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Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms.

Grangeon S, Fernandez-Martinez A, Warmont F, Gloter A, Marty N, Poulain A, Lanson B - Geochem. Trans. (2015)

Bottom Line: In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor.Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals.The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones.

View Article: PubMed Central - PubMed

Affiliation: BRGM, 3 Avenue Guillemin, 45060 Orléans Cedex 2, France.

ABSTRACT

Background: Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO6)(8-) octahedra connected through their edges and frequently contain vacancies and  (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO2) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy.

Results: Migration of Mn(3+) from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50-150 nm in the ab plane and ~10-50 nm along c*, that is a tenfold increase compared to fresh samples.

Conclusion: The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.

No MeSH data available.


Related in: MedlinePlus

TEM imaging of cryptomelane domains in type 4 crystals of MndBi3_10y. a, b TEM observations of two different crystals type 4 from sample MndBi3_10y. In both crystals, a tunnel-like structure is observed. It mainly has a tunnel size of 6.9 Å × 6.9 Å, but structures having heterogeneous tunnel size are also observed (arrow with dotted line). Areas with dotted lines indicate zones corresponding to the enlarged pictures (bottom right of a and top left of b)
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Fig10: TEM imaging of cryptomelane domains in type 4 crystals of MndBi3_10y. a, b TEM observations of two different crystals type 4 from sample MndBi3_10y. In both crystals, a tunnel-like structure is observed. It mainly has a tunnel size of 6.9 Å × 6.9 Å, but structures having heterogeneous tunnel size are also observed (arrow with dotted line). Areas with dotted lines indicate zones corresponding to the enlarged pictures (bottom right of a and top left of b)

Mentions: Similar to MndBi10_10y, crystal type 1 in MndBi3_10y exhibited SAED patterns typical for δ-MnO2, with only weak diffraction maxima at 2.42 and 1.40 Å (Fig. 8). Contrastingly, crystal type 4 exhibited additional reflections (e.g., at 2.19, 1.80, and 1.54 Å—Fig. 8) attributed to cryptomelane. Consequently, MndBi3_10y is built of four main types of crystals: crystal type 1 is a phyllomanganate, whereas crystal type 4 locally has a tectomanganate-like structure. We propose that crystals type 2 and 3, that could not be investigated by SAED as no isolated crystal or homogeneous aggregate could be found, have an intermediate structure. When viewed perpendicular to the c* axis, crystal type 4 systematically showed the presence of tunnel-like structures, with sizes of 6.9 Å × 6.9 Å typical for cryptomelane (Fig. 10). The minor presence of heterogeneous tunnel size (dotted arrows in Fig. 10) is likely. Finally, analysis of images collected on type 1, 2 and 3 crystals (Figs. 7a, b, 9) reveal distances of ~6.9–7.1 Å, consistent with layer-to-layer distance of phyllomanganates hosting a single plane of interlayer H2O molecules. 9.5 Å distances expected for phyllomanganates hosting two planes of water molecules [74] were also observed, although much less frequent. This uncommon persistence of highly hydrated states under TEM vacuum conditions [60, 75] is likely related to sample impregnation in resin prior to analysis.Fig. 10


Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms.

Grangeon S, Fernandez-Martinez A, Warmont F, Gloter A, Marty N, Poulain A, Lanson B - Geochem. Trans. (2015)

TEM imaging of cryptomelane domains in type 4 crystals of MndBi3_10y. a, b TEM observations of two different crystals type 4 from sample MndBi3_10y. In both crystals, a tunnel-like structure is observed. It mainly has a tunnel size of 6.9 Å × 6.9 Å, but structures having heterogeneous tunnel size are also observed (arrow with dotted line). Areas with dotted lines indicate zones corresponding to the enlarged pictures (bottom right of a and top left of b)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4556320&req=5

Fig10: TEM imaging of cryptomelane domains in type 4 crystals of MndBi3_10y. a, b TEM observations of two different crystals type 4 from sample MndBi3_10y. In both crystals, a tunnel-like structure is observed. It mainly has a tunnel size of 6.9 Å × 6.9 Å, but structures having heterogeneous tunnel size are also observed (arrow with dotted line). Areas with dotted lines indicate zones corresponding to the enlarged pictures (bottom right of a and top left of b)
Mentions: Similar to MndBi10_10y, crystal type 1 in MndBi3_10y exhibited SAED patterns typical for δ-MnO2, with only weak diffraction maxima at 2.42 and 1.40 Å (Fig. 8). Contrastingly, crystal type 4 exhibited additional reflections (e.g., at 2.19, 1.80, and 1.54 Å—Fig. 8) attributed to cryptomelane. Consequently, MndBi3_10y is built of four main types of crystals: crystal type 1 is a phyllomanganate, whereas crystal type 4 locally has a tectomanganate-like structure. We propose that crystals type 2 and 3, that could not be investigated by SAED as no isolated crystal or homogeneous aggregate could be found, have an intermediate structure. When viewed perpendicular to the c* axis, crystal type 4 systematically showed the presence of tunnel-like structures, with sizes of 6.9 Å × 6.9 Å typical for cryptomelane (Fig. 10). The minor presence of heterogeneous tunnel size (dotted arrows in Fig. 10) is likely. Finally, analysis of images collected on type 1, 2 and 3 crystals (Figs. 7a, b, 9) reveal distances of ~6.9–7.1 Å, consistent with layer-to-layer distance of phyllomanganates hosting a single plane of interlayer H2O molecules. 9.5 Å distances expected for phyllomanganates hosting two planes of water molecules [74] were also observed, although much less frequent. This uncommon persistence of highly hydrated states under TEM vacuum conditions [60, 75] is likely related to sample impregnation in resin prior to analysis.Fig. 10

Bottom Line: In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor.Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals.The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones.

View Article: PubMed Central - PubMed

Affiliation: BRGM, 3 Avenue Guillemin, 45060 Orléans Cedex 2, France.

ABSTRACT

Background: Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO6)(8-) octahedra connected through their edges and frequently contain vacancies and  (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO2) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy.

Results: Migration of Mn(3+) from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50-150 nm in the ab plane and ~10-50 nm along c*, that is a tenfold increase compared to fresh samples.

Conclusion: The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.

No MeSH data available.


Related in: MedlinePlus