Biotite ... biotite ... hmm ...
The only rocks I really remember from good ol' Earth Science class are bismuth and quartz and calcite.
What is biotite, and why are you it? Or why is it you, or whatever? ;-)
Why am I biotite? Isn't it obvious? Because The darker color and density increase with an increase in the iron content. Biotite tends to form in a wider range of conditions than phlogopite which is limited mostly to ultramafic rocks and magnesium rich marbles and pegmatites.