ABSTRACT
Brady and Weil (1999) show that biogeochemical weathering of rock is a function of water availability, the presence of organic acids, and complexation processes. Specifically, water is involved in hydration, hydrolysis, and dissolution. Hydration of oxides of iron and aluminum is an important process in rock degradation; for example, hematite (Fe2O3) is converted into ferrihydrate (Fe10O15·9H2O). Hydrolysis is important in the release of essential nutrients for plant growth. For example, potassium is released from microcline, a feldspar by the following reaction: KAlSi3O8+H2O HAlSi3O8+K++OH−. Dissolution allows the dissociation of anions and cations from complex materials. For example, gypsum dissolves to release calcium and sulfate ions. In dry areas, the structure of lichens acts as a point of condensation of water and a site on which atmospheric water can collect (Lange et al., 1994). They are therefore nuclei for water-related rock-weathering processes. A review of rock weathering by lichens is given by Chen et al. (2000).
