An urban model for dolomite precipitation: Authigenic dolomite on weathered building stones

Formation of authigenic dolomite within alteration crusts developing on limestone surfaces of a historical building is confirmed by XRD, polarising microscopy, and SEM with EDX data. Two different types of authigenic dolomite occur: limpid stoichiometric dolomite and Ca-rich 'protodolomite'. The presence of gypsum, calcite, pollution-derived particles, clay minerals, and organic material within the crusts confirms their recent formation, together with that of dolomite, in a micro-environment of high evaporation and ionic concentrations. Such environments are typically found in many urban centres. On the basis of these findings an 'Urban Model' for dolomite precipitation is proposed. There are two sources of Mg: (1) dissolution of nearby dolostones overlying the limestones, as a result of leaching by atmospheric SO2, oxidised and hydrolysed as H2SO4; and (2) Mg-rich metallic atmospheric particles. Elevated Mg/Ca values are achieved by precipitation of gypsum, which removes Ca from the solution. 'Protodolomite' then precipitates and later recrystallises as limpid dolomite due to changes in the pore fluids, related with the environmental conditions. CO32- and CO3H- are provided by leaching of both the limestone support and the nearby dolostones. Additional sources of CO2 are provided both by the polluted urban atmosphere, rain water, and the degradation of organic material within the crust. Organic matter (fungi and bacteria), pollution-derived particulate matter and clays seem to play a key role in promoting dolomite precipitation in this micro-environment by altering kinetic barriers and providing active sites for dolomite nucleation. Finally, the formation of urban dolomite is compared with Recent dolomites forming in natural environments with alternating evaporative/fresh-water conditions.