Mica minerals have perfect basal cleavage which makes it easy to separate them into many thin folia. ... Most biotite group minerals are black in color, although the color gets lighter (brownish) if the sheet is thin enough for the light to penetrate it.

Why is biotite dark?

Physical Properties

Its streak is white or gray, and it has a vitreous luster. The physical properties of biotite are affected by the amount of iron present. The specific gravity (G) is greater with increasing iron content, while the color in hand sample is darker with an increase in iron.

Is biotite light or dark?

The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness.