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| Titel |
Diffusion-controlled growth of bimineralic merwinite - diopside reaction rims between wollastonite - monticellite interfaces |
| VerfasserIn |
B. Joachim, E. Gardes, W. Heinrich, R. Abart |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026469
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| Zusammenfassung |
| At temperatures of 800 -C to 900 -C and 1.2 GPa, monticellite and wollastonite react to form
merwinite and diopside after the reaction:
2 monticellite (CaMgSiO4) + 2 wollastonite (CaSiO3)
- merwinite (Ca3MgSi2O8) + diopside (CaMgSi2O6)
We synthesized bimineralic merwinite - diopside reaction rims along the interfaces
of cylindric crystals of monticellite and wollastonite. The samples were loaded
in a platinum capsule and annealed at 800 -C to 900 -C and 1.2 GPa in a piston
cylinder apparatus for 5 to 65 hours. Natural CaF2 was used as pressure medium and
the charges were nominally dry. In all experiments, a single layer consisting of
bimineralic aggregates of merwinite and diopside was produced in about equal
molar amounts. Time series revealed that rim growth is parabolic, indicating that
the reaction kinetics is controlled by component diffusion. SEM analysis of the
microstructure showed that the original monticellite-wollastonite interface is located in
the centre of the reaction rim. This implies that rim growth primarily occurred by
transfer of the mobile component MgO from the rim-monticellite interface to the
rim-wollastonite interface. The bulk reaction is thus divided into two half reactions
occurring at the two reaction fronts. At the rim-monticellite interface the reaction is:
2 monticellite - 0.5 merwinite + 0.5 diopside + MgO,
and at the rim-wollastonite interface, it is:
2 wollastonite + MgO - 0.5 merwinite + 0.5 diopside
Using the model of Abart et al. (2009), the effective diffusion coefficients DMgO at 800 -C are
estimated at 1.55-
10-16± 2.18-
10-17 m2/s, and at 900 -C at 2.46-
10-16± 3.45-
10-17 m2/s.
This yields an activation energy of Ea= 45.6 ± 16.4 kJ/mol and a pre-exponential factor log
D0 = -13.59 ± 1.26 for the Arrhenius relations to describe the temperature-dependent
effective diffusivity of the MgO component in the bimineralic aggregate. IR-spectra revealed
distinctive OH-contents in the nominally dry phases monticellite and wollastonite after the
experiments, which had not been present in the reactants. Obviously, some hydrogen released
by the natural, water-containing CaF2 pressure medium diffused into the capsule,
thus producing traces of water inside. The presence of minute amounts of water
may strongly enhance the kinetics of the reaction. In fact, if completely waterfree
Al2O3-powder is used as pressure medium, no significant reaction occurred, and
accordingly, reactants remain OH-free. An Al2O3-layer of 3 mm thickness between the
capsule and the CaF2 surrounding delays the onset of rim growth for about one hour.
However, once initiated, the reaction progress is the same in instantaneous and delayed
reactions.
References: Abart R., Petrishcheva E., Fischer F.D., Svoboda J. (2009), Thermodynamic
model for diffusion controlled reaction rim growth in a binary system: application to
the forsterite-enstatite-quartz system, American Journal of Science, Vol. 309, pp.
114-131 |
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