T83 Charting Soils at the Atomic Scale I: A Tribute to the Careers of David L. Bish and Jeffrey E. Post

Monday, 10 October 2022
8:00 AM - 12:00 PM
607

THE CRYSTAL STRUCTURE OF FEITKNECHTITE (Β-MNOOH) AND A NEW MNOOH POLYMORPH

Paper number: T83-2
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Abstract

Feitknechtite (β-MnOOH) is a prevalent, and perhaps necessary, intermediate phase for the synthesis of birnessite-like phases, for the abiotic oxidation of Mn2+, and for the transformation of biogenic hexagonal phyllomanganates to more complex Mn oxides in laboratory and natural systems. Researchers have generally described feitknechtite as consisting of pyrochroite-like Mn3+-O octahedral layers, but a detailed crystal structure has not been reported. We used TEM/SAED and powder XRD and Rietveld refinements to derive the unit cell and, for the first time, report a complete structure description for feitknechtite (β-MnOOH). Our structure solution and refinement confirm that feitknechtite is constructed of parallel layers of Mn3+-(O,OH) octahedra, making it the only known Mn3+ layer structure. The Mn3+ Jahn-Teller distortions for the Mn-(O,OH) octahedra result in elongated axial bonds [2.286(1) Å] relative to the four axial distances [1.933(1) Å]. The monoclinic (C2/m) unit cell has a = 5.6541(4) Å, b = 2.89075(2) Å, c = 4.8347(3) Å, and β = 108.0709(1)°. Rietveld refinements were also completed for three natural feitknechtite/hausmannite samples, and time-resolved synchrotron XRD experiments were used to follow the thermal transformation of feitknechtite to hausmannite. Additionally, we identified and report the structure for a second, and perhaps novel, MnOOH polymorph, mixed with the synthetic feitknectite, that is similar to β-MnOOH but with a different layer stacking.

The lack of knowledge of the atomic structure of feitknechtite has limited understanding of its precise role and behavior in synthetic and natural redox reactions in manganiferous systems. This work provides the formal structure description required for model calculations that will provide a better understanding of this phase, and it make possible predictions about the behavior of β-MnOOH (feitknechtite) under a range of conditions. It also provides an essential starting point for analyzing powder diffraction data to determine and monitor unit-cell parameters and other structure details, and for quantitative phase analyses of Mn oxide samples that contain feitknechtite.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380829

© Copyright 2022 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.