ELUCIDATING THE STRUCTURAL STATE OF OPALINE SILICA BY RAMAN SPECTROSCOPY

Opaline silica (SiO₂ • nH₂O) is widespread in volcanic and sedimentary environments and forms when silicic acid is oversaturated in low-temperature aqueous solutions, which consequently produces a gel-like substance composed of colloidal silica that precipitates out of solution. Opal has a high degree of structural disorder, and as the silica matures via diagenesis the structure becomes more ordered. As such the degree of opal’s crystallinity can vary. The opaline silica phase name is based on the degree of disorder, that is, opal-A (amorphous), paracrystalline opal-CT (disordered cristobalite and tridymite), and microcrystalline opal-C (disordered cristobalite). By and large, opaline silica phase is determined by powder XRD by employing the measurement of the full-width at half-maximum intensity (FWHM) of reflection in XRD diffractogram at ~21.8 2θ. In past studies, FWHM range was determined to be 6.5-8 2θ for opal-A, 1-2 2θ for opal-CT + quartz, and 0.2-0.4 2θ for quartz. Though, impurities of opal can also impact the location of bands and the FWHM as well as the overall maturation process.

Here, we explored the feasibility of using Raman spectroscopy to elucidate silica maturation in amorphous opaline silica compared to measurements by XRD. Previously, Raman spectroscopy has been successfully used to determine the silica phase by examining Raman allowed bands at ~330–360 cm^-1 for opalA/CT, 230 cm^-1 and 420 cm^-1 for cristobalite, 305 cm^-1 and 350 cm^-1 for tridymite, 465 cm^-1 for quartz, and 500 cm^-1 for moganite. To date, Raman bands and XRD reflections have only been compared in a general sense, more qualitative than quantitative. This study will correlate subtle shifts in XRD FWHM reflection positions and FWHM of Raman bands in spectra acquired from natural opal samples. Here we show correlation between the spectral measurements obtained by the two analytical techniques. Furthermore, the benefit of using Raman spectroscopy, is that it affords a nondestructive and microscopic analysis furnishing high spatial resolution, in contrast to powder XRD.