The staining of blue stone limestones petrographically unraveled

This paper deals with the first results of a comparative petrographical study of un-aesthetically stained blue stone limestones, including Lower Carboniferous crinoidal limestones and Middle Cambrian oolitic limestones. Although it was commonly accepted that this staining resulted from weathering of ferroan dolomite, our study proved that it rather resulted from weathering of framboidal pyrite and subsequent dissolution of partially dedolomitized dolomite. Meteoric weathering resulted in a chemical reaction chain including oxidation of pyrite, formation of sulphate-rich water, dissolution of dedolomitized dolomite rhombs and precipitation of iron (hydr)oxide coatings in the intercrystalline dissolution voids. Weathering was probably accelerated by an increased permeability, related to the presence of open stylolites and/or by initial secondary dolomitization of the host limestone. Optical microscopy of fluorescent epoxy impregnated thin sections and of polished slabs proved to be particularly useful in elucidating the above staining mechanism, especially in combination with SEM and BSEM with on-line WDS/EDS.