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.

中文翻译：

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从纳米晶白云母前体形成隐甲烷：反应机理的高能X射线散射和透射电子显微镜观察。

背景技术白云母是在环境中普遍存在的纳米晶和层状叶绿锰酸盐。其层由通过其边缘连接的（MnO6）（8-）八面体构建，并经常包含空位和（或）同构取代。两者都会产生每层八面体超过1个价单元的层电荷不足，因此会引起强烈的化学反应性。另外，绿云母对许多痕量元素（例如，Co，Ni和Zn）具有高亲和力，并且具有氧化还原电势，其允许氧化还原敏感的元素（例如，As，Cr，Tl）被氧化。结果，白云石充当了许多痕量金属元素的接收器。在环境中，常发现白云母与认为是前驱体的tectoman锰酸盐（例如，todorokite和cryptmelaneane）有关。然而，尚未完全了解转变机理，并且在这种转变过程中最初包含在白云石结构中的金属的命运仍在争论中。在目前的工作中，在类似于土壤中普遍存在的条件下（干燥状态，室温和环境压力，在黑暗中）并且在约10年的时间范围内，将合成菱铁矿（δ-MnO2）转化为合成隐密三烷。使用高能X射线散射（具有Bragg-rod和对分布函数形式）和透射电子显微镜进行监测。结果Mn（3+）从层到层间迁移以释放菌株，随后它们在三角共享结构中在新形成的空位上随后吸附。反应以优先生长进行，形成针状晶体，随后聚集。最终，形成的板条形晶体堆叠，晶体之间旋转n×120°（n = 1或2）。所得隐甲烷晶体的尺寸在ab平面约为50-150 nm，沿c *约为10-50 nm，与新鲜样品相比增加了十倍。结论目前观察到的转变机理与其他研究中观察到的相似，该研究使用较高的温度和/或压力，并且所得的锰锰酸盐晶体具有许多与天然晶体相似的形态特征。这证明了拟议机制与环境条件的相关性。所得隐甲烷晶体的尺寸在ab平面约为50-150 nm，沿c *约为10-50 nm，与新鲜样品相比增加了十倍。结论目前观察到的转变机理与其他研究中观察到的相似，该研究使用较高的温度和/或压力，并且所得的锰锰酸盐晶体具有许多与天然晶体相似的形态特征。这证明了拟议机制与环境条件的相关性。所得隐甲烷晶体的尺寸在ab平面约为50-150 nm，沿c *约为10-50 nm，与新鲜样品相比增加了十倍。结论目前观察到的转变机制与其他研究中观察到的类似，该研究使用较高的温度和（或）压力，并且所得的锰锰酸盐晶体具有许多与天然晶体相似的形态特征。这证明了拟议机制与环境条件的相关性。所得的锰锰酸盐晶体具有许多与天然晶体相似的形态特征。这证明了拟议机制与环境条件的相关性。所得的锰锰酸盐晶体具有许多与天然晶体相似的形态特征。这证明了拟议机制与环境条件的相关性。