The Raman spectra of the Na2SO4-K2SO4 system: Applicability to soluble salts studies in built heritage

The Raman spectra of the Na2SO4-K2SO4-H2O system are not well-defined in the literature. Specifically, the proper identification of sodium and potassium sulphate (aphthitalite or glaserite, K3Na(SO4)(2)) and anhydrous sodium sulphate (thenardite, Na2SO4) is particularly problematic because their vibrational profiles present the same main Raman peak at 993 cm(-1) and very similar low frequency bands. As proved in bibliography, the similarity of their spectra can often lead to uncertain or erroneous identifications. Considering that aphthitalite and thenardite can be found as degradation products on built heritage materials and the degree of danger associated to them is not the same, being the second one the most harmful, the resolution of this problem has a critical importance. For this reason, in the present work, the Raman spectra of aphthitalite and thenardite are deeply studied to identify the vibrational fingerprints enabling their correct identification. The results here summarized and provided by two different Raman instruments highlight that the spectrum of aphthitalite displays characteristic bands at 1,084 and 1,202 cm(-1). In contrast, the bands at 1,100, 1,129, and 1,152 cm(-1) seem to be characteristic of thenardite. Furthermore, when those secondary bands are not observed or mixtures of both compounds are present, the ratio between their most intense bands at 452 and 993 cm(-1) is the key for their correct characterization. On the whole, this study fills the gaps observed in literature and gives the solution for the correct identification of aphthitalite and thenardite even when secondary bands are not observed.