Gypsum Raman

Wall painting in the Messer Filippo cell, Tower of Spilamberto (Modena, Italy). The painted surface is in an advanced state of deterioration the detachment of the pictorial matter from the plaster is evident, efflorescence is present, and the pictorial coating is almost completely covered by a grey-whitish film formed by microcrystalline gypsum, as proved by micro-Raman investigations. The FA profile, characterized by an A/P ratio of... 

Fjg, 9. Satellites near the O—H valency band in the Raman spectrum of gypsum crystal... 

This example illustrates the usefulness of the polarized Raman technique. For a further discussion on the analysis of calcite, see Nakamoto (37), Nakagawa and Walter (62) and Mitra (47) for data on gypsum. 

The INS spectra of hydrated minerals and those containing hydroxyls, are dominated by the librational modes. The complementarity of INS to infrared and Raman spectroscopies is apparent, the optical spectra below 2000 cm are dominated by the X-0 (X = e.g. C, S, Al, Si) stretch and bend vibrations of the framework. Gypsum, CaS04.2H20 [11,12], provides a classic example. 

Apart from the Raman spectroscopic identification of two different types of bacteria in a gypsum crystal Nostoc and Gloeocapsa), it was also possible to detect organic signatures from bacterial colonies sited several millimetres below the surface in a transparent crystal of selenite from a 26 Mya meteoritic impact crater [37] at Devon Island in the Canadian High Arctic (Figure 1-9). 

Figure 1-11. Antarctic endolith in gypsum, with Raman spectra of, upper spectrum, the surface (gypsum) and lower spectrum, the underlying cyanobacterial zone (scytonemin). The chemical structure of scytonemin is also given here...

Here, an example from the Rhub-al-Khalil in the Arabian Desert shows a cyanobac-terial colony in a dolomitic zone, several cm below the surface of a large gypsum crystal embedded in a halite matrix (Figure 1-17). The Raman spectra of the biological component clearly demonstrate the presence of photoprotective pigments such as scytonemin and carotenoids. 

Figure 1-17. Sabkha surface crust with gypsum and halite crystals, Rhub-al-Khalil, Arabian desert cyanobacterial colonisation at interface with subsurface dolomitized calcite. The Raman spectrum shows the presence of scytonemin and carotene in the biological zone...

Fig. 6.28 (a-f) Gated Raman spectra of different minerals = 248 nm, gate = 10 ns) at controlled experimental conditions of Terlingua-type pink calcite (a), dolomite (b), gypsum (c), barite (d) and quartz (e) in comparison with KNO3 (f). Data were used for absolute Raman cross-section evaluation using KNO3 as a reference... 

Fig. 8.51 (a-d) Gated Raman spectra with excitation by 532 nm of calcite, anhydrite, gypsum and quartz relevant for cement industry... 

Figure 9 illustrates the treatment performed in a red piece directly analyzed by micro-Raman with a X40 objective. Carbon, lime wash, and gypsum spectra are fitted and removed. Hematite is the only red pigment remaining in the sample. 

The IR spectrum obtained from a very small amount of powder detached from the piece is shown in Fig. 10. The spectrum consists mainly of gypsum substrate and no evidence for pigments is seen because our FTIR spectrometer range is at the limit of the hematite band positions. The same fitting analysis allows the identification of a small contribution of calcite, which cannot be seen in the Raman spectrum. 

Figure 9 Micro Raman spectrum of a red pigment from Convento de la Peregrina, Sahagun. (a) Synthesized spectrum after the fitting of the stored components of lime wash and black lamp (b) synthesized spectrum after the fitting of the stored components of gypsum (c) synthesized spectrum after the fitting of the stored components of hematite. The addition of parts (a), (b), and (c) matches exactly the obtained raw spectrum.

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