Structural characterization of labradorite-bytownite plagioclase from volcanic,plutonic and metamorphic environments |
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| Authors: | Timothy L Grove |
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| Institution: | (1) Hoffman Laboratory, Harvard University, 20 Oxford Street, 02138 Cambridge, Massachusetts, USA;(2) Present address: Department of Earth and Space Sciences, State University of New York, 11794 Stony Brook, New York, USA |
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| Abstract: | The transmission electron microscope and the electron microprobe are used to characterize calcic plagioclase (An65 to An85) from a variety of geological environments. The cooling histories of samples from volcanic, plutonic and metamorphic environments are estimated and the transformation and exsolution sequence is inferred from observations in the transmission electron microscope. Several distinctive textural modifications occur depending both on bulk composition and cooling history. (1) Exsolution occurs in increasingly calcic bulk compositions upon slower cooling, and the coexisting phases are An66 intermediate plagioclase and An85–90 P¯1, c=14 Å plagioclase in the sample from the metamorphic environment, (2) the morphology of b antiphase boundaries (APBs) in An75 to An85 plagioclase changes from smoothly curving (rapid cooling and calcic compositions) to zig-zag (slower cooling or sodic compositions). (3) The concentration of defects in the intermediate plagioclase superstructure changes from a high density in rapidly cooled plagioclase to a lower density in slowly cooled plagioclase. In all plagioclases except for the rapidly cooled, volcanic specimens there is evidence in images and diffraction patterns for short-range ordered domains with P¯1 symmetry. The observations allow the microstructure of a single zoned plagioclase to be used as an indication of the geologic environment under which it cooled. |
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