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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

Comparison of PDF of all studied samples. Main panel PDF of MndBi3_10y (orange), MndBi4_10y (purple), MndBi8_10y (green) and MndBi10_10y (blue). Inset at the top right shows alternations of decreasing (black arrows) and increasing (grey arrows) correlations, from MndBi10_10y to MndBi3_10y. Only MndBi10_10y and MndBi3_10y are shown in the inset to ease visualization
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Fig3: Comparison of PDF of all studied samples. Main panel PDF of MndBi3_10y (orange), MndBi4_10y (purple), MndBi8_10y (green) and MndBi10_10y (blue). Inset at the top right shows alternations of decreasing (black arrows) and increasing (grey arrows) correlations, from MndBi10_10y to MndBi3_10y. Only MndBi10_10y and MndBi3_10y are shown in the inset to ease visualization

Mentions: From a qualitative examination of the PDF data (Fig. 3), a systematic evolution is observed with sample equilibration pH. From MndBi10_10y to MndBi3_10y, correlations at 2.87, 4.95, 5.72, and 7.56 Å decrease in intensity (although the third appears less affected). These correlations are attributed to atomic pairs involving two layer Mn atoms and forming the first, second, third, and fourth Mn shells around a given layer Mn (Mn–MnL1, Mn–MnL2, Mn–MnL3 and Mn–MnL4 shells—Fig. 4a). Contrastingly, correlations at 3.45 and 5.32 Å increase in intensity. They are attributed to pairs formed by a layer Mn atom and a TCMn (Mn–TCMn pairs—Fig. 4b), with TCMn at vacancies belonging respectively to the first (Mn–TCMn1) and second (Mn–TCMn2) Mn–MnL shells (Fig. 4a, b, c). The number of layer vacancies thus increases from MndBi10_10y to MndBi3_10y, TCMn being sorbed above these vacancies. The first, second and fourth Mn–MnL shells are more affected than the third shell. All these shells contain, in a defect-free δ-MnO2 layer, 6 Mn atoms. Thus, a layer vacancy normally affects equally all of these shells, and the observed behavior can most straightforwardly be explained by an ordered layer vacancy distribution. Finally, the correlation at 7.22 Å increases in intensity from MndBi10_10y to MndBi3_10y. It corresponds to a TCMn–TCMn pair, with TCMn atoms being on opposite sides of layer vacancies separated from each other by one layer Mn atom (TCMn–TCMn1 in Fig. 4c). All these correlations and in particular the TCMn–TCMn one are typical for δ-MnO2 layers having 0.25 layer vacancy per layer octahedron and TCMn as in Fig. 4c. In addition, the correlation at 6.13 Å (Mn–TCMn3 pair, i.e., TCMn above a vacancy belonging to the Mn–MnL3 shell) may indicate the minor presence of domains with 0.33 vacancy per layer octahedron in low pH samples (Fig. 4d). Such domains could correspond to the cryptomelane-like structure or to their precursors.Fig. 3


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)

Comparison of PDF of all studied samples. Main panel PDF of MndBi3_10y (orange), MndBi4_10y (purple), MndBi8_10y (green) and MndBi10_10y (blue). Inset at the top right shows alternations of decreasing (black arrows) and increasing (grey arrows) correlations, from MndBi10_10y to MndBi3_10y. Only MndBi10_10y and MndBi3_10y are shown in the inset to ease visualization
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig3: Comparison of PDF of all studied samples. Main panel PDF of MndBi3_10y (orange), MndBi4_10y (purple), MndBi8_10y (green) and MndBi10_10y (blue). Inset at the top right shows alternations of decreasing (black arrows) and increasing (grey arrows) correlations, from MndBi10_10y to MndBi3_10y. Only MndBi10_10y and MndBi3_10y are shown in the inset to ease visualization
Mentions: From a qualitative examination of the PDF data (Fig. 3), a systematic evolution is observed with sample equilibration pH. From MndBi10_10y to MndBi3_10y, correlations at 2.87, 4.95, 5.72, and 7.56 Å decrease in intensity (although the third appears less affected). These correlations are attributed to atomic pairs involving two layer Mn atoms and forming the first, second, third, and fourth Mn shells around a given layer Mn (Mn–MnL1, Mn–MnL2, Mn–MnL3 and Mn–MnL4 shells—Fig. 4a). Contrastingly, correlations at 3.45 and 5.32 Å increase in intensity. They are attributed to pairs formed by a layer Mn atom and a TCMn (Mn–TCMn pairs—Fig. 4b), with TCMn at vacancies belonging respectively to the first (Mn–TCMn1) and second (Mn–TCMn2) Mn–MnL shells (Fig. 4a, b, c). The number of layer vacancies thus increases from MndBi10_10y to MndBi3_10y, TCMn being sorbed above these vacancies. The first, second and fourth Mn–MnL shells are more affected than the third shell. All these shells contain, in a defect-free δ-MnO2 layer, 6 Mn atoms. Thus, a layer vacancy normally affects equally all of these shells, and the observed behavior can most straightforwardly be explained by an ordered layer vacancy distribution. Finally, the correlation at 7.22 Å increases in intensity from MndBi10_10y to MndBi3_10y. It corresponds to a TCMn–TCMn pair, with TCMn atoms being on opposite sides of layer vacancies separated from each other by one layer Mn atom (TCMn–TCMn1 in Fig. 4c). All these correlations and in particular the TCMn–TCMn one are typical for δ-MnO2 layers having 0.25 layer vacancy per layer octahedron and TCMn as in Fig. 4c. In addition, the correlation at 6.13 Å (Mn–TCMn3 pair, i.e., TCMn above a vacancy belonging to the Mn–MnL3 shell) may indicate the minor presence of domains with 0.33 vacancy per layer octahedron in low pH samples (Fig. 4d). Such domains could correspond to the cryptomelane-like structure or to their precursors.Fig. 3

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