Linking feldspar luminescence phenomena and mineralogy using spatially resolved techniques

Feldspars, the most abundant mineral group in the Earth’s crust, exhibit a striking chemical and structural variety resulting from variations in their crystallisation environments and post-crystallisation processes. Since feldspars record both formation and alteration processes and conditions, they are key components in geological studies and valuable for developing tools to reconstruct past environmental change. Thermally and optically stimulated luminescence, the ability of minerals, such as feldspars, to emit light in different wavelengths upon excitation with light or heat, are sensitive to changes in the crystal lattice and thus may have the potential to be used as a tool for identifying crystallisation and alteration processes affecting feldspars. So far, the luminescence of feldspars has primarily been used as a numerical geochronological dating technique, constraining past geological and archaeological events and processes. However, the sensitivity of feldspar luminescence signals to changes in crystallisation and post-crystallisation processes allows for the development of luminescence-based proxies beyond geochronological studies. This study investigates the relationship between luminescence emission intensities in three emission bands and mineralogical features in 13 alkali feldspars and plagioclases using spatially resolved luminescence techniques, electron microprobe analysis, bulk geochemistry, and X-ray powder diffraction. The intensity of infrared photoluminescence (IRPL) emission (~ 900 and ~ 950 nm) is particularly sensitive to chemical and structural heterogeneities within the feldspar samples, such as the presence of K-rich phases, fractures, and alteration products. Blue (~ 400 nm) thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission intensities vary with feldspar type and perthitic texture, while yellow-green (~ 560 nm) TL and IRSL emissions are linked to fluid-induced alteration of the feldspar chemistry and structure. These findings highlight the potential of spatially resolved luminescence measurements as a non-destructive tool for identifying crystallisation and alteration processes in feldspars, as well as relationships between luminescence signal intensity and mineralogical properties, advancing both geochronological techniques which use these emissions, and mineralogical studies.