Does Biotite contain a mixture of two more different mineral types?

Answer 1

i don't think so

Answer 2

Biotite is a solid solution with end members of magnesium biotite and iron biotite. Also biotite alters to chlorite and both micas have similar chemicals with biotite having potassium and chlorite lacking it. These minerals co-exist, but it is not a mixture of biotite minerals. Phlogopite (magnesium biotite) might co-exist with the iron biotite, but that depends on whether there is exsolution in biotite. I do not know if there is.

Answer 3

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)3AlSi3O10(F, OH)2. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.

Biotite is a sheet silicate. Iron, magnesium, aluminum, silicon, oxygen, and hydrogen form sheets that are weakly bond 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.