Exploring variability in the luminescence properties of feldspars

Abstract

Feldspars are the most abundant minerals in the Earth's crust and they have the ability to store charge within defects in their crystal lattice over geological time scales. This allows their use as natural dosimeters in luminescence dating studies, which enables chronologies of past geomorphological, geological and archaeological events to be established. Despite the routine use of feldspars in luminescence dating studies over the past decades, many key questions regarding the physical processes leading to luminescence of feldspars remain unanswered. For example, the crystal defects functioning as electron trapping centres in feldspars are still unknown, and the causes of variability seen in anomalous fading rates of the blue emission (⇠410 nm) in feldspars have not been fully identified. Since feldspars are complex framework silicates with a variety of chemical compositions and mineralogical properties, linking properties inherent to the feldspar to particular luminescence characteristics is challenging. This thesis aims at establishing a better understanding of potential relationships between feldspar chemical composition, structural state and the number of phases present within a crystal and the luminescence properties of selected feldspar samples. To achieve this goal, this thesis investigates the luminescence properties of a selection of single crystal feldspars and feldspars of grain mixtures extracted from sediment and bedrock, by using a combination of excitation and emission spectroscopy, photo- radio-, thermo- and infrared stimulated luminescence and anomalous fading measurements. Electron trap depths ranging from 2 eV to 2.4 eV were found in chemically and structurally different feldspars. The IR resonance peak, likely reflecting the first excited state of IR-sensitive electron trapping centres, is located ⇠1.45 eV above the ground state of the trap. Similarities in energies measured for the ground and excited state energies across the sample suite indicate that defects functioning as electron trapping centres are likely located on the Si,Al-framework. Site-selective infrared photoluminescence (IRPL) excitation-emission spectroscopy revealed up to three different lattice environments in which the investigated electron trapping centres might be located. A comparison of chemically and structurally different alkali feldspars indicated the presence of K+ ions on M sites as one likely influence of the lattice environment of electron trapping centres in feldspars. Disorder of the tetrahedral site occupancy of Al3+ ions has only little effect on electron trapping centres and related IRPL emissions, and no correlation was found between the number of phases present in a single crystal feldspar (i.e. whether it is single phase or perthitic) and electron trapping centres. Thus, observed differences in electron trap depths and IRPL emissions are influenced by additional factors, not explored in this thesis. The width of the sub-conduction band-tail states range from 0.2 to 0.8 eV in the samples investigated. This suggests different impacts on charge mobility, and stability of trapped electrons in these samples. The intensity and stability of the blue luminescence emission (⇠410 nm), the emission most commonly used in luminescence dating studies, is suggested to be influenced by the degree of order of Al3+ ions on the framework. It is proposed that crystal defects giving rise to the blue luminescence emission are not only related to feldspars from geological environments where the high structural state (disorder of Al3+ ions on the framework) is retained during rapid cooling of the magma (e.g. volcanic origin), but is also related to perthites. In perthites the interfaces between K- and Na-feldspar lamellae are likely to host a high density of defects, resulting not only in intense blue emission, but also in high anomalous fading rates, making fading correction of luminescence ages inevitable. Research in this thesis shows the complexity of factors influencing luminescence properties in feldspars, which has to be kept in mind, when trying to further improve the use of feldspars as natural luminescence dosimeters.

Date of Award	2020
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Helen Roberts (Supervisor), Nick Pearce (Supervisor) & Geoff Duller (Supervisor)