Emerald green chromian 2M₁ mica occurs in metamorphosed and metasomatized limestones, related to serpentinic rocks from Sierra Nevada, Spain. Chemical analysis gave: SiO₂ 44.51, Al₂O₃ 27.60, TiO₂ 0.17, Cr₂O₃ 3.35, Fe₂O₃ 1.76, FeO 0.07, MnO 0.00, MgO 8.59, Li₂O 0.03, Rb₂O 0.01, BaO 0.01, CaO 1.71, Na₂O 0.40, K₂O 7.86, H₂O 4.21 = 100.28. Optic properties are: pleochroism X blue, Y green, Z bluish green; refractive indices: α=1.562, β=1.611, γ=1.616 (yellow light); inclined dispersion, r > v. Orientation: second order mica. Refined parameters are a = 5.2116(6), b = 9.045(2), c = 19.97(2)Å, β = 95.7(3)°. The structure is discussed and an order in the octahedral M(1) sites is observed, but no ordering in the tetrahedral T(1) and T(2) cations is detected. The mica described is a mariposite with anoma-lously high contents of Ca and Mg.