View source for Biotite

===Optical properties===

In [[thin section]], biotite exhibits moderate [[Optical relief|relief]] and a pale to deep greenish brown or brown color, with moderate to strong [[pleochroism]]. Biotite has a high [[birefringence]] which can be partially masked by its deep intrinsic color.<ref name="min-in-thin">{{cite web|url=http://funnel.sfsu.edu/courses/geol426/Handouts/mintable.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://funnel.sfsu.edu/courses/geol426/Handouts/mintable.pdf |archive-date=2022-10-09 |url-status=live|title=Identification Tables for Common Minerals in Thin Section|last=Faithful|first=John|year=1998|access-date=March 17, 2019}}</ref> Under [[Polarized light microscopy|cross-polarized light]], biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic [[Bird's eye maple (mineral property)|bird's eye maple extinction]], a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear [[isotropic]] under cross-polarized light.<ref name="min-rock-sec">{{cite book|title=Minerals in Rock Sections: The Practical Methods of Identifying Minerals in Rock Sections with the Microscope|url=https://archive.org/details/mineralsinrocks02luqugoog|quote=bird's eye extinction thin section grinding.|last=Luquer|first=Lea McIlvaine|year=1913|publisher=D. Van Nostrand Company|edition=4|location=New York|page=[https://archive.org/details/mineralsinrocks02luqugoog/page/n111 91]}}</ref>

<gallery widths="180px" heights="120px" >
File:Muscovite and Biotite2a.jpg|Biotite (in brown) and muscovite in an [[orthogneiss]] thin section under plane-polarized light.
File:Thin Section of Biotite (test) (cropped to Biotite).jpg|Biotite in thin section under cross-polarized light.
File:Sagenitic biotite.JPG|Basal section of biotite, with needle-like [[rutile]] inclusions, in thin section under plane-polarized light.
</gallery>