An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite is the most abundant of the mica group of minerals. The name is derived from that of the French chemist J. Biot. The formula for the ideal end member, phlogopite, is KMg₃AlSi₃O₁₀(OH)₂. The more general formula is AX₃Y₄O₁₂(Z)₂, where A (interlayer cation) = K, Na, Ca, Ba, or vacancies; X (octahedral cations) = Li, Mg, Fe²⁺, Fe³⁺, Al, Ti, or vacancies; and Y (tetrahedral cation) = Fe³⁺, Al, Si; Z = (OH), F, Cl, O^2-. This formula is more indicative of the wide range of compositions known for this mineral. Biotite has no commercial value, but vermiculite, an alteration product of magnesium-rich biotite, is used as insulation, as packing material, and as an ingredient for potting soils.