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| Titel |
The effect of water on the internal organization of bimineralic diopside-merwinite reaction rims |
| VerfasserIn |
Bastian Joachim, Emmanuel Gardés, Rainer Abart, Wilhelm Heinrich |
| 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 |
250048171
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| Zusammenfassung |
| Bimineralic diopside + merwinite reaction rims were produced in piston-cylinder
experiments along the interfaces of cylindrically shaped single crystals of monticellite and
wollastonite at 900 Ë C, 1.2 GPa and run durations between 5 and 65 h according to the
reaction:
2 monticellite (CaMgSiO4) + 2 wollastonite (CaSiO3) - 1 merwinite (Ca3MgSi2O8)
+ 1 diopside (CaMgSi2O6)
IR-spectra of periclase that was also loaded into the capsule show OH-defects after the
experiments that initially were not present. This indicates that H2O, released by natural,
water-containing CaF2 used as solid pressure medium, diffused into the Pt-capsule during
the experiment. The amount of water in the capsule increased with increasing run
duration, as indicated by an increase of the OH-defect concentration in periclase with
time.
During the first stage of rim growth, the reaction products form a cellular-microstructure
with palisade shaped merwinite and diopside microcrystals alternating at regular
intervals, whereby the long axes of the grains are oriented about normal to the original
monticellite-wollastonite interface (cellular type). At longer run durations, diopside and
merwinite start to segregate into layers with diopside accumulating in the centre and
merwinite at both sides of the reaction rim (multilayer type). After 65 h, segregation of the
product phases is almost complete and a triple layer rim with the sequence mtc / mw / di / mw
/ di is produced.
The two kinetic regimes where the multilayer and the cellular rim types are preferred are
separated in the λ versus LCaO/LMgO parameter space by the condition
°––––
24δHm–-
λ = qLCaOK
LMgO
where δ is the width of the reaction fronts at either side of the rim, H the rim thickness, and q
a scaling factor for the grain boundary area fraction (i.e., the GB-density normal to the flux);
m and K are constants calculated from the molar volumes and the stoichiometric coefficients.
Calculation of this model for different grain widths and mobility ratios of the components
show that, if all process parameters are kept constant, rim growth starts with a multilayer
structure and is replaced by a cellular microstructure at later growth stages. The
reverse evolution is observed, which is only possible by a significant change of the
relative mobility ratios, where LCaO/LMgO is increasing from |
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