It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite.

Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica.

Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions.

Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off.

Biotite dissolves in both acid and alkaline aqueous solutions, with the highest dissolution rates at low pH.

[12] Like other micas, biotite has a crystal structure described as TOT-c, meaning that it is composed of parallel TOT layers weakly bonded to each other by cations (c).

Biotite is occasionally found in large cleavable crystals, especially in pegmatite veins, as in New England, Virginia and North Carolina USA.

It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature.

[12] The largest documented single crystals of biotite were approximately 7 m2 (75 sq ft) sheets found in Iveland, Norway.

Because argon escapes readily from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks.