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

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.