End-Cretaceous akaganéite as a mineral marker of Deccan volcanism in the sedimentary record

Abstract

An enigmatic chloride-rich iron (oxyhydr)oxide has been recently identified together with mercury anomalies in End-Cretaceous marine sediments coeval with the Deccan Traps eruptions. The mineral was observed in Bidart (France) and Gubbio (Italy), suggesting a widespread phenomenon. However, the exact nature and origin of this Cl-bearing mineral remained speculative. Here, we characterized the accurate composition and nanostructure of this chloride-rich phase by using micro-Raman spectroscopy, Transmission (TEM) and Scanning (SEM) Electron Microscopy on Focused Ion Beam foils. We also provide new evidence of its occurrence in Zumaia, a reference KPg section from Spain. Results confirm akaganéite (β-FeOOH) as the main phase, with chloride content of 3–5 atomic weight %. Akaganéite particles are constituted by the aggregation of nanorods of akaganéite. Internal structures contain empty spaces, suggesting formation in a low-density (atmospheric) environment. This new mineralogical evidence supports the hypothesis that the observed akaganéite was formed in the Deccan volcanic plume and was transported to the Atlantic and Tethysian realms through the stratosphere. Therefore, akaganéite provides a potential new sedimentary marker to identify the imprint of the Deccan eruptions in the stratigraphic record and is evidence of volcanic halogen degassing and its potential role for the Cretaceous-Tertiary mass extinction.

Effects of Environmental Fe Concentrations on Formation and Evolution of Allophane in Al‐Si‐Fe Systems: Implications for Both Earth and Mars

“…On Earth, akaganeite often appears as the corrosion product of metallic Fe or Fe(II) in acidic Cl‐rich oxidation conditions (Bibi et al., 2011) and has been found in Cl‐rich environments including soils drained of acidic water, hot brines, marine environments, sediment of sulfur‐rich oxides, carbonate concretion, Fe‐Ni meteorites and even Fe‐rich archeological sites (Buchwald & Clarke, 1989; Font et al., 2017; Réguer et al., 2007). These conditions are distinctly different from the conditions under which allophane typically occurs.…”

Micro‐Raman spectroscopy and complementary techniques for the study of iron weapons from Motya and Lilybaeum (Sicily, Italy): Corrosion patterns in lagoon‐like and calcarenitic hypogea environments

“…[ 9 ] The formation of akaganeite occurs in environments under acidic oxidizing conditions (pH 1–3) as precipitate in chloride rich and acid environments. [ 10 ] Instead, in alkaline media, akageneite can be transformed to goethite and hematite. [ 11 ] Some studies [ 12,13 ] indicated that chlorine does not participate directly in the mechanism of electrochemical reactions, but it plays an indirect role in the corrosion process due to its high hygroscopicity that increases the conductivity of the solution.…”

Effect of Solution pH and Chloride Concentration on Akaganeite Precipitation: Implications for Akaganeite Formation on Mars

“…Solution pH and Cl − concentration are the dominant factors controlling akaganeite formation in terrestrial environments (Cornell & Schwertmann, ). Natural akaganeite usually forms through oxidation of Fe(II) and metallic Fe to Fe(III) followed by Fe(III) hydrolysis to akaganeite under chloride‐rich acidic conditions in hot brines, marine environments, oxidized sulfide‐rich sediments, carbonate concretions, meteorites, volcanic rocks, and plumes (Bibi et al, ; Buchwald & Clarke, ; Font et al, ; Johnston, ; Mackay, ; Morris et al, ; Pye, ). Synthetic akaganeite forms through forced hydrolysis of 0.1–2 M Fe(III) chloride solutions at room temperature (RT) or at elevated temperatures (40–120 °C) under acidic (initial pH < 2) conditions (Zhao et al, ).…”