Optimized laser-induced luminescence imaging of terbium³⁺ in natural calcites

An optimized laser-induced luminescence analysis protocol for terbium detection was developed and successfully tested on Finnish natural calcite samples. These results were also compared with photoluminescence spectra and to the key element contents determined with LA-ICP-MS. When the excitation laser wavelength is chosen carefully, even slight variations in terbium content can be detected and complex interactions with other luminescent activators, sensitizers, and quenchers can be revealed. At an optimal excitation wavelength of 226 nm, luminescence of Tb³⁺ can be observed in calcite at contents as low as 0.01 ppm. An additional benefit of this specific wavelength is that it does not efficiently excite broad Mn²⁺ emission, which would otherwise dominate the total luminescence and affect image interpretation. When analysing the results of Tb³⁺ luminescence imaging, two different approaches were used: the developed normalized G map method and t-SNE. The Tb³⁺ luminescence imaging protocol presented here may prove to be a valuable technique in mineral prospecting, as it provides a proxy for the overall REE potential of the analysed minerals and may even reveal chemical distributions in minerals (e.g. zoning) that would otherwise remain undetected. Nevertheless, although luminescence imaging is a fast and highly sensitive method, the quantitative detection of terbium in natural calcites remains challenging.