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| Titel |
Determination of component mobilities in bimineralic reaction rims using isotopically doped starting materials |
| VerfasserIn |
Bastian Joachim, Rainer Abart, Carmen Höschen, Wilhelm Heinrich |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072657
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| Zusammenfassung |
| Rim growth experiments were performed between monticellite (CaMgSiO4) single crystals
and wollastonite (CaSiO3) powder at 900Ë C and 1.2 GPa to produce bimineralic diopside
(CaMgSi2O6) + merwinite (Ca3MgSi2O8) reaction rims.
Symmetrical makeup of the internal rim microstructure implies that rims grow from the
original interface towards both reactants at identical rates, indicating that solely
MgO-diffusion controls overall rim growth with logD (MgO) = -16.3 ± 0.2 m2s-1 (Joachim
et al. 2012).
Presence of ppm-amounts of water significantly affects the internal rim microstructure. At
“very dry” condition, a lamellar microstructure of alternating palisade-shaped diopside and
merwinite grains elongated normal to the reaction front is generated, indicating that CaO and
SiO2-mobilities are significantly smaller compared to the MgO-mobility. In presence of
minute amounts of water a segregated multilayer microstructure with almost perfectly
monomineralic merwinite - diopside - merwinite layers oriented parallel to the reaction front
develops, indicating a sufficient additional mobility of either CaO or SiO2 compared to
MgO.
We used isotopically doped wollastonite (44Ca29SiO3) to identify, which component
mobility, CaO or SiO2, is enhanced in presence of ppm amounts of water. Both, 44Ca
stemming from the wollastonite as well as 40Ca stemming from the monticellite are
distributed across the entire rim. In addition to that, small amounts of 40Ca are found within
the wollastonite and substantial amounts of 44Ca are found in the monticellite starting
material.
In contrast to that, 28Si and 29Si remain in the regions that were originally occupied by
their respective source materials monticellite and wollastonite, indicating that the
SiO2-mobility is comparatively low. This suggests that the presence of small amounts of
water significantly enhances the relative mobility of CaO.
Consequently minute amounts of water may not only affect overall rim growth
kinetics but also the relative component mobilities and accordingly the internal rim
organization.
B. Joachim, E. Gardés, B. Velickov, R. Abart, W. Heinrich (2012) Experimental growth
of diopside + merwinite reaction rims: the effect of water on microstructure development. Am
Min 97, pp. 220-230. |
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