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Share this pageAnhydrite: Crystal structure | Sample photo | Raman spectrum | Interpretation | References
Crystal structure of Anhydrite
Formula: |
CaSO4 |
|
Crystal Data: |
Crystal System: Orthorhombic - Dipyramidal |
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Point Group: 2/m 2/m 2/m |
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Cell Data: |
Space Group: Amma, a = 6.991, b = 6.996, c = 6.238, Z = 4 |
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Density (calc.) = 2.96 and V = 305.09 Å3 |
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| Element color: Ca, S, O | ||
Anhydrite sample
 |
Sample no. 5536 from the "Mineralogy and Petrography Museum Grigore Cobălcescu" of "Alexandru Ioan Cuza" University, Iaşi. Origin: Bleiberg, Carinthia - Austria. Click image to enlarge |
Raman spectrum and vibrations of Anhydrite
Interpretation of Raman spectrum of Anhydrite
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.
| Anhydrite | Assignments | ||
| Buzgar et al., 20091 | Liu et al., 2009 | Bhagavantam, 1938 | |
| 235 | Ca-O | ||
| 419 | 416 | 415 | ν2 SO4 |
| 503 | 499 | 499 | |
| 612 | 609 | ν4 SO4 | |
| 630 | 629 | 628 | |
| 678 | 676 | 674 | |
| 1017 | 1017 | 1018 | ν1 SO4 |
| 1110 | ν3 SO4 | ||
| 1129 | 1128 | 1128 | |
| 1160 | 1159 | 1160 | |
| 3395 | νH2O | ||
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). |
References
• 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]
