Effect of LIBS-Induced Alteration on Subsequent Raman Analysis of Iron Sulfides

Abstract

Mineral alteration is a possible side effect of spectroscopic techniques involving laser ablation, such as laser-induced breakdown spectroscopy (LIBS), and is related to the interaction of the generated plasma and ablated material with samples, dust, or ambient atmosphere. Therefore, it is essential to understand these interactions for analytical techniques involving laser ablation, especially for space research. In this combined LIBS–Raman analytical study, pyrite (FeS 2 ) and pyrrhotite (Fe 1– x S) samples have been consecutively measured with LIBS and Raman spectroscopy, under three different atmospheric conditions: ∼10 –4 mbar (atmosphereless body), ∼7 mbar, and Martian atmospheric composition (Martian surface conditions), and 1 bar and Martian atmospheric composition. Furthermore, a dust layer was simulated using ZnO powder in a separate test and applied to pyrite under Martian atmospheric conditions. In all cases, Raman spectra were obscured after the use of LIBS in the area of and around the formed crater. Additional Raman transitions were detected, associated with sulfur (pyrite, 7.0 mbar and 1.0 bar), polysulfides (all conditions), and magnetite (both minerals, 1.0 bar). Magnetite and polysulfides formed a thin film of up to 350–420 and 70–400 nm in the outer part of the LIBS crater, respectively. The ZnO-dust test led to the removal of the dust layer, with a similar alteration to the nondust pyrite test at 7.0 mbar. The tests indicate that recombination with the CO 2 -rich atmosphere is significant at least for pressures from 1.0 bar and that plasma–dust interaction is insignificant. The formation of sulfur and polysulfides indicates fractionation and possible loss of volatile elements caused by the heat of the LIBS laser. This should be taken into account when interpreting combined LIBS–Raman analyses of minerals containing volatile elements on planetary surfaces.

Alteration in the Raman spectra of characteristic rock‐forming silicate mixtures due to micrometeorite bombardment

“…The samples were held in a high‐vacuum chamber, simulating planetary bodies without atmosphere, during irradiation and as long as possible after it. While atomic emission from the laser‐ablated ejecta in previous simulation studies 31 reveal a residual abundance of ambient atmosphere obviously trapped in the remained cavities between particles in the pressed pellets, its influence on the Raman spectra of silicates is vanishing. The pressure to at least 10 −4 Pa was achieved with a newly designed Praying Mantis™ high‐temperature reaction chamber 9 .…”

“…(A) Iron oxides in the Raman spectra of the irradiated sample (ID 368). Wüstite (purple) with its typical band centred peak 30 at about 590 cm À1 and maghemite 31 (brownish) with the typical band centred DB at around 700 cm À1 (693/706 cm À1 ) can be identified in the spectra of the pellet with the ID 368. In both spectra, also, olivine is detected by the typical DP at 826/857 cm À1 .…”

Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance

“…3). Several studies have been dedicated to determining the influence of LIBS analysis on subsequent Raman observations 38,43,44 . All of these methods highlight potential modifications of the targets due to LIBS analysis within but also close to the ablation crater, such as amorphization, melting and/ or phase transition.…”