The composition of phlogopite is KMg3[AlSi3O10](F,OH)2. It crystallizes in the monoclinic crystal system. The crystal form is columnar or pseudohexagonal blocks, and it usually has a conical shape, sometimes with crystal twinning. It can also appear in a scale-like blocky aggregate form. It can be colourless, yellowish brown, light green or white with colourless streaks. The diaphaneity is transparent to opaque with a pearly lustre. The specific gravity is 2.76–2.90, the Mohs hardness is 2–2.5, and the fractures are uneven. It mainly forms in ultrabasic and contact-metamorphosed rocks.