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Contributions to Mineralogy and Petrology

, Volume 114, Issue 4, pp 519–532 | Cite as

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

  • Christos Evangelakakis
  • Herbert Kroll
  • Gerhard Voll
  • Hans-Rudolf Wenk
  • Hu Meisheng
  • Jürgen Köpcke
Article

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.

Keywords

Heterogeneous Nucleation Atomic Resolution Alkali Feldspar Interior Part Resolution Microscopy 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • Christos Evangelakakis
    • 1
  • Herbert Kroll
    • 1
  • Gerhard Voll
    • 2
  • Hans-Rudolf Wenk
    • 3
  • Hu Meisheng
    • 3
  • Jürgen Köpcke
    • 4
  1. 1.Institut für MineralogieWestfälische Wilhelms-UniversitätMünsterGermany
  2. 2.Institut für MineralogieUniversität zu KölnKölnGermany
  3. 3.Department of Geology and GeophysicsUniversity of CaliforniaBerkeleyUSA
  4. 4.Institut für Physikalische ChemicHannoverGermany

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