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| Titel |
Experimental exsolution of alkali feldspar |
| VerfasserIn |
Gregor Neusser, Rainer Abart, Richard Wirth, Nicholas Norberg |
| 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 |
250058051
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| Zusammenfassung |
| Alkali feldspars of intermediate composition have been exsolved experimentally during
annealing at 500 and 550oC and 1 bar. The starting materials were gem quality sanidine from
the Eifel with an original composition of XOr = 0.85 and gem quality orthoclase from
Madagaskar with an original composition of XOr = 0.95. These feldspars were exchanged
with an NaCl-KCl melt with a molar NaCl/KCl proportion of 80/20 at 850oC and 1 bar to
produce chemically homogeneous alkali feldspar with a composition of XOr = 0.4.
This composition corresponds to the temperature maximum in the alkali feldspar
solvus, which is at about 650oC at 1 bar. The intermediate feldspars were then
annealed in the unstable region at 500 and 550oC and 1 bar to provoke unmixing. Run
durations were 8, 16, 32, and 64 days. In all runs the feldspars developed a lamellar
microstructure, which is visible on TEM bright field images in cuts taken normal to the [010]
direction. This microstructure is interpreted to result from spinodal decomposition
as is expected to occur during anneling in the unstable region. The characteristic
wavelength of the lamellae increases with time, where the wavelength seems to be
proportional to the cube root of time. From linear extrapolation the initial wavelength
is estimated at 15 nm for annealing at 550oC and at about 8 nm for annealing at
500oC, where the initial wavelength is similar for both materials prepared from
sanidine and orthoclase. The rate of coarsening is significantly higher for the 550oC
experiments than for the runs at 500oC testifying to the temperature dependence of Na-K
interdiffusion. The initial wavelength of the lamellar microstructure is related to the energy
associated with phase boundaries, whereas the rate of coarsening reflects the rate
of Na-K interdiffusion. Our unmixing experiments thus allow to calibrate diffuse
interface models for spinodal decomposition as presented by Petrishcheva and Abart
(2009). With this calibration a tool is provided to extract cooling rates from the
characteristic grain size and from the compositions of the exsolved phases in perthitic alkali
feldspar.
Petrishcheva E, Abart R (2009): Exsolution by spinodal decomposition: I: evolution equation
for binary mineral solutions with anisotropic interface energy. Am Journ Sci 309:431–449 |
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