Characterization of SEM-CL red emission in quartz from various types of rocks

Despite being one of the purest minerals, quartz presents different types of defects, intrinsic or due to impurities, that might contain important genetic information. Its common occurrence is making it a promising tool for provenance studies. This study explores scanning electron microscopy (SEM) coupled with cathodoluminescence (CL) wavelength resolved spectroscopy for investigating luminescence emissions of quartz sourced from rocks of different types, spanning diverse geological ages.

All examined samples display two distinct broad emissions: a blue emission centred at approximately 440 nm (2.7 eV) and a dominant red emission around 650 nm (1.9 eV). The 650 nm (1.9 eV) emission present in quartz samples is attributed to the non-bridging oxygen hole centres (NBOHC). The formation of NBOHCs involves multiple mechanisms, including processes associated with radiation damage in quartz (intrinsic mechanisms) as well as hydrogen- or alkali-passivated precursor defects (extrinsic mechanisms) (Skuja et al., 2020). Making a distinction between the two mechanisms of NBOHC formation is a challenging task that has been rarely adressed. In the long run, we aim to demonstrate that NBOHC defects, which may be inherent to a certain extend since crystallization, could serve as reliable indicators of age, particularly for similar types of samples.

Here we study the dynamics of this defect under 15 keV electron irradiation in SEM. The samples were irradiated for different exposure times. Preliminary results indicate a saturating exponential growth of the red emission. While a growth of the NBOHC under irradiation was previously reported in the literature (e.g. Götze et al., 2021), here we describe this increase in a quantitative manner.

The observed behaviour is well-described by equation S(t) = S(0) * (1- exp(-(t-t₀)/t_c)).

Here, t_c represents the critical time, a measure of the time required for the signal to increase by a factor of 1/e. By determining t_c​, a characteristic saturation time, hence dose can be calculated. While experiments are still in progress, for our experimental setup, t_c values range from 300 to 500 s, depending on the sample. The maximum intensity is not showing significant variation across most samples, with a significant exception for the oldest sample investigated. A noteworthy observation is the non-zero value of t₀​, the time intercept, which suggests that NBOHC exists in the quartz samples since crystallization, before irradiation. In the end, quantifying the exposure to the electron beam in terms of radiation dose (energy delivered per unit mass expressed in Gy) is attempted for facilitating direct comparisons with results obtained by other experimental techniques such as electron paramagnetic resonance or optically stimulated luminescence.

Acknowledgement: This research is funded by European Research Council ERC grant PROGRESS-CoG “Reading provenance from ubiquitous quartz: understanding the changes occurring in its lattice defects in its journey in time and space by physical methods”

References:

Skuja, L., Ollier, N., Kajihara, K. 2020. Luminescence of non-bridging oxygen hole centers as a marker of particle irradiation of α-quartz. Radiation Measurements, Volume 135, 106373.

Götze J., Pan Y. & Müller A. 2021. Mineralogy and mineral chemistry of quartz: A review. Mineralogical Magazine. 85(5):639-664. DOI:10.1180/mgm.2021.72