Coloured mineral coatings on monument surfaces as a result of biomineralization: the case of the Tarragona cathedral (Catalonia)

Successive coloured coatings on the Taragona cathedral (Catalonia) were analyzed using chemical, microbiological and mineralogical techniques. The coatings consist of mainly biogenic minerals (calcite, several oxalates, phosphates) and their fabric and composition is independent of the underlying rock. The origin of the crusts or patinas is attributed to bygone (sub-fossil) microflores which formed biofilms and microbial mats on and within the surface of the limestones and marbles of the cathedral. The staining of the patina layers is not related to the mineral colours, nor to anthropogenic dust or fly-ashes. The colours are most probably organic pigments embedded between the crystals and Fe- and Mn-organic compounds remaining from bioactivity. A sequential hypothesis of the origin(s) of the patina layers is given.