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| Titel |
Thermodynamic model for microstructure evolution during reaction rim growth |
| VerfasserIn |
Rainer Abart, Elena Petrishcheva, Bastian Joachim |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057023
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| Zusammenfassung |
| The formation of different microstructural types of reaction rims in a closed system is
investigated theoretically. In particular multilayer types that are comprised of a
sequence of monomineralic layers and cellular (symplectite) types are addressed. A
thermodynamic model for cellular reaction rim growth in a ternary model system is derived.
It is found that either the chemical mass transfer across the rim or the material
re-distribution that must occur within a reaction front, at which a cellular microstructure is
produced, may be rate limiting. Based on the thermodynamic extremum principle,
parameter domains can be discerned, where reaction rims preferably produce cellular
or multilayer microstructural types. The controlling factors are the characteristic
length scale (wavelength) of mineral intergrowth in the cellular microstructure and
the relative efficiencies of chemical mass transfer across the rim and within the
reaction fronts of cellular layers. For a given set of kinetic parameters formation of
the multilayer type is preferred during the initial growth stages, and the cellular
type is preferred at later growth stages. If component mobilities remain constant
throughout reaction rim growth, a transition from the multilayer to the cellular
type is expected to occur as the rim thickness increases. The reverse transition is
unlikely.
Both transitions have been observed in experiments done in the CaO - MgO- SiO2
system. The transition from multilayer to cellular type was observed in experiments,
where monticellite and wollastonite reacted to akermanite and diopside and some
low-SiO2 phase at 1200oC, 0.5 GPa and durations between 5 min and 60 h at very dry
conditions. The reverse transition was observed in experiments, where monticellite and
wollastonite reacted to diopside and merwinite at 900oC , 1.2 GPa and durations
between 5 h and 65 h. In the latter case the reverse transition clearly indicates a
change in the relative mobilities of MgO and CaO during the experiment, where CaO
became more mobile relative to MgO due to the successive diffusion of water into the
capsule. The microstructural evolution during solid state transformations is very
sensitive to the kinetic parameters underlying the respective reaction. Irreversible
thermodynamics provides a tool to infer kinetic parameters from observed microstructures. |
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