Shirozulite, KMn2+3 (Si3Al)O10(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 = 1.592(2), n_ß n = 1.635(2). The structural formula is (K_0.90Ba_0.09) (Mn2+_1.53Mg_0.94Fe2+_0.20Ti_0.04Al_0.29) (Si_2.54 Al_1.46) O₁₀ [(OH)_1.97F_0.03], 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, ß = 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, (31); 3.386, 51, (003); 1.556, 48, (13); 2.467, 46, (32); 2.202, 36, (33). The crystal structure has been refined to an R value of 4.1% based on 663 observed reflections collected with MoK 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 Å, = 8.36°, _M1 = 58.5°, _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 Mn2+ relative to Mg and Fe2+ than in olivine, pyroxenoid, and garnet.