Biotite is a very unique mineral that can be easily broken apart into very thin sheets. It is a form of mica that is very soft. In fact, it can be scratched with your fingernail. Biotite is usually black or brown and is used in a variety of ways. It is usually ground up and added to other things such as paint, waterproofing sealers, and lubricants, or greases. Since biotite is fireproof, it is also used in fire extinguishers.