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Chemical Evidence for Exsolution in a Labradorite

Nature Physical Science volume 245, pages 135–137 (1973)Cite this article

Abstract

The plagioclase feldspars are important constituents and genetic indicators in most metamorphic and plutonic rocks. Complete solid solution exists between the end members, albite (Ab), NaAlSi3O8 and anorthite (An), CaAl2SiO8 at high temperatures, but considerable evidence has been accumulated for the presence of limited solid solubility at the low temperatures representative of near-equilibrium conditions in plutonic and metamorphic rocks. Electron microscopic investigations of specimens of various bulk compositions show lamellar textures a few tens to hundreds of nm in thickness indicative of exsolution (phase separation) into two phases. In addition, doubled X-ray and electron diffraction spots have been obtained from samples in the composition ranges An2-An15 (known as peristerites)1−5 and An67–An83 (the bytownites)6–9. The characteristic faint blue iridescence or “schiller” displayed by peristerites was shown to be the result of Bragg diffraction of light from the boundaries between the two phases10. Similar but more intense colours covering the whole visible spectrum are produced by labradorites (An42-An58) from coarsely crystalline anorthosites11, and this has led to speculations about the existence of “reflecting planes”12 and hence the possibility of a solubility gap13 similar to that which occurs in the peristerite and bytownite composition fields.

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Author information

Authors and Affiliations

  1. Lahor für Elektronenmikroskopie 11, ETH (Swiss Federal Institute of Technology), Cfl-8049, Zürich

    H.-U. NISSEN

  2. Department of Geology,

    P. E. CHAMPNESS

  3. Department of Metallurgy, University of Manchester,

    G. CLIFF & G. W. LORIMER

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  1. H.-U. NISSEN
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  2. P. E. CHAMPNESS
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  4. G. W. LORIMER
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NISSEN, HU., CHAMPNESS, P., CLIFF, G. et al. Chemical Evidence for Exsolution in a Labradorite. Nature Physical Science 245, 135–137 (1973). https://doi.org/10.1038/physci245135a0

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