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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

Experimental and calculated XRD patterns of MndBi10_10y and MndBi8_10y. Experimental (black solid line) and calculated (red solid line) XRD patterns of MndBi10_10y (top) and MndBi8_10y (bottom). Structure parameters are available in Table 1
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Fig2: Experimental and calculated XRD patterns of MndBi10_10y and MndBi8_10y. Experimental (black solid line) and calculated (red solid line) XRD patterns of MndBi10_10y (top) and MndBi8_10y (bottom). Structure parameters are available in Table 1

Mentions: Quantitative modeling of hk bands from MndBi8_10y and MndBi10_10y XRD patterns is shown in Fig. 2. Modeling of MndBi3_10y and MndBi4_10y patterns was not undertaken owing to the presence of cryptomelane, whose most intense reflections overlap δ-MnO2 [11, 20] band (Fig. 1). Relative intensity ratios calculated for cryptomelane and δ-MnO2 having similar CSD sizes (~6 nm) indicate that cryptomelane represent <5 % of the crystalline phases in the sample. MndBi10_10y has a CSD size in the ab plane of 5.8 nm and contains 0.11(1) TCMn3+ per layer octahedron (Table 1), lower than the value obtained 2 years sooner [0.165(10) per layer octahedron]. Similar decrease of TCMn3+ with time was previously observed [70] and attributed to TCMn3+ to Mn4+ oxidation with time, followed by migration to the layer. This phenomenon may also be at play in other samples but could not be identified either because the structures were not refined (MndBi3_10y and MndBi4_10y) or because the structure of the same sample was not determined 2 years sooner owing to a restricted amount of sample available for analysis with conventional XRD instruments (MndBi8_10y). Finally, the number of interlayer H2O molecules in MndBi10_10y was refined to 0.12 per interlayer site, slightly more than 2 years sooner (0.10). MndBi8_10y has CSD size in the ab plane of 6.2 nm, contains 0.02 more TCMn3+ and 0.02 more layer vacancies per layer octahedron than MndBi10_10y.Fig. 2


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)

Experimental and calculated XRD patterns of MndBi10_10y and MndBi8_10y. Experimental (black solid line) and calculated (red solid line) XRD patterns of MndBi10_10y (top) and MndBi8_10y (bottom). Structure parameters are available in Table 1
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig2: Experimental and calculated XRD patterns of MndBi10_10y and MndBi8_10y. Experimental (black solid line) and calculated (red solid line) XRD patterns of MndBi10_10y (top) and MndBi8_10y (bottom). Structure parameters are available in Table 1
Mentions: Quantitative modeling of hk bands from MndBi8_10y and MndBi10_10y XRD patterns is shown in Fig. 2. Modeling of MndBi3_10y and MndBi4_10y patterns was not undertaken owing to the presence of cryptomelane, whose most intense reflections overlap δ-MnO2 [11, 20] band (Fig. 1). Relative intensity ratios calculated for cryptomelane and δ-MnO2 having similar CSD sizes (~6 nm) indicate that cryptomelane represent <5 % of the crystalline phases in the sample. MndBi10_10y has a CSD size in the ab plane of 5.8 nm and contains 0.11(1) TCMn3+ per layer octahedron (Table 1), lower than the value obtained 2 years sooner [0.165(10) per layer octahedron]. Similar decrease of TCMn3+ with time was previously observed [70] and attributed to TCMn3+ to Mn4+ oxidation with time, followed by migration to the layer. This phenomenon may also be at play in other samples but could not be identified either because the structures were not refined (MndBi3_10y and MndBi4_10y) or because the structure of the same sample was not determined 2 years sooner owing to a restricted amount of sample available for analysis with conventional XRD instruments (MndBi8_10y). Finally, the number of interlayer H2O molecules in MndBi10_10y was refined to 0.12 per interlayer site, slightly more than 2 years sooner (0.10). MndBi8_10y has CSD size in the ab plane of 6.2 nm, contains 0.02 more TCMn3+ and 0.02 more layer vacancies per layer octahedron than MndBi10_10y.Fig. 2

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