The crystal structure of feitknechtite (β-MnOOH) and a new MnOOH polymorph

Journal Article · Wed Nov 01 00:00:00 UTC 2023 · American Mineralogist

DOI: https://doi.org/10.2138/am-2022-8729 · OSTI ID:2496218

Post, Jeffrey E. ^[1]; Heaney, Peter J. ^[2]; Ilton, Eugene S. ^[3]; Elzinga, Evert J. ^[4]

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1. Smithsonian Institute, Washington, DC (United States)
2. Pennsylvania State Univ., University Park, PA (United States)
3. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
4. Rutgers University, Newark, NJ (United States)

Studies suggest that feitknechtite (β-MnOOH) is a prevalent, and perhaps necessary, intermediate phase during the synthesis of birnessite-like phases, the abiotic oxidation of Mn2+, and 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 (or cadmium iodide-like) Mn-O octahedral layers, but a detailed crystal structure has not been reported. For this work, 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). 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 (proposed designation ε-MnOOH), mixed with the synthetic feitknechtite, that is similar to β-MnOOH but with a different layer stacking.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-06CH11357; AC05-76RL01830; FG02-94ER14466
• OSTI ID: 2496218
• Report Number(s): PNNL-SA--175790
• Journal Information: American Mineralogist, Journal Name: American Mineralogist Journal Issue: 11 Vol. 108; ISSN 0003-004X
• Publisher: Mineralogical Society of AmericaCopyright Statement
• Country of Publication: United States
• Language: English