Manganese Oxides:  Parallels between Abiotic and Biotic Structures

Abstract

A large number of microorganisms are responsible for the oxidation of Mn(2+)((aq)) to insoluble Mn(3+/4+) oxides (MnO(x)()) in natural aquatic systems. This paper reports the structure of the biogenic MnO(x)(), including a quantitative analysis of cation vacancies, formed by the freshwater bacterium Leptothrix discophora SP6 (SP6-MnO(x)()). The structure and the morphology of SP6-MnO(x)() were characterized by transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), including full multiple-scattering analysis, and powder X-ray diffraction (XRD). The biogenic precipitate consists of nanoparticles that are approximately 10 nm by 100 nm in dimension with a fibrillar morphology that resembles twisted sheets. The results dem-onstrate that this biogenic MnO(x)() is composed of sheets of edge-sharing of Mn(4+)O(6) octahedra that form layers. The detailed analysis of the EXAFS spectra indicate that 12 +/- 4% of the Mn(4+) layer cation sites in SP6-MnO(x)() are vacant, whereas the analysis of the XANES suggests that the average oxidation state of Mn is 3.8 +/- 0.3. Therefore, the average chemical formula of SP6-MnO(x)() is M(n)()(+)(y)()Mn(3+)(0.12)[ square(0.12)Mn(4+)(0.88)]O(2).zH(2)O, where M(n)()(+)(y)() represents hydrated interlayer cations, square(0.12) represents Mn(4+) cation vacancies within the layer, and Mn(3+)(0.12) represents hydrated cations that occupy sites above/below these cation vacancies.

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

“…[52][53][54] In a perfectly ordered layer formed from edge-sharing Mn octahedra (that is, di-m-oxo bridging), for each Mn there are 6 neighboring Mn ions at 2. The various protocols used for deposition at constant potential result in inactive oxides with XANES and EXAFS spectra resembling closely the spectra of the inactive Mn oxide shown in Fig.…”

Atomic-Scale Structure of Biogenic Materials by Total X-ray Diffraction: A Study of Bacterial and Fungal MnOx

“…This relatively high water content is consistent with the fact that this biogenic material was formed in an aqueous medium. 15 As can be seen in Figures 1 and 4, the experimental PDF for fungal MnO x has a very strong third peak at about 3.45 Å. This is the typical separation between Mn atoms from octahedra that are connected through their vertices.…”

“…Biogenic MnO x studied here were produced by L. discophora SP-6 bacteria 15 and fungi from the Acremonium strictum family. 22 High-energy XRD patterns for bacterial, fungal, and crystalline MnO 2 (birnessite) standard are shown in Figure 1 (the upper part).…”

“…The relatively high concentration of interlayer {Mn}, and associated layer vacancies, is consistent with an average Mn oxidation state lower than ϩ4 in bacterial MnO x , as found by XANES. 15 The interlayer {Mn} atoms sit on both sides of the layers and share oxygen atoms from both the [MnϪO] layers and the water molecules positioned in between these layers. Thus the oxygen coordination of all Mn atoms in bacterial MnO x appears to be octahedral and the whole material may be viewed as a highly defective "network" of both edge and corner-sharing MnϪO octahedra as shown in Figure 2e.…”

New insight into the origin of manganese oxide ore deposits in the Appalachian Valley and Ridge of northeastern Tennessee and northern Virginia, USA

“…To date, all of the known Mn oxides formed by microorganisms either in the laboratory or via in situ field cultures have been composed of poorly crystalline birnessite-group minerals (layer structure Mn oxides) or poorly crystalline todorokite group minerals (3x3 tunnel structure Mn oxides) (see reviews by Saratovsky et al, 2006;Spiro et al, 2010;Tebo et al, 2004). It is possible that gradual reordering of tiny, poorly crystalline Mn oxide crystals may lead to increased crystallinity over time (described in laboratory experiements by Hinkle et al, 2016;Spilde et al, 2005).…”