It's a type of mica, specifically a dark-colored mica, and it fits the definition of a mineral:
* Naturally occurring: Biotite is found in nature, not created in a lab.
* Inorganic: It's not derived from living organisms.
* Solid: It has a definite shape and volume at room temperature.
* Crystalline structure: Biotite has a specific, repeating arrangement of atoms.
* Definite chemical composition: Its formula is K(Mg,Fe)3AlSi3O10(OH)2, meaning it's composed of specific elements in a fixed ratio.
