Shirozulite, KMn_3~(2+)(Si_3Al)O_(10)(OH)_2, a new manganese-dominant trioctahedral mica: Description and crystal structure

摘要

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_(1.53)~(2+)Mg_(0.94)Fe_(0.20)~(2+)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_3~(+2)AlSi_3O_(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) A, β = 100.186(9)°, V= 507.8(1) A~3. The six strongest lines in the powder X-ray diffraction pattern areas follows: d(A), l(%), (hkl):10.16, 100, (001); 2.654, 96, ((1-bar)31); 3.386, 51, (003); 1.556, 48, ((3-bar)13); 2.467, 46, ((1-bar)32); 2.202, 36, ((1-bar)33). 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: = 1.668, = 2.118, = 2.103, (inner) = 2.995, and (outer) = 3.376 A, 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.