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| Titel |
Influence of deformation on dolomite rim growth kinetics |
| VerfasserIn |
Vanessa Helpa, Erik Rybacki, Luiz Fernando Grafulha Morales, Georg Dresen |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101571
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| Publikation (Nr.) |
 EGU/EGU2015-735.pdf |
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| Zusammenfassung |
| Using a gas-deformation apparatus stacks of oriented calcite (CaCO3) and magnesite
(MgCO3) single crystals were deformed at T = 750Ë C and P = 400 MPa to examine the
influence of stress and strain on magnesio-calcite and dolomite (CaMg[CO3]2) growth
kinetics. Triaxial compression and torsion tests performed at constant stresses between 7 and
38 MPa and test durations between 4 and 171 hours resulted in bulk strains of 0.03-0.2 and
maximum shear strains of 0.8-5.6, respectively. The reaction rims consist of fine-grained (2-7
μm) dolomite with palisade-shaped grains growing into magnesite reactants and
equiaxed granular dolomite grains next to calcite. In between dolomite and pure calcite,
magnesio-calcite grains evolved with an average grain size of 20-40 μm. Grain
boundaries tend to be straighter at high bulk strains and equilibrium angles at grain triple
junctions are common within the magnesio-calcite layer. Transmission electron
microscopy shows almost dislocation free palisades and increasing dislocation
density within granular dolomite towards the magnesio-calcite boundary. Within
magnesio-calcite grains, dislocations are concentrated at grain boundaries. Variation of time
at fixed stress (≈17 MPa) yields a parabolic time dependence of dolomite rim width,
indicating diffusion-controlled growth, similar to isostatic rim growth behavior.
In contrast, the magnesio-calcite layer growth is enhanced compared to isostatic
conditions. Triaxial compression at given time shows no significant change of dolomite
rim thickness (11±2 μm) and width of magnesio-calcite layers (33±5 μm) with
increasing stress. In torsion experiments, reaction layer thickness and grain size
decrease from the center (low stress/strain) to the edge (high strain/stress) of samples.
Chemical analysis shows nearly stoichiometric composition of dolomite palisades, but
enhanced Ca content within granular grains, indicating local disequilibrium with
magnesio-calcite, in particular for twisted samples. The shift from local equilibrium is
≈3 mol% in triaxial compression and ≈7 mol% in torsion. Electron backscatter
diffraction analysis reveals a crystallographic preferred orientation (CPO) within the
reaction layers with [0001] axes parallel to the compression/rotation axis and poles of
{2-1-10} and {10-10} prismatic planes parallel to the reaction interface. Compared
to isostatic annealing, the CPO is more pronounced and the amount of low-angle
grain boundaries is increased. At the imposed experimental conditions, most of
the bulk deformation is accommodated by calcite single, which is stronger than
magnesite. Application of flow laws for magnesio-calcite and dolomite suggest that the
fine-grained reaction products should deform by grain boundary diffusion creep, resulting
in lower flow strength than the single crystal reactants. However, microstructural
observations indicate that deformation of granular dolomite and magnesio-calcite is at least
partially assisted by dislocation creep, which would result in an almost similar strength
to calcite. Therefore, flattening of the reaction layers during triaxial compression
may be counterbalanced by enhanced reaction rates, resulting in almost constant
layer thickness, independent of the applied stress. For simple shear, the reduced
reaction kinetics in the high stress/strain region of twisted samples may be related
to increased nucleation rates, resulting in a lower grain size and rim thickness. |
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