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| Titel |
Na-K-Interdiffusion in Alkali Feldspars |
| VerfasserIn |
A.-K. Schaeffer, E. Petrishcheva, D. Rhede, R. Abart |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066708
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| Zusammenfassung |
| Exchange experiments between crystallographically oriented plates of gem-quality alkali
feldspar (XOr 0.85) with NaCl/KCl melts have been conducted at 850Ë and 920Ë C. The
melt composition was varied systematically between XKCl 0.6 and 1 to induce a shift of the
feldspar composition towards more Na-rich as well as K-rich compositions (XOr 0.74 to 1).
We applied 40-times molar excess of cations in the melt to ensure constant concentration
boundary conditions for cation exchange.
Depending on the direction of the composition shift diffusion profiles with different
geometries develop. After a shift towards more K-rich compositions the diffusion profile
shows two plateaus representing the unexchanged core and the exchanged rim in equilibrium
with the melt, respectively. The exchange front between the two plateaus has an inflection
point and propagates through the crystal with t1-2. Its width depends on the composition
difference between the exchanged and unexchanged domains of the grain as well as
crystallographic direction.
The profiles that develop during a shift towards more Na-rich compositions lack an
inflection point. If XOr is shifted by more than 10 mole-% crack systems begin to develop
due to composition strain associated with the substitution of K+ by the smaller
Na+ion.
While the propagation rate of the fronts is roughly equal in all crystallographic directions,
the profiles measured in the direction normal to (001) are always narrower than those normal
to (010). This indicates a marked anisotropy in interdiffusion which appears fastest in the
direction perpendicular to (001), i.e. c*.
The observed geometry of the diffusion fronts can be explained by a composition
dependence of the interdiffusion coefficient. This dependence was first described by
Christoffersen et al. (1983) but these authors used a different experimental setup and only had
reliable data for intermediate compositions. From our data we could extract the interdiffusion
coefficient for high XOr with help of the Boltzmann Transformation. The Na-K
interdiffusion coefficient increases with a gentle slope from 0.3 to 0.8 x 10-15m2s-1 for the
composition interval between XOr 0.80 and 0.97 and then rises steeply to values of
2.2 x 10-15m2s-1. In the composition interval between XOr 0.74 and 0.80 the
interdiffusion coefficient decreases slightly from 0.4 to 0.3 x 10-15m2s-1. On the
whole this results in a “bowl shaped” composition dependence of the interdiffusion
coefficient in the composition range XOr 0.74 to 1. This deviates from what is
expected from theoretical calculations using the Manning relation for interdiffusion
(Christoffersen et al., 1983). The strong direction dependence of the diffusion profiles may
indicate that interdiffusion is influenced by the coherency stress across the diffusion
front.
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References
Christoffersen et al. (1983): Interdiffusion of K and Na in alkali feldspar: diffusion couple
experiments, -American Mineralogist, Vol. 68, pp. 1126-1133 |
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