biotite
noun
bi·o·tite ˈbī-ə-ˌtīt 
: a generally black or dark green form of mica that is a constituent of crystalline rocks and consists of a silicate of iron, magnesium, potassium, and aluminum
biotitic
ˌbī-ə-ˈti-tik
adjective 
Word History
Etymology
German Biotit, from Jean B. Biot †1862 French mathematician
First Known Use
1848, in the meaning defined above