Mineralogical transformation and neoformation in granite caused by the lichens Tephromela atra and Ochrolechia parella

Tephromela atra and Ochrolechia parella are among the most abundant lichens colonizing granitic monuments in the region of Galicia (northwest Spain). In this work, their interaction with a two-mica granite used in the construction of the Toxosoutos Monastery (Noia, Galicia) was studied, using optical microscopy, scanning electron microscopy (with back-scattered-electron and energy-dispersive X-ray detection), X-ray diffractometry and atomic absorption spectroscopy to evaluate their physical, mineralogical and chemical effects. Both lichens contributed to physical weathering by penetrating intermineral voids and mineral cleavage planes, disaggregating the rock and entrapping the loosened mineral grains in their thalli. Significant chemical and mineralogical weathering also occurred, including depletion of potassium from biotite, transformation of this mica into hydroxyaluminium-vermiculite, and neoformation of whewellite and calcite in the lichen thalli. Neoformation of these calcium minerals on a calcium-poor rock such as granite is noteworthy, and this is the first time calcium carbonate has been detected within a lichen colonizing a granitic rock. Precipitation of the calcium carbonate was attributed to the local pH in the thalli having been raised due to release of sodium from nearby plagioclase during weathering. (C) 1997 Elsevier Science Ltd. All rights reserved.