Size-Driven Structural and Thermodynamic Complexity in Iron Oxides

Navrotsky, Alexandra; Mazeina, Lena; Majzlan, Juraj

doi:10.1126/science.1148614

Cited by 707 publications

(744 citation statements)

References 25 publications

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“…The amount of organic matter, which is often correlated to bacteria, may effectively promote the elevated rainfall-driven processes [Cornell and Schwertmann, 2003], but the presence of organic matter is not a prerequisite. From the point of view of thermodynamics, the dissolution of Mgh can either precede or follow the dissolution of Hm, since Mgh and Hm are the least stable phases among the studied iron oxides and since these Fe 2 O 3 polymorphs have similar chemical and mineralogical characteristics [Cornell and Schwertmann, 2003;Navrotsky et al, 2008]. Therefore, this study partly verifies the abiotic mechanism of magnetic change by the genetic connection between Hm and Mgh in aerobic soils across a wide climate range.…”

Section: Mechanism Of the Magnetic Reduction With Excessive Rainfallmentioning

confidence: 52%

Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Long

Balsam

2011

J. Geophys. Res.

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[1] The iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate. Previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall; the correlation seems to be reversed under extreme climates with high rainfall. Here we present a transect of saprolitic soils from Hainan Island, South China, with high rainfall (1440∼2020 mm/yr), little temperature variation (23∼24°C), and extreme weathering. Along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall, whereas goethite concentration displays a large increase. However, there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect. Goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall-driven processes. These pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite, magnetic susceptibility and rainfall control of an inflection point. This study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range. A conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point.

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Section: Mechanism Of the Magnetic Reduction With Excessive Rainfallmentioning

confidence: 52%

Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Long

Balsam

2011

J. Geophys. Res.

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“…In other words, ACC dehydration is an irreversible process, with the dehydrated form more stable than the hydrated. This behavior is in contrast to the exothermic hydration (with respect to liquid water) of oxides to form oxyhydroxides (43,44), and of dehydrated zeolites to form hydrated ones (45). The DSC measurements indicate that the vaporization of water from synthetic hydrated ACC is endothermic.…”

Section: Discussionmentioning

confidence: 89%

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Radha

Forbes

Killian

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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420

449

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Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO 2 sequestration. amorphous calcium carbonate (ACC) | calorimetry | crystallization enthalpy | sea urchin larval spicules | synthetic and biogenic ACC

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“…Particle size-dependent phase behavior is well known at the nanoscale, where the structure of the bulk becomes influenced by the particle surface energy. [40,41] Such phenomena are more unusual in macroscopic materials, however, where defect structures and domain sizes can influence the stability of crystal phases as well as surface effects. [42,43] Notably the variable temperature unit cell, DSC and susceptibility data all imply that SCO proceeds similarly in single crystalline and powdered 1.…”

Section: Resultsmentioning

confidence: 99%

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Cook

Shepherd

Comyn

et al. 2015

Chemistry A European J

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Crystalline [Fe(bppSMe)2][BF4]2 (1; bppSMe=4‐(methylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine) undergoes an abrupt spin‐crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit‐cell a axis to one‐third of its original value (high‐temperature phase 1; Pbcn, Z=12; low‐temperature phase 2; Pbcn, Z=4). The SCO‐active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140–300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)2][BF4]2 (2; bppBr=4‐bromo‐2,6‐di(pyrazol‐1‐yl)pyridine) and [Fe(bppI)2][BF4]2 (3; bppI=4‐iodo‐2,6‐di(pyrazol‐1‐yl)‐pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1–3 reveals that the more cooperative spin transition in 1, and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light‐induced excited‐spin‐state trapping (LIESST) effect with T(LIESST=70–80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.

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Size-Driven Structural and Thermodynamic Complexity in Iron Oxides

Cited by 707 publications

References 25 publications

Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

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