ISHIDA ET AL.: SHIROZULITE 233 American Mineralogist, Volume 89, pages 232–238, 2004 2+ Shirozulite, KMn3 (Si3Al)O10(OH)2, a new manganese-dominant trioctahedral mica: Description and crystal structure KIYOTAKA ISHIDA,1* FRANK C. HAWTHORNE,2 AND FUMITOSHI HIROWATARI3 1Department of Evolution of the Earth and the Environment, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan 2Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 3 2734-1-1 Fukuma, Fukuoka 811-3213, Japan ABSTRACT 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) 2+ 2+ (Mn 1.53Mg0.94Fe 0.20Ti0.04Al0.29) (Si2.54 Al1.46) O10 [(OH)1.97F0.03], and the end-member composition is 2+ 3 3 KMn 3AlSi3O10(OH)2. Density: obs. = 3.20(3) g/cm by pycnometer, calc. = 3.14(2) g/cm . 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, (131); 3.386, 51, (003); 1.556, 48, (313); 2.467, 46, (132); – 2.202, 36, (133). The crystal structure has been refined to anR 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. INTRODUCTION SAMPLE DESCRIPTION Trioctahedral micas along the phlogopite-annite join occur as rock-forming minerals in a wide range of geological environ- Occurrence ments. They usually contain only minor Mn2+, and micas con- The Taguchi mine is located at Yatuhashi, Kita-shitara taining dominant octahedral Mn2+ have not been reported until County, Aichi Prefecture, Japan. The mine is in a strata-bound now. Synthesis work on trioctahedral micas with Mn-bearing Mn ore deposit in the Ryoke metamorphic belt and is renowned starting materials suggests that micas with a tetrahedral composi- for an abundance of mineral species: e.g., yoshimuraite (Hi- VI 2+ tion of (Si3Al) cannot contain more than about one Mn pfu rowatari and Isono 1963), richterite (Shoda and Bunno 1973), (per formula unit) (Hazen and Wones 1972). However, a new alleghanyite, sonolite, manganosite, jacobsite, and galaxite. Mn2+-dominant trioctahedral mica has been found at the Taguchi Manganese-rich layers are up to 2 m wide and show a zonal mine, a regionally and thermally metamorphosed strata-bound arrangement: hausmannite + rhodochrosite, tephroite, and Mn ore deposit in Aichi Prefecture, Japan. This mineral is named pyroxenoid (rhodonite, pyroxmangite) ores (Hirowatari and shirozulite in honor of Dr. Haruo Shirozu, Professor Emeritus Isono 1963). Shirozulite formed during low P/T-type regional of Kyushu University, for his outstanding contributions to the metamorphism of Cretaceous age, and thereafter, was thermally crystal-chemistry of sheet-silicate minerals, particularly the metamorphosed by an adjacent granodiorite. Shirozulite occurs in chlorite group. Shirozulite has been approved as a new mineral tephroite-rhodochrosite ores in contact with a Ba-bearing K-feld- by the International Mineralogical Association's Committee on spar vein, as subhedral grains closely associated with tephroite, New Minerals and Mineral Names. Type material is deposited rhodochrosite, and apatite. Grains of barite (a few μm across) at the Graduate School of Social and Cultural Studies, Kyushu and bementite (about 5 μm in diameter) are dispersed throughout University, Japan. the rhodochrosite, giving it a turbid appearance in thin section. A back-scattered electron image is shown in Figure 1; the mottled appearance of the mica is due to minor chemical heterogeneity XII IV XII IV * E-mail: [email protected] involving the substitution Ba Al K-1 Si–1. 0003-004X/04/0001–232$05.00 232 ISHIDA ET AL.: SHIROZULITE 233 TABLE 1. Chemical analysis (in wt%) and formula unit* for shirozulite SiO2 31.40 Si 2.536 Al2O3 18.45 Al 1.464 TiO2 0.71 Σ 4.000 FeO 2.90 MnO 22.38 Ti 0.043 MgO 7.83 Al 0.292 2+ BaO 2.77 Fe 0.196 K2O 8.75 Mg 0.943 2+ F 0.11 Mn 1.531 H2O† 3.66 Σ 3.005 O = F –0.05 Σ 98.91 K 0.902 Ba 0.088 Σ 0.990 OH 1.972 FIGURE 1. Back-scattered-electron image of shirozulite. Shr = F 0.028 shirozulite; Tp = tephroite; Ap = apatite; Rds = rhodochrosite; Bm = Σ 2.000 bementite. * Calculated on the basis of 12 anions with (OH + F) = 2.00 apfu. † Determined by stoichiometry. Physical properties TABLE 2. Unit-cell dimensions and data pertaining to crystal-struc- ture refinement of shirozulite Grains of shirozulite are up to 0.5 mm across and have a typi- a (Å) 5.3791(7) crystal size (mm) 0.06 × 0.10 × 0.16 cal micaceous appearance. Color: dark reddish brown. Cleavage: b (Å) 9.319(1) radiation MoKα c (Å) 10.2918(9) No. of reflections 1564 (001) perfect. Optical properties: biaxial negative, 2Vx = very β(°) 100.186(9) No. of unique reflections 788 small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, 3 V (Å ) 507.8(1) No. |Fo| > 4σFo 663 absorption X < Y ≈ Z. Refractive indices: nα = 1.592(2), nβ ≈ nγ= Space group C2/m Raz (%) 2.3 → 1.2 1.635(2). Density: obs. = 3.20 g/cm3 by pycnometer, calc. = 3.14 Z 2 R1 (|Fo|> 4σFo) (%) 4.1 wR (F2) (%) 5.2 3 2 o g/cm . Vickers hardness was measured on the (001) surface with 2 2 2 2 2 1/2 2 Notes: R1 = Σ (|Fo| – |Fc|)/Σ|Fo|; wR2 = [Σw(Fo – Fc) /Σw(Fo) ] , w = 1/σFo. an Akashi model MVK: VHN = 100~130; Mohs hardness ≈ 3. Chemical composition Intensities were corrected for background, absorption, Lorentz, Preliminary chemical analyses of the mica and coexisting minerals and polarization effects, and reduced to structure factors. Details were done using a scanning electron microscope JEOL SEM35CF-II concerning these procedures are given in Table 2. equipped with a Link System 800-2-500 energy-dispersive spectrom- The crystal structure was refined using the SHELXTL-PC eter and a ZAF-4/FLS quantitative-analysis software system. Oper- system of programs; R indices are of the form given in Table 2. ating conditions were as follows: accelerating voltage 15 kV, beam The structure was refined in the space groupC 2/m to an R-index of current 1.5 nA on Fe metal, and 100 s collecting time. 4.1% for 663 observed reflections. Final atom parameters are given The crystal used to collect single-crystal XRD data was in Table 3, selected interatomic distances and angles are given in mounted in epoxy, polished, and analyzed using a CAMECA Table 4, and refined site-scattering values (epfu) and assigned site- SX-50 electron-microprobe operating in wavelength-dispersion populations (apfu) are given in Table 5. Calculated stereochemical mode, with a beam diameter of 5 μm and an accelerating voltage parameters for shirozulite are given in Table 6, and observed and 1 of 15 kV. The following standards were used: albite (NaKα), calculated structure factors are listed in Table 7 . fayalite (FeKα), diopside (CaKα, SiKα), kyanite (AlKα), spes- X-ray powder pattern sartine (MnKα), orthoclase (KKα), zinnwaldite (FKα), forsterite (MgKα), titanite (TiKα), and barite (BaLα). The mean com- Powder diffraction data were collected with a Rigaku Rint- position is given in Table 1. The value for H2O was calculated 2100V diffractometer using curved-graphite monochromatized assuming OH + F = 2 apfu (atom per formula unit). The structural CuKα X-rays and operating at 40 kV and 40 mA with a step-width 2+ 2+ θ formula for shirozulite is (K0.90Ba0.09) (Mn1.53Mg0.94Fe0.20Ti0.04Al0.29) of 0.01 °2 and a step-interval of 16 s. A Rietveld analysis was (Si2.54 Al1.46) O10 [(OH)1.97F0.03], and its end-member composition done using the program Rietan-2000 (Izumi and Ikeda 2000): the 2+ is KMn3 (AlSi3)O10(OH)2. single-crystal structural data described above were used as the initial parameters, and the isotropic-displacement factors were Single-crystal diffraction and structure refinement used. Table 8 shows the observed and calculated d-values, and A crystal was mounted on a Siemens P4 automated four-circle the observed intensities; Figure 2 shows the fitted powder-dif- diffractometer with a graphite monochromator and a Mo X-ray fraction pattern. Within the estimated standard deviations, the tube. Twenty-five reflections were centered; a constrained mono- resultant unit-cell parameters are consistent with the values from clinic cell was determined from the setting angles and refined the single-crystal data.