The interaction of the functional molecular group, SO42-, in sulfate minerals with its extramolecular environment is usually manifest in the splitting of degeneracies of vibrational modes of the ion. Although symmetry prescribes the number of components of degenerate vibrational modes such as va, the spectra of sulfate minerals do not always observably reflect symmetry effects, and it is, therefore, not always possible to obtain information about molecular symmetry of sulfate minerals by counting the number of absorptions. The absorption spectra of most sulfate minerals studied, however, give evidence of the modification of the symmetry of the sulfate ion from Td to various lower symmetries. The spectral data for alunite and jarosite show more vibrational components for v2 than permitted by the site symmetry of the sulfate ion, thus indicating the presence of non-equivalent ions.

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