Shirozulite is a new Mn-dominant trioctahedral mica from the Taguchi mine, Aichi Prefecture, Japan. The mineral occurs in tephroite-rhodochrosite ores in contact with a Ba-bearing, K-feldspar vein. Shirozulite formed during regional low-P/T metamorphism and, thereafter, suffered thermal metamorphism from a local granodiorite. Grains of shirozulite are up to 0.5 mm across and have a typical micaceous habit. Color: dark reddish brown. Cleavage: (001), perfect. Optical properties: biaxial negative, 2V x = very small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, absorption X < Y ≈ Z. Refractive indices: n α = 1.592(2), n β ≈ n γ = 1.635(2). The structural formula is (K 0.90 Ba 0.09 ) (Mn 2+ 1.53Mg 0.94 Fe 2+ 0.20 Ti 0.04 Al 0.29 ) (Si 2.54 Al 1.46 ) O 10 [(OH) 1.97 F 0.03 ], and the end-member composition is KMn 2+ 3 AlSi 3 O 10 (OH) 2 . Density: obs. = 3.20(3) g/cm 3 by pycnometer, calc. = 3.14(2) g/cm 3 . Shirozulite is monoclinic, C2/m, 1M polytype, a = 5.3791(7), b = 9.319(1), c = 10.2918(9) Å, β = 100.186(9)°, V = 507.8(1) Å 3 . The six strongest lines in the powder X-ray diffraction pattern are as follows: d (Å), l (%), (hkl): 10.16, 100, (001); 2.654, 96, (1̅31); 3.386, 51, (003); 1.556, 48, (3̅̅13); 2.467, 46, (1̅32); 2.202, 36, (1̅33). The crystal structure has been refined to an R value of 4.1% based on 663 observed reflections collected with MoKα X-radiation from a single crystal. The mean bond lengths, tetrahedral rotation, and octahedral flattening angles are as follows: <T-O> = 1.668, <M1-O> = 2.118, <M2-O> = 2.103, <K-O>(inner) = 2.995, and <K-O>(outer) = 3.376 Å, α = 8.36°, ψ M1 = 58.5°, ψ M2 = 58.2°. The apparent element distribution coefficient analyses among coexisting manganese or manganoan silicate minerals indicate that the trioctahedral mica structure cannot contain larger amounts of Mn 2+ relative to Mg and Fe 2+ than in olivine, pyroxenoid, and garnet.