Raman spectra of mirabilite, Na2SO4·10H2O and the rediscovered metastable heptahydrate, Na2SO4·7H2O

Hamilton, Andrea and Menzies, Robert I. (2010) Raman spectra of mirabilite, Na2SO4·10H2O and the rediscovered metastable heptahydrate, Na2SO4·7H2O. Journal of Raman Spectroscopy, 41 (9). pp. 1014-1020. ISSN 0377-0486

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Abstract

Salt crystallisation in pores is known to cause serious damage to masonry. Sodium sulphate, often regarded as one of the most damaging salts, has a rich hydrate chemistry including one rediscovered metastable hydrate and a new high pressure octahydrate plus five known polymorphs of the anhydrous phase. The difficulty in working with these hydrates lies in their strong tendency to dehydrate or to convert to the stable phase, in the case of the heptahydrate. We present Raman spectra and a table of peak wave numbers for randomly oriented crystals of mirabilite and the metastable heptahydrate, sufficient to distinguish between these phases that have SO4 ν1 values of 989.3 and 987.6 cm−1, respectively. Mirabilite has a Raman spectrum very similar to the free sulphate anion in solution, which is probably due to the mobility of oxygen atoms within the sulphate tetrahedron. The oxygen atoms in the heptahydrate sulphate groups have no partial occupancy, and predicted peak splitting is observed in the region 400–1200 cm−1.