The mechanism of thermal decomposition of dolomite: New insights from 2D-XRD and TEM analyses
-
Carlos Rodriguez-Navarro
,
Krzysztof Kudlacz
Abstract
Despite being studied for more than one century, no consensus exists regarding the ultimate mechanism(s) of the thermal decomposition of dolomite [(CaMg(CO3)2]. To shed light on such a reaction, dolomite single crystals were calcined in air between 500 and 1000 °C, and in situ, in a TEM (high vacuum), following irradiation with the electron beam. In situ TEM shows that the decomposition involves the initial formation of a face centered cubic mixed oxide (Ca0.5Mg0.5O) with reactant/product orientation relationships [001]dolomite//<111>oxide, <4̄41>dolomite//<100>oxide, {112̄0}dolomite//{110}oxide, {112̄8}dolomite//{110}oxide and {101̄4}dolomite^{100}oxide~12°. This phase undergoes de-mixing into oriented crystals of Mg-poor CaO and Ca-poor MgO solid solutions upon long-term e-beam exposure. Ex situ TEM, XRD, 2D-XRD, and FESEM analyses show the formation of porous pseudomorphs made up of oxide nanocrystals with similar parent/product orientation relationships, but with limited Ca/Mg substitution (up to ~9-11%) due to de-mixing (spinodal decomposition) of the metastable (Ca,Mg)O precursor. High ion diffusivity at T > 500 °C (ex situ experiments) favors the formation of pure CaO and MgO crystals during coarsening via oriented aggregation and sintering. These results show that the thermal decomposition of dolomite is topotactic and independent of pCO2. Formation of Mg-calcite nanocrystals (up to ~8 mol% Mg) during the so-called “half decomposition” is observed at 650-750 °C. This transient phase formed topotactically following the reaction of CaO nanocystals (solid solution with ~9 mol% Mg) with CO2 present in the air and/or released upon further dolomite decomposition. With increasing T, Mg-calcite transformed into calcite, which underwent decomposition following the known topotactic relationship: {101̄4}calcite//{110}CaO and <4̄41>calcite//<110>CaO. These observations solve the long standing controversy on the mechanism of the “two-stage” decomposition of dolomite, which assumed the direct formation of calcite during the so-called “half decomposition.”
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
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- Silician magnetite from the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia
- The mechanism of thermal decomposition of dolomite: New insights from 2D-XRD and TEM analyses
- A revised diamond-graphite transition curve
- Insights into the crystal and aggregate structure of Fe3+ oxide/silica co-precipitates
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- Kinetics of evaporation of forsterite in vacuum
- X-ray absorption near edge structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3 perovskite
- Rehydration of dehydrated-dehydroxylated smectite in a low water vapor environment
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- OH group behavior and pressure-induced amorphization of antigorite examined under high pressure and temperature using synchrotron infrared spectroscopy
- Single-crystal Raman spectroscopy of natural paulmooreite Pb2As2O5 in comparison with the synthesized analog
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- Implications of ferrous and ferric iron in antigorite
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Articles in the same Issue
- Boron in natural type IIb blue diamonds: Chemical and spectroscopic measurements
- Mejillonesite, a new acid sodium, magnesium phosphate mineral, from Mejillones, Antofagasta, Chile
- Silician magnetite from the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia
- The mechanism of thermal decomposition of dolomite: New insights from 2D-XRD and TEM analyses
- A revised diamond-graphite transition curve
- Insights into the crystal and aggregate structure of Fe3+ oxide/silica co-precipitates
- Compositional dependence of alkali diffusivity in silicate melts: Mixed alkali effect and pseudo-alkali effect
- Kinetics of evaporation of forsterite in vacuum
- X-ray absorption near edge structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3 perovskite
- Rehydration of dehydrated-dehydroxylated smectite in a low water vapor environment
- Effect of high pressure on the crystal structure and electronic properties of magnetite below 25 GPa
- OH group behavior and pressure-induced amorphization of antigorite examined under high pressure and temperature using synchrotron infrared spectroscopy
- Single-crystal Raman spectroscopy of natural paulmooreite Pb2As2O5 in comparison with the synthesized analog
- The dissolution of laumontite in acidic aqueous solutions: A controlled-temperature in situ atomic force microscopy study
- Crystal structure of CaRhO3 polymorph: High-pressure intermediate phase between perovskite and post-perovskite
- Oxide melt solution calorimetry of Fe2+-bearing oxides and application to the magnetite–maghemite (Fe3O4–Fe8/3O4) system
- Static compression of (Mg0.83,Fe0.17)O and (Mg0.75,Fe0.25)O ferropericlase up to 58 GPa at 300, 700, and 1100 K
- Implications of ferrous and ferric iron in antigorite
- Markascherite, Cu3(MoO4)(OH)4, a new mineral species polymorphic with szenicsite, from Copper Creek, Pinal County, Arizona, U.S.A.
- Natural hydrous amorphous silica: Quantitation of network speciation and hydroxyl content by 29Si MAS NMR and vibrational spectroscopy
- Lead-tellurium oxysalts from Otto Mountain near Baker, California: VII. Chromschieffelinite, Pb10Te6O20(OH)14(CrO4)(H2O)5, the chromate analog of schieffelinite
- Experimental growth of diopside + merwinite reaction rims: The effect of water on microstructure development
- Thermodynamic model for growth of reaction rims with lamellar microstructure
- The high-pressure behavior of micas: Vibrational spectra of muscovite, biotite, and phlogopite to 30 GPa
- Critical evaluation of the revised akdalaite model for ferrihydrite—Discussion
- Critical evaluation of the revised akdalaite model for ferrihydrite—Reply
