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Gypsum: Crystal structure | Sample photo | Raman spectrum | Interpretation | References

Crystal structure of Gypsum

Formula:

CaSO4•2(H2O)

Crystal Data:

Crystal System: Monoclinic - Prismatic

Point Group: 2/m

Cell Data:

Space Group: A2/a, a = 5.68, b = 15.18, c = 6.29, Z = 4

Using the mouse (click here for more information)

Density (calc.) = 2.31 and V = 496.09 Å3
Element color: Ca, S, O, H
Gypsum sample
Gypsum

Sample no. 5492 from the "Mineralogy and Petrography Museum Grigore Cobălcescu" of "Alexandru Ioan Cuza" University, Iaşi.

Origin: Cavnic - Romania.




Click image to enlarge
Raman spectrum and vibrations of Gypsum
Element color: S, O Toggle Grid Toggle Coordinates Reverse Spectrum

Click to in the Raman spectrum. To see the vibrations click on one of them (highlight region) and it (the selected vibration) will appear on the right side of the Raman spectrum.
Below spectra are various settings, and other vibrations of gypsum (which are not present in this spectrum).
You have the possibility to zoom in the spectrum by selecting a spectral region you need to be increased (along axis x); to zoom keep the left mouse-click continously pressed, drag (to left or to right) and release the left button. To return to the initial size spectrum, press the right click on the spectrum -> Zoom -> Reset View.
Model type:
Sticks Ball-and-Sticks (1) Ball-and-Sticks (2) CPK (Spacefill)

Orientation:
Reset view Front view Back view Left view Right view Top view Bottom view 		Background color:
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Spin on Spin off
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SO4 polyhedra on SO4 polyhedra off 		Vibrations:
ν2 SO4 ν4 SO4 ν1 SO4 ν3 SO4

Vibration on Vibration off 		Download spectrum:
Raman spectrum .txt Raman spectrum .spc


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Interpretation of Raman spectrum of Gypsum

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).


Anhydrite (Buzgar et al., 20091) Bassanite (Liu et al., 2009) Gypsum Assignments
Buzgar et al., 20091 White, 2008
235 T(H2O, Ca)
316
419 427 416 415 ν2 SO4
503 489 495 494
630 628 622 620 ν4 SO4
678 668 674 671
1017 1015 1010 1008 ν1 SO4
1129 1128 1143 1136 ν3 SO4
1160
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]

• 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]