Iron oxides and hydroxides in the weathering interface between Stereocaulon vesuvianum and volcanic rock

The effectiveness of lichens in degrading their rock substrates, leading to the first stages in the formation of soil, has long been discussed and opinions have varied widely (Jones, 1988; Syers & lskandar, 1973). Nevertheless, it is generally recognized that lichen encrusted rocks provide an ideal environment for studying the biological weathering of minerals since, unlike the situation in soil, there are fewer complicating and interacting factors. The interaction between saxicolous lichens and their substrates can be seen as etch markings on the constituent rock minerals and can result in the formation of a range of biogenic minerals which usually crystallize extracellularly. Among these minerals are the iron oxides which were referred to in review articles by Hawksworth (1973) and James (1973). Until recently, only two lichenhosted iron oxides have been reported. The first is ferrihydrite identified, as a poorly crystallized form of Fe oxide, in thin weathering crusts beneath Stereocaulon vulcani colonizing recent lava flow outcrops in Hawaii (Jackson & Keller, 1970) and, as a hydrous Fe oxide with a formula close to FesHOs-4H20, in a thin ochreous layer at the interface between Pertusaria coralIina and a weathered basalt substrate in Western Scotland (Jones et al., 1980). The second one is goethite detected by Ascaso et al. (1976) at the interface between Rhizocarpon geographicum and granite, and by Galvan et al. (1981) associated with various lichens on metamorphic rocks. It seems likely that the biotic formation of iron oxides in the weathering zone between lichen and rock involves oxidation, possibly microbially, of organic molecules complexing Fe 3+. This leads to an immediate release of Fe 3§ with subsequent hydrolysis and precipitation on rock surfaces or within lichen thalli (Schwertmann & Taylor, 1989). The apparent absence of ferrous iron oxalate in lichens could be ascribed to such a general process (Jones & Wilson, 1986). The objective of this study was to determine the nature and composition of the Fe phases occurring O 1997 The Mineralogical Society