IRON-OXIDES AND HYDROXIDES IN THE WEATHERING INTERFACE BETWEEN STEREOCAULON VESUVIANUM AND VOLCANIC ROCK

The nature and composition of the Fe minerals in the rusty ferruginous material frequently located in the zone of contact between the thallu s of Stereocaulon vesuvianum and the leucite bearing rock of Mt. Vesuv ius have been studied by microscopic and submicroscopic investigations (SEM, TEM with EDXRA) complemented by X-ray diffraction and chemical analysis. Ferrihydrite is likely to be the main component of the short -range order iron oxyhydroxide pool, although the possibility that sig nificant feroxyhite may be present cannot be completely excluded. Hema tite and goethite are the crystalline iron oxides. The organic acids p roduced by lichens play a key role in the enrichment of poorly-ordered phases at the rock-lichen interface. It is suggested that the peculia r morphology of the Stereocaulon vesuvianum thallus and the irregulari ty of the volcanic rock surface lead to different microsites in the ro ck-lichen interfaces which have their own conditions of pH, humidity a nd redox potential, favourable for the neoformation of crystalline pha ses.