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Synthesis and structural characterization of ferrous trioctahedral smectites: Implications for clay mineral genesis and detectability on Mars
Author(s) -
Chemtob Steven M.,
Nickerson Ryan D.,
Morris Richard V.,
Agresti David G.,
Catalano Jeffrey G.
Publication year - 2015
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1002/2014je004763
Subject(s) - clay minerals , x ray absorption spectroscopy , silicate , saponite , ferrous , chemistry , absorption spectroscopy , infrared spectroscopy , ferrihydrite , analytical chemistry (journal) , mineralogy , materials science , adsorption , physics , organic chemistry , quantum mechanics , chromatography
Widespread detections of phyllosilicates in Noachian terrains on Mars imply a history of near‐surface fluid‐rock interaction. Ferrous trioctahedral smectites are thermodynamically predicted products of basalt weathering on early Mars, but to date only Fe 3+ ‐bearing dioctahedral smectites have been identified from orbital observations. In general, the physicochemical properties of ferrous smectites are poorly studied because they are susceptible to air oxidation. In this study, eight Fe 2+ ‐bearing smectites were synthesized from Fe 2+ ‐Mg‐Al silicate gels at 200°C under anoxic conditions. Samples were characterized by inductively coupled plasma optical emission spectrometry, powder X‐ray diffraction, Fe K‐edge X‐ray absorption spectroscopy (XAS), Mössbauer spectroscopy, and visible/near‐infrared (VNIR) reflectance spectroscopy. The range of redox states was Fe 3+ /ΣFe = 0 to 0.06 ± 0.01 as determined by both XAS and, for short integration times, Mössbauer. The smectites have 060 distances ( d (060) ) between 1.53 and 1.56 Å, indicating a trioctahedral structure. d (060) and XAS‐derived interatomic Fe‐(Fe,Mg,Al) distance scaled with Fe content. Smectite VNIR spectra feature OH/H 2 O absorption bands at 1.4 and 1.9 µm, (Fe 2+ ,Mg,Al) 3 ‐OH stretching bands near 1.4 µm, and Fe 2+ Fe 2+ Fe 2+ ‐OH, MgMgMg‐OH, AlAl(Mg,Fe 2+ )‐OH, and AlAl‐OH combination bands at 2.36 µm, 2.32 µm 2.25 µm, and 2.20 µm, respectively. The spectra for ferrous saponites are distinct from those for dioctahedral ferric smectites, permitting their differentiation from orbital observations. X‐ray diffraction patterns for synthetic high‐Mg ferrosaponite and high‐Mg ferrian saponite are both consistent with the Sheepbed saponite detected by the chemistry and mineralogy (CheMin) instrument at Gale Crater, Mars, suggesting that anoxic basalt alteration was a viable pathway for clay mineral formation on early Mars.