Zones of chemical and physical interaction at interfaces between microbial communities and minerals: A model

Four zones of biogeochemical weathering of silicate mineral assemblages colonized by lithobiontic communities (lichens) are proposed based on electron microscopic studies. Zone I (photosynthetic), represented by the upper thallus of lichens, is devoid of mineral transformations. In the lower thallus, communities of microorganisms physically disaggregate minerals to a depth of similar to 10 mm. This physical disruption delineates Zone 2 (direct biochemilithic), defined by the intimate association of all mineral surfaces with microorganisms and their extracellular products. Complex mixtures of organic polymers, nontopotactic Ca, K, Fe clay minerals with a 1-nm basal spacing, and nanocrystalline aluminous Fe oxyhydroxides coat corroded mineral surfaces extensively. Mineral weathering reactions in Zone 3 (indirect biochemilithic) show clear evidence of biological impact, yet surfaces are not coated with organic polymers. This zone occurs below the direct biochemilithic zone and also within mineral fragments in the direct biochemilithic zone. Topotactically oriented smectite and minor goethite form along corroded cracks. Zone 4 (physiochemilithic) is dominated by incipient, predominantly inorganically controlled weathering and unweathered rock.