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.
/smacks forehead
Of course! I should've thought of that right away, my apologies ;-)
Actually, I have a small piece of a really neat type of rock nicknamed "TV rock" (the true name is ulexite). It was given the nickname because it acts as a natural fiberoptic thanks to many many tiny 'tubules' (for lack of a better description) that permeate the rock. If you place it over the page of a book or newspaper, the text appears to float on the surface of the rock, several inches above the paper. Very cool :)