The mineral streak of biotite is white. Moreover, its optical properties can vary from transparent to translucent to opaque. The mineral is also useful in determining the age of rocks and in assessing the temperature history of metamorphic rocks.

How can you identify biotite?

Biotite is very easy to identify, and with a little experience a person will be able to recognize it on sight. It is a black mica with perfect cleavage and a vitreous luster on the cleavage faces. When biotite is separated into thin sheets, the sheets are flexible but will break upon severe bending.

How can you tell the difference between biotite and muscovite?

Muscovite is clear, silvery, or coppery silver in color (depending on the thickness of the sample and presence of impurities) whereas fresh biotite is black. When biotite weathers, it can become dark golden or coppery in color.