BIOLOGICALLY VERSUS INORGANICALLY MEDIATED WEATHERING REACTIONS - RELATIONSHIPS BETWEEN MINERALS AND EXTRACELLULAR MICROBIAL POLYMERS IN LITHOBIONTIC COMMUNITIES

Biophysical and biogeochemical weathering of amphibole syenite associa ted with the Stettin complex near Wausau, Wisconsin, has been examined by HRTEM, WDS, LM, and XRD. The rock consists of microperthitic felds par, ferriannite, quartz, and ferrohastingsite. Crustose saxicolous Ii chens, Rhizocarpon grande and Porpidea albocaerulescens, penetrate the rock surface to a depth of 10 mm. Within the intact rock, amphibole s urfaces along hyphae-filled cracks are highly corroded. Fungal hyphae exploit grain boundaries, cleavages, and cracks to gain access to mine ral surfaces, resulting in accumulations of cleavage-bound mineral fra gments as small as 5 mu m within the lower thallus. Bacterial microcol onies are common and all mineral surfaces are completely coated in ext racellular acidic mucopolysaccharides from fungal and bacterial source s. In the cases of amphibole, quartz, and feldspar, dissolution does n ot appear to involve pervasive leaching, for even the smallest mineral fragments retain their chemical and structural identity. Biotite dire ctly in contact with the lichen thallus is intimately interpenetrated by fungal hyphae growing along (001) cleavages and is partially conver ted to vermiculite. No siliceous relies have been identified. Biologic ally mediated weathering involves a complex dissolution/selective tran sport/recrystallization mechanism occurring within the acidic extracel lular gels coating all mineral surfaces. A specialized weathering micr oenvironment around each mineral grain initially produces minute phyll osilicate crystallites. A rind of clay minerals forms around the disso lving parent phase, eventually culminating in abundant 5-10 mu m diame ter polymer-bound aggregates of face-to-face oriented clay minerals of homogeneous composition. Physiochemical weathering of ferrohastingsit e produces topotactically oriented smectite and goethite. The cleavage -controlled reaction is neither isochemical nor isovolumetric.