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Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka

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Abstract

In the alkali feldspars of the amphibolite- and granulite-facies rocks of Sri Lanka, a late-stage, final exsolution event is observed which produced film lamellae and fine-scale spindles. These were investigated by optical, microprobe, single-crystal, transmission electron microscopy and atomic resolution microscopy techniques. The lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350° C. Precession photographs and ARM micrographs show that the intergrowth is perfectly coherent. In sections ‖ (010) the rhombic section of the Pericline twins corresponds to analbite or high albite. The albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed-orthoclase matrix and became periodically twinned analbite or high albite, which subsequently developed only a slight increase in Al, Si order. The relationship between twin periodicity and lamellar width, predicted for coherent intergrowths by Willaime and Gandais (1972), is obeyed. In Or-rich grains, in which coherent exsolution is the only exsolution event, the film lamellae tend to be restricted to the rim, the fine-scale spindles to the centre of the grains. The films nucleated heterogeneously at grain boundaries and grew towards the grain centres. Fine-scale spindles probably nucleated homogeneously in the interior part of grains. Heterogeneous nucleation and coherent growth are not mutually exclusive.

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Authors and Affiliations

  1. Institut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstrasse 24, D-48149, Münster, Germany

    Christos Evangelakakis & Herbert Kroll

  2. Institut für Mineralogie, Universität zu Köln, Zülpicher Strasse 49, D-50674, Köln, Germany

    Gerhard Voll

  3. Department of Geology and Geophysics, University of California, 94720, Berkeley, CA, USA

    Hans-Rudolf Wenk & Hu Meisheng

  4. Institut für Physikalische Chemic, Callinstrasse 3-3A, D-30167, Hannover, Germany

    Jürgen Köpcke

Authors
  1. Christos Evangelakakis
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  2. Herbert Kroll
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  3. Gerhard Voll
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  4. Hans-Rudolf Wenk
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  5. Hu Meisheng
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  6. Jürgen Köpcke
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Evangelakakis, C., Kroll, H., Voll, G. et al. Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka. Contr. Mineral. and Petrol. 114, 519–532 (1993). https://doi.org/10.1007/BF00321756

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