Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type

Abstract

An extensive infrared (IR) spectroscopy study using transmission, specular and diffuse reflectance, and attenuated total reflection (ATR) was undertaken to characterise opal-AG, opal-AN (hyalite), opal-CT and opal-C, focussing on the Si-O fingerprint region (200–1600 cm−1). We show that IR spectroscopy is a viable alternative to X-ray powder diffraction (XRD) as a primary means of classification of opals even when minor levels of impurities are present. Variable angle specular reflectance spectroscopy shows that the three major IR bands of opal are split into transverse optical (TO) and longitudinal optical (LO) components. Previously observed variability in powder ATR is probably linked to the very high refractive index of opals at infrared wavelengths, rather than heterogeneity or particle size effects. An alternative use of ATR using unpowdered samples provides a potential means of non-destructive delineation of play of colour opals into opal-AG or opal-CT gems. We find that there are no special structural features in the infrared spectrum that differentiate opal from silica glasses. Evidence is presented that suggests silanol environments may be responsible for the structural differences between opal-AG, opal-AN and other forms of opaline silica. Complementary studies with Raman spectroscopy, XRD and scanning electron microscopy (SEM) provide evidence of structural trends within the opal-CT type.

Comparison of Natural and Dyed Fire Opal

“…All the opals exhibited strong Raman signals in the range of 100-2000 cm −1 , which is typical of framework silicates (Figures 7 and 8) [8,9]. These broad bands also indicated that the more crystalline samples are significantly disordered (Table 4).…”

“…These broad bands also indicated that the more crystalline samples are significantly disordered (Table 4). The major absorptions of opal and silica occurred predominantly in the region of 200-500 cm −1 , showing broad bands with varying degrees of fine detail for opal-A and opal-CT [8,9]. According to the literature [20], the position of the maximum peak of the most intense band in this range is a key factor in distinguishing opal-A and opal-CT. For opal-A, the broad band is centered at around 430 cm −1 , while opal-CT presented a maximum at around 320-330 cm −1 [20], which is attributed the superposition of a typical maximum of disordered tridymite modifications and cristobalite [25].…”

“…In a recent spectroscopic test of opals, three major silicon-oxygen fingerprint bands in the 450-500, 780-820, and 1000-1300 cm −1 regions have been identified that are common to all types of opal [8][9][10]. The three bands may be split into transverse optical (TO) and longitudinal optical (LO) components [11][12][13].…”

“…In this study, TO 1 /LO 1 , TO 2 /LO 2, and TO 3 /LO 3 are used to assign the three bands described above. It is worth mentioning that a shoulder at approximately 1250 cm −1 is presumably due to either the LO 3 bands or a mixed LO-TO mode with dominant LO 3 character [8,9,11,12].…”

“…FTIR diffuse reflectance spectra were collected in the low-frequency region of 400-2000 cm −1 (Figure 5), characterized by the three strong peaks typical of framework silicates (Si-O-Si bending) [10,[16][17][18], with the main peaks in the range of 1099-1115 cm −1 (Table 2). According to previous studies [8,9,[19][20][21], the infrared spectra may also be instrumental in classification and provenance. The frequencies of the band near 790 cm −1 are related to the degree of crystallinity, which is always at higher frequencies in opal-A (non-crystalline opal, 796-800 cm −1 ) and opal-C (793-794 cm −1 ) compared to those of opal-CT (micro-crystalline opal, 788-792 cm −1 ) [21].…”