A dioctahedral member of the brittle mica group. The end-member formula is: CaAl₂vAl₂Si₂O₁₀(OH)2, where v = vacancy. Typical site substitutions include: Ca > Na,K; v > Li; ^ivAl ≈ Si; ^ivAl > Be. Margarite is a rock-forming mineral and occurs in low- and medium-grade metamorphic rocks and at nearly all pressure grades. In addition, margarite commonly occurs as pseudomorphs, indicating retrograde reactions and/or polymetamorphism. Margarite most commonly is a 2M₁ polytype and, less often, as 1M and 1M_d. Tetrahedral site occupancies alternate with Si and Al around the tetrahedral ring, thereby producing a non-centric structure in space group Cc.
Cf., bityite