The biogeophysic altcration of rocks by lichens has been extensively investigated (4, 5, 6. 8, 17, 18, 20, 23). It has been claimed that drying of lichens produced mechanical alteratiuns that were more important than the chemical weathering of rocks by lichenic compounds (20). More recently, the role of lichens in the chemical alteration of rocks has also been investigated (26). It is currcntly thought that lichenic substances form complexes with soil cations. It has been shown that oxalic acid from lichens is an effective "solvent" both of clay materials and iron-oxides (27). More recently, it has beeu observed that oxalic acid is able to dissolve the Si02 from soils (12). In spite of these findings, the role of lichens in the chemical weathering of minerals and rocks has largely been discounted since it is generally considered that lichen compounds are insoluble in water (9, 10, 21, 24). It was shown, that some lichen substances have a water solubility high enough to be responsible of the formation of metalic complexes (13) that can be detected spectrophotometrically (14, 22, 25). The formation of colored complexes by some lichen substances is presumed to be a consequence of the polar groups acting as electrón donors (7, 16). Recently, the amount of complexed cations by some lichenic compounds has been estimated (11, 14). In these reports, data on the morphological and structural alterations induced in the minerals and rocks as a consequence of the described treatment are not presented, however. The purpose of this study was to correlate the metal-complex formation with the structural weathering of granite, gneiss and their primary minerals when lichen compounds are allowed to react with them.

We gnitefully acknowledge skilfnll technical assistance of Mr. FERNANDO PINTO.

11 pages, figures, and tables statistics.

Peer reviewed