The strongest Raman spectrum of gypsum was found at 1010 cm^-1 and corresponds to the ν₁ symmetric stretch vibration mode of the SO₄ tetrahedra. Gypsum exhibits doublet for ν₂ symmetric bending, at 416 cm^-1 and 495 cm^-1. The peak at 1143 cm^-1 was assigned to the ν₃ asymmetric stretch vibration mode, and the peaks at 622 cm^-1 and 674 cm^-1 to the ν₄ asymmetric bending vibration modes. Another band that appears at 316 cm^-1 has been assigned to the translational modes T(H₂O, Ca) (Sarma et al., 1998).

A comparison between the gypsum and anhydrite spectra. It can be observed that the internal modes frequencies of SO₄ group are similar. In anhydrite the ν₃ mode is split in two bands and can be explained by the distortion of SO₄ tetrahedra. The oxygen atoms of SO₄ groups are in different crystallographic environments. In gypsum, they are coordinated with a calcium atom and two water molecules, and in anhydrite with two calcium atoms. Therefore, the changes in the vibrational modes of the sulfate group could be a consequence of the dehydration process (Sarma et al., 1998). In other words, the wavenumbers of the bands increase with a decrease of the hydration degree, from gypsum (CaSO₄•2H₂O) to bassanite (CaSO₄•0.5H₂O) to anhydrite (CaSO₄).

Because the measurements were made in the spectral range 200 – 3400 cm^-1, we could not the characteristic bands for the stretching vibration modes of water in gypsum. They have been reported in literature at values of 3406 cm^-1 and 3494 cm^-1 (White, 2008).

• The Mineralogy Database [link]

• Crystal data (.cif file) from the American Mineralogist Crystal Structure Database [link]

• ¹BUZGAR N., BUZATU A., SANISLAV I. V. (2009) - The Raman study on certain sulfates. Analele Stiintifice ale Universitatii “Al. I. Cuza” - Iasi, Tome 55, issue 1, 5-23 [link]

• Sarma, L. P., Prasad, P. S. R., Ravikumar N. (1998) - Raman Spectroscopic Study of Phase Transitions in Natural Gypsum. Journal of Raman Spectroscopy, 29, 851-856 [link]

• Liu, Y., Wang, A., Freeman, J. J. (2009) - Raman, MIR, and NIR spectroscopic study of calcium sulfates: gypsum, bassanite, and anhydrite. 40th Lunar and Planetary Science Conference. The Woodlans, Texas. [link]

• White, S. N. (2008) - Laser Raman spectroscopy as a technique for identification of seafloor hydrothermal and cold seep minerals. Chemical Geology, 15589. [link]