# Bird's eye extinction

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This article is about the mineral property. For the type of lumber, see [Bird's eye figure](/source/Bird's_eye_figure).

**Bird's eye extinction**, or **bird's eye maple**, is a specific type of [extinction](/source/Extinction_(optical_mineralogy)) exhibited by [minerals](/source/Mineral) of the [mica](/source/Mica) group[1] under cross [polarized](/source/Polarization_(waves)) light of the [petrographic microscope](/source/Petrographic_microscope). It gives the [mineral](/source/Mineral) a pebbly appearance as it passes into extinction. This is caused when the grinding tools used to create [petrographic](/source/Petrography) [thin sections](/source/Thin_section) of precise thickness alter the alignment of the previously perfect basal [cleavage](/source/Cleavage_(crystal)) planes which split micas up into its characteristic thin sheets. The resulting, slightly roughened surface alters the extinction angle of various parts of the [crystal lattice](/source/Crystal_lattice), leading to this type of extinction. Since it is not a natural feature of the mineral, bird's eye extinction is not observed in all mica crystals, nor from all angles, but it is quite common, and is used as a diagnostic feature for micas.

Common [micas](/source/Micas) which exhibit this include [biotite](/source/Biotite) (and the magnesium end-member [phlogopite](/source/Phlogopite)) and [muscovite](/source/Muscovite).

## References

1. [Smith College - Geology 222b - Petrology - Petrographic Data File - Biotite](http://www.science.smith.edu/geosciences/petrology/petrography/biotite/biotite.html) [Archived](https://web.archive.org/web/20130510071953/http://www.science.smith.edu/geosciences/petrology/Petrography/Biotite/Biotite.html) 2013-05-10 at the Wayback Machine Retrieved 2015-07-02

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