Weathering of the black limestone of historical monuments (Ljubljana, Slovenia): Oxygen and sulfur isotope composition of sulfate salts

The black limestone widely used in Slovenian monuments, particularly in the baroque architecture, is deteriorating extensively due to salt crystallization. Samples of soluble salts from two important historical monuments (in Ljubljana, Slovenia) were investigated in terms of their mineral and isotopic (S and O) compositions. Results revealed the presence of gypsum and soluble salts of the MgSO4·nH2O series, such as starkeyite (MgSO4·4H2O), pentahydrite (MgSO4·5H2O) and hexahydrite (MgSO4·6H2O). Whereas black crusts and subflorescences consisted of gypsum, efflorescences appeared to be an assemblage of gypsum and MgSO4 hydrates. Sample δ18Osulfate values varied from −1.9‰ to +5.5‰ vs. V-SMOW and δ34Ssulfate values from −19.8‰ to +3.2‰ vs. V-CDT. The respective isotopic composition of analysed outdoor and indoor monument samples indicated different sources of contamination.