A comprehensive spectroscopic study of synthetic Fe²⁺, Fe³⁺, Mg²⁺ and Al³⁺ copiapite by Raman, XRD, LIBS, MIR and vis–NIR

Abstract

The identification of iron sulfates on Mars by the Mars Exploration Rovers (MERs) and the Mars ReconnaissanceOrbiter emphasized the importance of studying iron sulfates in laboratory simulation experiments. The copiapite group of minerals was suggested as one of the potential iron sulfates occurring on the surface and subsurface on Mars, so it is meaningful to study their spectroscopic features, especially the spectral changes caused by cation substitutions. Four copiapite samples with cation substitutions (Fe 3+ , Al 3+ , Fe 2+ , Mg 2+ ) were synthesized in our laboratory. Their identities were confirmed by powder X-ray diffraction (XRD). Spectroscopic characterizations by Raman, mid-IR, vis-NIR and laser-induced-breakdown spectroscopy (LIBS) were conducted on those synthetic copiapite samples, as these technologies are being (and will be) used in current (and future) missions to Mars. We have found a systematic ν 1 peak shift in the Raman spectra of the copiapite samples with cation substitutions, a consistent atomic ratio detection by LIBS, a set of systematic XRD line shifts representing structural change caused by the cation substitutions and a weakening of selection rules in mid-IR spectra caused by the low site symmetry of (SO 4 ) 2− in the copiapite structures. The near-infrared (NIR) spectra of the trivalent copiapite species show two strong diagnostic water features near 1.4 and 1.9 µm, with two additional bands near 2.0 µm. In the vis-NIR spectra, the position of an electronic band shifts from 0.85 µm for ferricopiapite to 0.866 µm for copiapite, and this shift suggests the appearance of a Fe 2+ electronic transition band near 0.9 µm.

Applying portable Raman spectrometers for field discrimination of sulfates: Training for successful extraterrestrial detection

“…However, the observed band positions are close to the band positions of magnesiocopiapite reported by Kong et al. This is consistent with the XRD analysis, which shows dominant magnesiocopiapte, with gypsum and alunogen as minor constituents.…”

“…Raman spectroscopy has frequently been used for the rapid and unambiguous discrimination of sulfates, even those with similar compositions and structures, such as minerals from the jarosite group or copiapite group . This method is able to track the hydration state of closely related simple sulfates, observing the difference in the positions of the most intensive ν 1 stretching mode bands, as has been shown in the case of sulfates containing magnesium and iron .…”

Spatial distributions of secondary minerals in the Martian meteorite MIL 03346,168 determined by Raman spectroscopic imaging

“…For future planetary missions especially for Mars surface exploration, Raman spectroscopy has a great advantage due to its power in fast molecular (organic and inorganic) phase identification in mixtures, characterization of mineral chemistry, and the minimum‐to‐no sample preparation requirement [e.g., Rull et al ., ; Sharma et al ., ; Wang et al ., , ]. The applications of Raman spectroscopy in planetary sciences and its potential in planetary explorations have been developed since 1995, for igneous mineralogy and petrology that target the Moon, Mars, and Venus [ Freeman et al ., ; Haskin et al ., ; Kuebler et al ., ; Ling et al ., ; Wang et al ., , , , , , , ; Wang , ]; and for secondary mineralogy and alteration processes that target Mars, asteroids, and other primary planetary bodies [ Chio et al ., , ; Chou et al ., ; Gough et al ., , ; Nuding et al ., ; McSween and Treiman , ; Kong et al ., , ; Kong and Wang , ; Ling and Wang , ; Wang et al ., ; Wang and Valentine , ; Wang et al ., , , , ; Y. W. Wang et al ., ; Wang et al ., ; Wang and Ling , ; Wang and Zhou , ; Wei et al ., ; Liu and Wang , ; Wang et al ., submitted to Journal of Raman Spectroscopy ]. Recently, three Raman units have been selected as scientific payloads for 2018 ExoMars and 2020 Mars rover, with the mission goals of fine‐scale definitive mineralogy and biosignature detection.…”

Raman and FTIR spectroscopy applied to the conservation report of paleontological collections: identification of Raman and FTIR signatures of several iron sulfate species such as ferrinatrite and sideronatrite

“…High concentrated sulfuric acid solutions were investigated, but also solid state sulfate salts, such as magnesium, aluminum, and iron sulfates. Recently, a great effort was devoted to the Raman characterization of mixed iron sulfates such as jarosite (X + Fe 3+ 3 (SO 4 ) 2 (OH) 6 ; X=K, NH 4 , Na, Pb…), copiapite (X 2+ Fe 3+ 4 (SO 4 ) 6 (OH) 2 •20H 2 O; X=Mg, Fe, Zn, …), halotrichite (Fe 2+ Al 2 (SO 4 ) 4 •22H 2 O), botryogen (MgFe 3+ (SO 4 ) 2 (OH)•7H 2 O), etc. As a result, many sulfate species are now characterized in Raman and infrared databases .…”