Definition of "biotite" [bi•o•tite]

• A dark-brown or dark-green to black mica, K(Mg,Fe)3(Al,Fe)Si3O10(OH)2, found in igneous and metamorphic rocks. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "In certain minerals, notably the brown variety of mica known as biotite, the microscope reveals minute circular marks occurring here and there, quite irregularly."
• "Furthermore, their model included a second mineral called biotite, surrounding a zircon core."
• "And micas such as biotite have been found in regions containing evidence of the earliest life-forms, which are believed to have existed about 3.8 million years ago."