Chemical Formula: K(Mg, Fe3)(Al, Fe)Si3O10(OH, F)2
Color: Black to darker brown
Hardness: 2.5 – 3
Importance: The main use for biotite is as an insulator in circuits due to its low conductivity. Also used in paints and other industrial products.
Other Interesting Information: Always forms in very thin sheets, which makes it easy to identify. Has some elasticity, meaning it can bend to a certain degree without breaking.
My Take: Biotite is really cool. It is amazing that it forms in sheets that are almost paper thin. Only a few other minerals have this same trait.
For More Information: http://www.minerals.net/mineral/biotite.aspx