T138 Effects of Mineral-Water Interface Complexity on Geochemical Processes: A Session in Honor of Mineralogical Society of America Awardee for 2018, Laura Nielsen Lammers

Monday, 23 September 2019
1:30 PM - 5:30 PM
Room 229B, North Building

STRUCTURAL AND CHEMICAL COMPLEXITY OF PYRITE (001) SURFACES

Paper number: T138-14
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

Pyrite, FeS2, is a widespread sulfide mineral that is thermodynamically stable under reducing conditions and in saturated sediments provides surfaces for the adsorption of aqueous species. When exposed to oxidizing environments, pyrite oxidation leads to the generation of low pH solutions termed acid mine drainage. Detailed understanding of the reactivity of pyrite surfaces requires knowledge of their atomic structure under relevant conditions. We developed an anaerobic chemical–mechanical polishing method that creates low-roughness pyrite (001) surfaces shown by atomic force microscopy (AFM) to be dominated by irregularly shaped (001) terraces. We studied the structure of the hydrated pyrite (001) surface using the crystal truncation rod (CTR) method, obtaining reproducible results from three crystals. Optimal fitting of the CTR data requires the incorporation of two structurally distinct termination surfaces that differ in the coverage, and likely the chemical speciation, of surface sulfur atoms. In each case, surface iron atoms and disulfur groups are significantly displaced from bulk positions. The adsorption of aqueous metal ions provides a complementary approach for assessing the chemical speciation of sites on the (001) surface. We observed modulation in the CTR scattering patterns following exposure to Fe3+ and Pb2+ ions, confirming that the termination surface is accessible and chemically reactive. Grazing incidence extended X-ray absorption fine structure (EXAFS) studies of surface-adsorbed Pb2+ revealed at least two near-neighbor distances consistent with a fraction of lead atoms adsorbing via inner-shell interactions with surface sulfur sites. We used ambient pressure X-ray photoelectron spectroscopy (XPS) with synchrotron soft-X-ray excitation to characterize the chemical state of S and Fe on pristine (001) surfaces. The data confirm the FeS2 (001) surface to be terminated solely by Fe2+ and S2- groups and constrain the stoichiometry. Following exposure to partial pressures of water (up to 1 mTorr) and oxygen (up to 1 µTorr) the surface exhibits both reversible and irreversible changes in S and Fe oxidation state. The studies reveal key aspects of a molecular model for pyrite surface structure and reactivity.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-338158

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.