A dioctahedral member of the true mica group. The ideal end-member formula is KFe3+(Mg, Fe2+)□Si4O10(OH)2, where □ = vacancy. The typical range in composition is: viR2+/(viR2+ + viR3+) ≥ 0.25, viAl/(viAl + viFe3+) < 0.5, Mg/(Mg + viFe2+) > 0.5 (Rieder et al., 1998). Celadonite is commonly an alteration product of pyroxenes and other Fe,Mg minerals found in basalts either during low-grade metamorphism or hydrothermal activity.