a specimen of biotite mica from Jamestown, Boulder Co, Colorado, USA
|| Jamestown, Boulder Co, Colorado, USA
This is a sheet silicate. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called “iron mica” because it is more iron-rich than phlogopite. It is also sometimes called “black mica” as opposed to “white mica” (muscovite) – both form in some rocks, and in some instances side-by-side.
Named in 1847 by Johann Friedrich Ludwig Hausmann in honour of the French physicist, mathematician, meteoriticist, astronomer, and mineralogist, Jean Baptiste Biot [April 21, 1774 Paris, France – February 3, 1862 Paris, France], who studied the optical properties of the micas. Biot and his associate, Félix Savart, discovered that an electric current in a wire produced a magnetic field. Biot received many awards in his lifetime in recognition of the value of his scientific researches.
Because of Biotite’s abundance, its presence is usually lacking in collections except for it being an accessory mineral to other minerals. Biotite can come in enormous crystal sheets that can weigh several hundred pounds. Thin sheets can be peeled off as layers, and the thinner a layer is peeled the greater its transparency becomes.
In 1998, the IMA removed the status of Biotite as an individual mineral species, and instead declared it as a group name for the following individual members: Phlogopite, Annite, Siderophyllite, and Eastonite. However, mineral collectors still refer to Biotite by its traditional name and rarely make a distinction among its members except for Phlogopite.
Biotite is very hard to clean because if washed it will absorb water internally and start to break apart. The best way to wash Biotite and other Micas is with a dry electric toothbrush.