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A Scanning and Transmission Electron Microscopic Study of Oxidation Weathering of Ferrous Minerals: Chlorate and Bromate as Effective Oxidants on Mars

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe (Posters)

Presenting Author:

Amy Schoenenberger

Authors:

Schoenenberger, Amy Larissa¹, Zoesch-Wiegel, Zoe², Mitra, Kaushik³

(1) Earth and Planetary Science, University of Texas at San Antonio, New Braunfuls, TX, USA, (2) Earth and Planetary Science, University of Texas at San Antonio, San Antonio, Texas, USA, (3) Earth and Planetary Science, University of Texas at San Antonio, San Antonio, Texas, USA,

Abstract:

Ferrous [Fe(II)-bearing] minerals such as olivine, pyroxene, pyrite, pyrrhotite, magnetite, and Fe/Mg smectites are commonly found on the surface of Mars and in Martian meteorites. Aqueous alteration of these minerals is believed to be a major source of dissolved Fe(II) in Martian aqueous systems. Previous studies have shown that dissolved Fe(II) is rapidly oxidized by oxyhalogen species (e.g., chlorate, bromate). Additionally, chlorate and bromate brines have been shown to be important oxidizing agents of ferrous minerals on the surface of Mars. Experimental studies on pyrite (FeS₂) and magnetite (Fe₃O₄) have demonstrated that chlorate and bromate brines are effective oxidants. However, the relative amounts of mineral alteration observed at room-temperature conditions in short-term (~100 days) experiments were often minimal at high pH.

The most common analytical tool used to detect and characterize mineral products is an X-ray Diffractometer (XRD). However, if the extent of alteration of the primary minerals is not substantial or the amount of new mineral product is either minor or amorphous, then XRD is not an ideal tool for such an experimental setup. Oxidative weathering studies of ferromagnesian silicate minerals, including olivine, pyroxene, and smectites, experience such slow rates of alteration mainly due to their low dissolution rates. Therefore, in order to accurately investigate the weathering of these minerals by chlorate and bromate, we adopt a microscopy-based approach that would prove instrumental in determining the products of alteration.

Experimental work was conducted using greenalite, Fe/Mg smectite, siderite, forsterite, and fayalite. These minerals were exposed to chlorate and bromate solutions under anoxic conditions for approximately 100 days at 24°C (1 atm). Experiments were performed in an anaerobic chamber using sealed serum bottle reactors with no stirring, simulating stagnant Martian porewater conditions as function of oxidant type, initial pH (10, 7, 5, 3), and background fluid composition (MgCl₂ or MgSO₄). Microscopic analysis (SEM and TEM) shows definite signs of alteration on the surface of primary minerals at microscopic scale. Clear evidence of phyllosilicate minerals coating entire mineral grains was found on olivine grains. The XRD analysis of the same mineral grains proved futile as the scans were nearly identical for both starting and altered minerals. Our preliminary analyses demonstrate that microscopic techniques are more suitable to study low-temperature alteration of primary minerals on Mars.