Natural and synthetic quartz crystals with different OH concentrations were investigated by NIR (5500-3800 cm−1) and IR (3800-3125 cm−1) absorption spectroscopy at room temperature. These crystals exhibit differences in their OH spectra both in the IR and in the NIR. Quantitative relations between NIR bands at ∼ 4500 and ∼ 4000 cm−1 and IR bands are given. Explanations for the origin of the NIR bands proposed by previous authors for samples with medium to high H concentrations are not always compatible with our observations. Furthermore, for low H concentrations, we found:
i) a small 4000 cm−1 residual three-peak band which we attribute to intrinsic SiO2 absorption modes (the linear absorption coefficient of the prominent peak is 0.0017 ± 0.0002 cm−1 at ca. 4000 cm−1);
ii) the 4500 cm−1 band is absent (i.e., linear absorption coefficient < 0.0001 cm−1) for low, but non-vanishing H concentrations (< ∼ 8 ppm H/Si, i.e., α3410 < ∼ 0.09 cm−1 for synthetic, α3484+ α3380 (e2) < ∼ 0.20 cm−1 for natural samples), which calls into question the nature of the OH defects at these low values.
We show further that irradiation can produce synthetic quartz that displays NIR and IR spectra similar to those of natural quartz.