Laser-produced craters in minerals of a palladium ore sample

Abstract

Laser-induced breakdown spectroscopy (LIBS) is an emerging technique in geochemistry that allows rapid in-situ analysis of the elemental composition and concentration of minerals by laser ablation of the material surface and measurement of the light emitted by the resulting plasma. However, this type of application is still under development for geochemical analyses. Indeed, it is still difficult to know how minerals are ablated under laser pulses in the context of LIBS geochemical analysis using a high-power Q-switched Nd:YAG laser operating at 1064 nm with pulse durations on the order of nanoseconds. Important questions remain unanswered regarding the volume sampled by the laser beam on the minerals to be analyzed, as well as the plasmas induced by the laser on the minerals in air at atmospheric pressure. The objective of this work is to provide insight into laser-mineral interactions within the framework of LIBS geochemical analysis of ore samples with emphasis on the characterization of plasmas and laser ablation craters under ambient air at atmospheric pressure. We study the crater morphology in the three main phases of a palladium ore fragment (Lac des Iles mine, Canada), namely plagioclase feldspar, amphibole and sulfides [Mohamed et al., Geostand Geoanal Res 45:539, (2021)] We performed four series of laser shots (50, 250, 500 and 1000 shots) in the three mineral phases and characterized the morphology of the craters obtained by scanning electron microscopy and optical coherence tomography. It turns out that laser ablation is most effective in plagioclase, presumably due to its lower thermal conductivity. In addition, the temperature and electron density of the plasma were determined for each phase from the iron and nickel lines of LIBS spectra taken 4 µs after the laser shots. They are between 6300 and 8600 °C and about 2 × 1017 cm−3, respectively.