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, 2Vx = very small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, absorption X <Y ≈ Z. Refractive indices: nα = 1.592(2), nβ ≈ nγ = 1.635(2). The structural formula is (K0.90Ba0.09) (Mn2+1.53Mg0.94Fe2+0.20Ti0.04Al0.29) (Si2.54 Al1.46) O10 [(OH)1.97F0.03], and the end-member composition is KMn2+3AlSi3O10(OH)2. Density: obs. = 3.20(3) g/cm3 by pycnometer, calc.= 3.14(2) g/cm3. 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) Å3. 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, (3̅13); 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°, ΨM1 = 58.5°, ΨM2 = 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 Mn2+ relative to Mg and Fe2+ than in olivine, pyroxenoid, and garnet.