Don't you have an optical mineralogy text that has mineral characteristics tabulated or otherwise listed? I do, somewhere, but it isn't my work so I am not digging it out. Instead, I did a quick search on optical mineralogy. My initial thought on seeing your question is that biotite is brown and muscovite is clear. But then you asked about characteristics, so: Muscovite - Very fine cleavage in one set, high birefringence, straight extinction, mottling in extinction position, Biaxial negative with moderate 2V. Biotite - Strong pleochroism in shades of yellow, brown and red- brown, high birefringence, mottling in extinction position, straight extinction, Biaxial negative with low 2V.(R. I. high in some instances). Merry Christmas.