Gem News International Gems & Gemology, Winter 2023, Vol. 59, No. 4

FTIR Identification of Carbon Dioxide Fluids in Sapphire

Kazuko Saruwatari

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Figure 1. Various negative crystals in pink sapphire from Sri Lanka (A, enlarged in B) and fingerprints in blue sapphire from Myanmar (C, enlarged in D). Image B shows bubbles in flat negative crystals, and the center negative crystal in D has a bubble. Photomicrographs by Momo Matsumura (A and B) and Shunsuke Nagai (C and D); fields of view 4.05 mm (A), 8.15 mm (B), 8.47 mm (C), and 1.71 mm (D).
 
Figure 1. Various negative crystals in pink sapphire from Sri Lanka (A, enlarged in B) and fingerprints in blue sapphire from Myanmar (C, enlarged in D). Image B shows bubbles in flat negative crystals, and the center negative crystal in D has a bubble. Photomicrographs by Momo Matsumura (A and B) and Shunsuke Nagai (C and D); fields of view 4.05 mm (A), 8.15 mm (B), 8.47 mm (C), and 1.71 mm (D).

Carbon dioxide is well known as a trapped fluid in negative crystals in sapphire. From room temperature to cooler temperatures, the negative crystals change from a single supercritical fluid phase to two phases of gas and liquid. This feature is fascinating to observe under the microscope (e.g., Spring 2016 G&G Micro-World, pp. 78–79). These fluid inclusions are considered proof that a sapphire has not undergone thermal treatment (e.g., J.I. Koivula, “Carbon dioxide fluid inclusions as proof of natural-colored corundum,” Fall 1986 G&G, pp. 152–155).

However, the Summer 2020 issue of G&G reported on a sapphire heated with pressure that contained a carbon dioxide gas bubble even after treatment (Summer 2020 Lab Notes, pp. 287–288). Most previous reports about carbon dioxide fluid inclusions were based on microscopic observation, but this one addressed the presence of carbon dioxide fluid using Fourier-transform infrared (FTIR) spectroscopy.

Figure 2. FTIR spectra of the two sapphires from figure 1 indicate dense carbon dioxide–assigned peaks from 2000 to 4000 cm<sup>–1</sup> and 4500 to 5500 cm<sup>–1</sup>. The two peaks at 3601 and 3705 cm<sup>–1</sup> are easily mistaken for kaolinite peaks at 3619 and 3698 cm<sup>–1</sup>.
 
Figure 2. FTIR spectra of the two sapphires from figure 1 indicate dense carbon dioxide–assigned peaks from 2000 to 4000 cm–1 and 4500 to 5500 cm–1. The two peaks at 3601 and 3705 cm–1 are easily mistaken for kaolinite peaks at 3619 and 3698 cm–1.
Table 1. Carbon dioxide vibration band position (Buback et al., 1986; Oancea et al., 2012).
 

Figure 1 shows two sapphires from Sri Lanka and Myanmar with fluid inclusions of primary and secondary negative crystals. FTIR spectra of these two sapphires are presented in figure 2. The two peaks at 3601 and 3705 cm–1 are easily confused with kaolinite-related peaks at 3619 and 3698 cm–1. All of the spectra show several peaks assigned to dense carbon dioxide vibration and combination overtone bands. The 2342 cm–1 peak is C=O asymmetric stretching vibration ν3. The 3601 and 3705 cm–1 peaks are 2ν2 + ν3 and ν1 + ν3 first Fermi resonance, respectively. The 4838, 4967, and 5088 cm–1 peaks are 4ν2 + ν3, ν1 + 2ν2 + ν3, and 2ν1 + ν3 second Fermi resonance peaks, respectively, where ν1 is a symmetric stretching and ν2 is a doubly degenerate bending (table 1). Those peaks generally present double absorption bands, P- and R-contour, under atmospheric pressure and temperature due to the rotational transitions that occur for the stretching band, as seen in the bottom spectrum of figure 2. The double peaks change to a single peak, increasing the carbon dioxide fluid density by more than ~0.4 g/cm3 (M. Buback et al., “Near infrared absorption of pure carbon dioxide up to 3100 bar and 500 K. I. Wavenumber range 3200 cm–1 to 5600 cm–1,” Zeitschrift für Naturforschung A, Vol. 41, 1986, pp. 505–511; A. Oancea et al., “Laboratory infrared reflection spectrum of carbon dioxide clathrate hydrates for astrophysical remote sensing applications,” Icarus, Vol. 221, No. 2, 2012, pp. 900–910). 

Figure 3. Raman spectrum of carbon dioxide showing the Fermi diad (or doublet) from a negative crystal inclusion in a sapphire.
 
Figure 3. Raman spectrum of carbon dioxide showing the Fermi diad (or doublet) from a negative crystal inclusion in a sapphire.

In order to prove the high density of carbon dioxide fluids, this author used micro-Raman spectroscopy. Raman spectra of carbon dioxide in fluid inclusions show a Fermi diad (or doublet) (figure 3), which is useful in estimating carbon dioxide fluid pressure and density in negative crystals (e.g., J. Yamamoto et al., “Paleo-Moho depth determined from the pressure of CO2 fluid inclusions: Raman spectroscopic barometry of mantle- and crust-derived rocks,” Earth and Planetary Science Letters, Vol. 253, 2007, pp. 369–377; H.M. Lamadrid et al., “Reassessment of the Raman CO2 densimeter,” Chemical Geology, Vol. 450, 2016, pp. 201–222). Using the equations reported by Lamadrid et al. (2016), carbon dioxide fluid densities in sapphires measured by FTIR spectra were estimated at 0.54–0.74 g/cm3. This density is consistent with the results derived from FTIR spectra in figure 2.

According to the Summer 2020 Lab Notes entry, sapphire with heat and pressure showed a strong broad peak at 3047 cm–1 with C=O asymmetric stretching vibration ν3 at 2342 cm–1 and no other peaks at wavenumbers higher than 3600 cm–1. The Raman spectra also revealed a carbon dioxide Fermi diad (N. Ng-Pooresatien, pers. comm., 2023), and the density was estimated at 0.11–0.18 g/cm3, which is about one-fifth lower. The existence of carbon dioxide bubbles in a negative crystal is therefore no longer definitive proof of the absence of thermal treatment. This study also showed that FTIR analysis is a useful method for detecting the high density of carbon dioxide fluid in sapphire, and resulting FTIR peaks above 3600 cm–1 could indicate the absence of heat treatment.

Kazuko Saruwatari is manager of colored stone identification at GIA in Tokyo.