Weathering of rocks and neogenesis of minerals associated with lichen activity

The weathering action of lichens on rocks and the biogeophysical and biogeochemical alteration of rock-forming minerals, their influence in dissolution and precipitation reactions and in bioformation of new minerals are reviewed. The intimate adhesion of lichen thalli to the rock surface and the hyphal penetration in less coherent areas of the rock cause a physical disaggregation and fragmentation of the mineral surface. Chemical weathering is essentially due to the excretion of organic acids. Depending on the nature of the minerals, etching patterns and decomposition features are formed by biosolubilisation processes. The presence of calcium, magnesium, manganese and copper oxalate crystals at the rock-lichen interface and in the lichen thalli suggests that oxalic acid, secreted by the mycobiont, is one of the most active agents of chemical alteration. Transformation of minerals to siliceous relicts, precipitation of iron oxides and hydroxides acid formation of alumino-silicates are presumably related to the activity of organic acids in complexing and removing elements from the substrate. The involvement of the so-called 'lichen acids', a group of mainly polyphenolic compounds, as bioweathering agents has been only recently well documented. Present and further progresses in the study of the lichen-rock relationship rely on application of modern instrumental and analytical techniques. The relevance of lichen weathering to the biodeterioration of stoneworks of artistic value is considered. (C) 2000 Elsevier Science B.V. All rights reserved.