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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 observations of two different type 3 crystals from MndBi3_10y. a Shows the presence of a dislocation perpendicular to c*, highlighted with arrows in c which corresponds to the area delimited with a dotted line in a. b Shows evidence for the disruption of lattice fringes perpendicular to ab and thus certainly witnesses aggregation of two crystals, as highlighted with a dotted line in d which is a magnified view of the area delimited with a dotted line in b. In both crystals presented here, lattice fringes separated by 6.9 and 9.5 Å could be observed, close to the values expected for the layer-to-layer distances in δ-MnO2 having respectively one and two planes of interlayer water molecules
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Fig9: TEM observations of two different type 3 crystals from MndBi3_10y. a Shows the presence of a dislocation perpendicular to c*, highlighted with arrows in c which corresponds to the area delimited with a dotted line in a. b Shows evidence for the disruption of lattice fringes perpendicular to ab and thus certainly witnesses aggregation of two crystals, as highlighted with a dotted line in d which is a magnified view of the area delimited with a dotted line in b. In both crystals presented here, lattice fringes separated by 6.9 and 9.5 Å could be observed, close to the values expected for the layer-to-layer distances in δ-MnO2 having respectively one and two planes of interlayer water molecules

Mentions: Observation of MndBi3_10y samples revealed a much more complex assemblage. The sample contained at least four types of crystals distinguishable on the basis of their morphology and size (Figs. 7, 8). Crystal type 1 had sizes of 5–20 nm, similar to fresh samples (data not shown) and MndBi10_10y. Crystal type 2 had a needle-like morphology, typically 10–30 nm in length. Crystal type 3 had lath-like morphology, with its long distance in the ab plane (20–150 nm) up to ~5–10 times larger than crystal type 2. It was also typically 2–10 times wider, and interference fringes were frequently disrupted, as if it was built of aggregated type 2 crystals (Fig. 9). Aggregation was observed to occur within the ab plane, in agreement with the aggregation mechanism proposed previously [50, 60, 72]. It was also observed along c*, with disruption of interference fringes that had geometrical shapes identical to previous observations [60, 72, 73] and indicate that growth may also take place through stacking along c* of two or more type 2 crystals. Finally, crystal type 4 often had typical sizes of 50–150 nm in the ab plane and 10–50 nm along c*, and was built of stacks of lath-like layers resembling type 3 crystals rotated by n × 120° (n being equal to 1 or 2) relative to each other.Fig. 7


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 observations of two different type 3 crystals from MndBi3_10y. a Shows the presence of a dislocation perpendicular to c*, highlighted with arrows in c which corresponds to the area delimited with a dotted line in a. b Shows evidence for the disruption of lattice fringes perpendicular to ab and thus certainly witnesses aggregation of two crystals, as highlighted with a dotted line in d which is a magnified view of the area delimited with a dotted line in b. In both crystals presented here, lattice fringes separated by 6.9 and 9.5 Å could be observed, close to the values expected for the layer-to-layer distances in δ-MnO2 having respectively one and two planes of interlayer water molecules
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig9: TEM observations of two different type 3 crystals from MndBi3_10y. a Shows the presence of a dislocation perpendicular to c*, highlighted with arrows in c which corresponds to the area delimited with a dotted line in a. b Shows evidence for the disruption of lattice fringes perpendicular to ab and thus certainly witnesses aggregation of two crystals, as highlighted with a dotted line in d which is a magnified view of the area delimited with a dotted line in b. In both crystals presented here, lattice fringes separated by 6.9 and 9.5 Å could be observed, close to the values expected for the layer-to-layer distances in δ-MnO2 having respectively one and two planes of interlayer water molecules
Mentions: Observation of MndBi3_10y samples revealed a much more complex assemblage. The sample contained at least four types of crystals distinguishable on the basis of their morphology and size (Figs. 7, 8). Crystal type 1 had sizes of 5–20 nm, similar to fresh samples (data not shown) and MndBi10_10y. Crystal type 2 had a needle-like morphology, typically 10–30 nm in length. Crystal type 3 had lath-like morphology, with its long distance in the ab plane (20–150 nm) up to ~5–10 times larger than crystal type 2. It was also typically 2–10 times wider, and interference fringes were frequently disrupted, as if it was built of aggregated type 2 crystals (Fig. 9). Aggregation was observed to occur within the ab plane, in agreement with the aggregation mechanism proposed previously [50, 60, 72]. It was also observed along c*, with disruption of interference fringes that had geometrical shapes identical to previous observations [60, 72, 73] and indicate that growth may also take place through stacking along c* of two or more type 2 crystals. Finally, crystal type 4 often had typical sizes of 50–150 nm in the ab plane and 10–50 nm along c*, and was built of stacks of lath-like layers resembling type 3 crystals rotated by n × 120° (n being equal to 1 or 2) relative to each other.Fig. 7

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