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. Biotite, like other micas, has a layered structure of iron magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Biotite is rarely considered a valuable mineral specimen, but it can accompany other minerals and compliment them. In Bancroft, Ontario Biotite forms large crystals with green apatite and hornblende.
Color is black to brown and yellow with weathering.
Luster is vitreous to pearly. Transparency crystals are transparent to translucent. Also as lamellar or granular rock forming masses providing the luster for most schists and gneiss. Cleavage is perfect in one direction producing thin sheets or flakes. Fracture is not readily observed due to cleavage but is uneven.