The Effect of Laser-induced Heating on Moganite, Silanole and Quartz during Raman Spectroscopy

Moganite is a silica polymorph found intergrown with microcrystalline quartz. Raman spectroscopy is used to detect moganite using a band at 501 cm^-1, generated by the vibration of a four-membered tetrahedral SiO₂. This band is numerically distinct from the most intense band used to identify quartz at 465cm^-1_, and thus Raman spectroscopy might be considered a reliable methodology to detect moganite. However, the moganite detection using Raman spectra must be done with caution since a band at 503 cm^-1 can be caused by a Si-O vibration of silanole (SiOH), and thus the two bands interfere and may mingle. Such interference might be mitigated or increased by sample preparation, but it has not previously been shown with certainty whether powdered or intact rock surfaces, would exhibit the greatest interference. Here, we present a Raman spectroscopic study of different particle sizes on moganite and flint to investigate how it affects moganite detection. We found a Raman band in pristine flint with a similar peak position to moganite, but subsequent to heat treatment at 700 ˚C for 6 hours, the band disappeared indicating the presence of silanole rather than moganite. Powdering the sample in combination with the use of higher laser powers increased this effect and the relative intensity of the silanole band. Overall, the Raman spectrum of flint was found to be more sensitive to laser-power-induced artifacts than moganite. Aggregated quartz powder is known to be affected by laser-induced heating during Raman spectroscopy. However, the effect of the heating on silanole and moganite bands is not as well documented. The peak shift of moganite has a similar trend to the phase transition detected by Raman Spectroscopy and XRD with heat thus the two approaches are consistent. Furthermore, the silanole band is known to change its position by 6 cm^-1 at heating from room temperature to 600 ˚C. Based on the results from other research, the peak shift and broadening in the present study can be interpreted as an effect of laser-induced heating. To date, for mineral analyses, the number of studies reporting the effects of laser-induced heating on minerals is limited, which contrasts strongly with Raman spectroscopy of organic materials. The result in the present study suggests that the band shift of silanole and the transition of α-β moganite can be caused by the heat of the laser should be taken into consideration especially when small particle size moganite is being identified by Raman Spectrum. However, this same sensitivity to temperature may indicate potential as a measure of paleotemperature.