Spectral and morphological characteristics of synthetic nanophase iron (oxyhydr)oxides

Abstract

Nanophase iron (oxyhydr)oxides are ubiquitous on Earth, globally distributed on Mars, and likely present on numerous other rocky solar system bodies. They are often structurally and, therefore, spectrally distinct from iron (oxyhydr)oxide bulk phases. Because their spectra vary with grain size, they can be difficult to identify or distinguish unless multiple analysis techniques are used in tandem. Yet, most literature reports fail to use multiple techniques or adequately parameterize sample morphology, making it difficult to understand how morphology affects spectral characteristics across techniques. Here, we present transmission electron microscopy, Raman, visible and near-infrared, and mid-infrared attenuated total reflectance data on synthetic, nanophase akaganéite, lepidocrocite, goethite, hematite, ferrihydrite, magnetite, and maghemite. Feature positions are tabulated and compared to those for bulk (oxyhydr)oxides and other nanophase iron (oxyhydr)oxides from the literature. The utility and limitations of each technique in analyzing nanophase iron (oxyhydr)oxides are discussed. Raman, mid-infrared, and visible near-infrared spectra show broadening, loss of some spectral features, and shifted positions compared to bulk phases. Raman and mid-infrared spectroscopies are useful in identifying and distinguishing akaganéite, lepidocrocite, goethite, and hematite, though ferrihydrite, magnetite, and maghemite have overlapped band positions. Visible near-infrared spectroscopy can identify and distinguish among ferrihydrite, magnetite, and maghemite in pure spectra, though akaganéite, lepidocrocite, and goethite can have overlapping bands. It is clear from this work that further understanding of variable spectral features in nanophase iron (oxyhydr)oxides must await additional studies to robustly assess effects of morphology. This study establishes a template for future work.

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions

“…The nanolite peak (see Fig. 1A) represents the vibration of Fe─O bonds, but the position and sharpness are consistent with those bonds being in the FeO oxide mineral (possibly magnetite), suggesting that the nanoparticles are "crystalline" rather than features of the melt structure [e.g., (66)]. This crystallinity has also been confirmed by lattice images observed during HAADF imaging and by selected area electron diffraction during our STEM imaging.…”

Reduction and Morphological Transformation of Synthetic Nanophase Iron Oxide Minerals by Hyperthermophilic Archaea

“…Ten nanophase Fe(III) (oxyhydr)oxides were synthesized for this study: two-line ferrihydrite (Fh), akaganéite (Ak102315), goethite (Goet011515 and Goet012315), lepidocrocite (Lep030415 and Lep100615), hematite (Hem100915 and Hem022015), maghemite (Magh061815), and magnetite (Mag060516) (Supplementary Figure S1 ). The synthesis procedures for these minerals, except Lep100615 and Hem022015, were described previously ( Sklute et al, 2018 ). Hem022015 was synthesized based on hematite “Method 5” and Lep100615 following the main synthesis method in Schwertmann and Cornell (2000) .…”

“…Hem022015 was synthesized based on hematite “Method 5” and Lep100615 following the main synthesis method in Schwertmann and Cornell (2000) . All mineral syntheses were confirmed for phase purity and identity using TEM; visible and near infrared, mid-infrared, and Raman spectroscopies ( Sklute et al, 2018 ); powder XRD; and Mössbauer spectroscopy. All the synthetic Fe(III) (oxyhydr)oxides were kept in solution at 4°C to maintain fluid-mineral properties and prevent drying induced changes in phase and crystallinity.…”

Amorphous salts formed from rapid dehydration of multicomponent chloride and ferric sulfate brines: Implications for Mars

“…Mixtures are molar ratios and abbreviations are FS for Fe 2 (SO 4 ) 3 , MgCl for MgCl 2 , FeCl for FeCl 3 , CaCl for CaCl 2 , and BC for NaHCO 3 . The akaganéite and hematite are synthetic nanophase samples Ak102315 and Hem100915, respectively Sklute et al, 2017); the rhomboclase is sample JB-JLB-74A aka MLS84; the kornelite is sample R16185 aka R16783; and the lausenite is sample 102,923. (a) Halite, gypsum, and calcite ATR spectra are from the RRUFF database (sample IDs R070292, R040029, and R040070, respectively (Lafuente et al, 2015;Dyar, 2015a)).…”