Neutralization and neoformation: Analogous processes in the atmosphere and in lichen thalli-A review

The present review discusses the concept of mineral neoformation which describes the formation of gypsum as the end-product of analogous processes occurring in the atmosphere, in the lichen thallus and on lithic substrates colonized by lichens. The formation of gypsum occurs as a result of H₂SO₄ reacting with CaCO₃. Climate change, e.g. global warming and desertification led to an increasing number of annual dust storms originating in North African deserts, from 5 days in 1958 to 30 days in 2006, i.e. sixfold in 48 years. Calcium is a major constituent of desert dust. The enlarged dust load in the Eastern Mediterranean coincides with the amount of calcium accumulated in lichens: from 6300μgg^-1 in 1978 to 36,603±7563μgg^-1 in 2002, i.e. about sixfold in 24 years. Desert dust carrying Ca-containing particles reacts with sulfuric acid produced in the atmosphere following both volcanic and anthropogenic activities. Gypsum produced via neoformation in the atmosphere, precipitates in varying formations, e.g. iberulites. A process of neutralization leads to the formation of biominerals. CaCO₃-containing particles entrapped in the lichen thallus react with sulfuric acid to produce gypsum as its end-product, detected in the form of crystals on the outer surface of the thallus. The formation of biogenic gypsum occurs also via the reaction of intra- and extracellular Ca²⁺ cations with SO₄^2- anions, penetrating into, or found inside thallial cells. The catalytic capability of lichens is evident in the process of neoformation of gypsum on lithic substrates, e.g. granite.