The pedogenic action of Parmelia Conspersa, Rhizocarpon Geographicum and Unbilicaria Pustulata

Abstract

The weathering of rock surfaces and soil formation are the result of two clearly distinguished processes, cheluviation and soluviation (Swindale & Jackson, 1956). In the presence of chelating agents, minerals are decomposed and then eluviated (Schatz et al., 1954; Schatz, 1963). Lichens excrete various compounds the solubility of which has generally been discounted (Smith, 1921; Hale, 1961; Smith, 1962; Haynes, 1964). However, more recent studies have shown that at least certain lichen substances, namely depsides and depsidones, are in fact slightly soluble in water (Iskandar and Syers, 1971) and that the presence of electrón donor groups in their respective molecules (Ginzburg et al., 1963; Kononova et al., 1964) might explain the formation of coloured complexes when either the thalli of certain lichens, or relevant lichen compounds, are allowed to react with water suspensions of minerals and rocks (Schatz, 1963; Syers, 1969). Moreover, it is known that certain lichen compounds can extract varying amounts of aluminium, calcium, iron and magnesium from silicates (Iskandar and Syers, 1972). More recent observations have shown that certain rock silicates are not only structurally modified in the presence of lichen compounds but that these substances, after promoting the reléase of cations, can give rise to new minerals under natural conditions (Ascaso and Calvan, in press). That lichens can alter rock surfaces in nature producing new minerals has been discussed by several authors (Bachman, 1907; Polynov, 1945; Aidinyan, 1949; Yarilova, 1950; Schatz et al, 1956) although in all these studies the possibility of extraneous contamination cannot be entirely ruled out. Laboratory studies under rigorously controlled conditions are therefore indispensible for a better understanding of the processes of chemical alteration of rocks and their consequent weathering and the promotion of soil formation. This paper observes and compares rock modifications by lichens in nature and in conjunction with carefully monitored studies in the laboratory.