated by the activity of specific lichens. It was concluded that l ichens that produce l ichen acids could be significant weathering agents in nature. Deplet ion of Mg, Fe, K and A1 on the mica surface was reported only one t ime by Wilson and Jones (1983) for naturally l ichenized granite. Given that activities of at least some lichens could lead to deterioration of their immedia te micaceous substrates, the quest ion is raised as to the exact nature of the processes involved and to the tendency of mineral transformation in natural conditions. The aim of this work was to: 1) improve the knowledge of natural biological weathering processes o f the micaceous component of granitic rocks; and 2) identify pathways for the morphologica l and chemical changes o f biodegraded granitic biotite. M A T E R I A L S A N D M E T H O D S Pieces of granitic rock covered by the fol iose thallus of Parmelia conspersa (Ehrht) Ach. and crustose AsCopyright 9 1996, The Clay Minerals Society 652 Vol. 44, No. 5, 1996 Bioweathering of the biotite induced by lichens 653 pic i l ia in termutans (Nyl.) Am. and coexisting pieces without lichen cover were collected at Bustarviejo de la Sierra, near Madrid. Previous petrographic and X-ray diffraction (XRD) studies of this material showed the presence of quartz, orthoclase and plagioclase in the granitic rock as major rock-forming minerals, and biotite, zircon and apatite as accessory minerals (Ascaso 1985). Part of lichen-colonized rocks was processed according to conventional transmission electron microscopy (TEM) procedures for the preparation of lichen material (Ascaso et al. 1986) applying LR White resin as the impregnate medium. Afterward, the blocks were cut and fine polished. Wierzchos and Ascaso (1994) give detailed information concerning this preparation technique. Those samples were then examined with SEM in backscattered electron (BSE) emission mode using a DSM 960 Zeiss apparatus. The subsequent microprobe analysis as points analysis, spatial element distribution and microprobe profile with Energy Dispersive Spectrometry (EDS) Link ISIS microanalytical system were made on the SEM-BSE samples simultaneously during SEM observation operating at a 35 ~ take-off angle, an accelerating voltage of 15 kV, a working distance of 25 mm and a specimen current of 1-5 nA. Quantitative mineral analyses were obtained using standard ZAF correction procedures. Muscovite was used as a reference to standardize the analysis condition. Taking into account multipoint EDS analyses performed on SEM-BSE samples, the geochemical mass balance of the major elements was calculated by the Barth method (1948). The number of ions contained within a single unit cell network based on 22 O atoms has also been transformed into the weights of the corresponding elements and the loss, or gain, of each element, and the total loss of matter has thus been obtained (Lineares et al. 1987; Pozzuoli et al. 1994).