A study of the weathering phenomena brought about by the growth of Lecanora alra on a substrate of magnesium silicate minerals (serpentinite) has been carried out mainly by X-ray diffraction and scanning electron microscopy. The lichen thallus contains appreciable amounts of crystalline magnesium oxalate dihydrate, which occurs as an insoluble extra-cellular precipitate and derives from the decomposition of magnesium silicates (particularly chrysotile) by oxalic acid secreted by the mycobiont. In theory magnesium oxalate dihydrate should be capable of incorporating large amounts of heavy metal ions into its structure, as indeed the electron probe evidence indicates has happened, thus suggesting a mechanism for enabling some lichens to cope with environments that are high in these generally harmful ions. The only weathering product detected in the lichen weathering crust is an X-ray amorphous silica gel which often retains the fibrous morphology of the chrysotile from which it forms.