biotite

Biotite is a common phyllosilicate mineral within the mica group. 

Biotite 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, in some instances side-by-side.