Raman spectrum for anhydrite shows the ν₁ mode at 1017 cm^-1. The ν₂ and ν₃ modes of sulfate tetrahedra split in two bands: one at 419 cm^-1 and 503 cm^-1 and the other one at 1129 cm^-1 and 1160 cm^-1. The values attributed to ν₄ vibration mode are at 630 cm^-1 and 678 cm^-1. An additional band has been observed at 235 cm^-1, which was attributed to an external mode of vibration in anhydrite crystal.

The ν₄ asymmetric bending mode at 609 - 612 cm^-1 reported by Bhagavantam (1938) and Liu et al. (2009), has a very weak intensity and it is integrated in 630 cm^-1 band. Bhagavantam also reported a frequency at 1110 cm^-1. In our spectrum, this line is overlaped by the 1129 cm^-1 band. The spectrum of anhydrite presents a weak band at 235 cm^-1, which may be assigned to the vibrational mode of Ca-O bonding. The band at 3395 cm^-1, was assigned to the water stretching mode. Most probably the water was absorbed on the mineral surface from the air.

Figure 1. Wavenumber of the ν1 vibrational Raman mode vs. atomic mass of the cations.

If we consider the Raman spectra of the barite group (barite, celestine and anglesite) and the results presented above few considerations can be made: the wavenumber increases with an increase in the force; the stretching force constant between S-O for anhydrite it is K = 6.41 md/Å (Miyake et al., 1978); the wavenumber decreases with an increase of the atomic mass of the cations (figure 1).

• 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]

• 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]

• Bhagavantam, S. (1938) - Interpretation of Raman Spectra in Crystals: Anhydrite and Gypsum. Proceedings of the Indian Academy of Science A, 8, 345-348. [link]

• Miyake, M., Minato, I., Morikawa, H., Iwai, S. (1978) - Crystal structures and sulphate force constants of barite, celestite and anglesite. American Mineralogist, 63, 506–510. [link]