Journal of Mineralogical and PetrologicalIseite, a new Sciences, mineral Volume 108, page 37─ 41, 2013 37 LETTER Iseite, Mn2Mo3O8, a new mineral from Ise, Mie Prefecture, Japan * ** ** *** Daisuke NISHIO-HAMANE , Norimitsu TOMITA , Tetsuo MINAKAWA and Sachio INABA * The institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan ** Department of Earth Science, Faculty of Science, Ehime University, Matsuyama, Ehime 790-8577, Japan *** Inaba-Shinju Corporation, Minamiise, Mie 516-0109, Japan Iseite, Mn2Mo3O8, a new mineral that is a Mn-dominant analogue of kamiokite, is found in the stratiform ferro- manganese deposit, Shobu area, Ise City, Mie Prefecture, Japan. It is the first mineral species that includes both Mn and Mo as essential constituents. Iseite is iron-black in color and has a submetallic luster. It occurs as ag- gregates up to about 1 mm in size made of minute crystals (<20 μm). Iseite has a zoned structure closely associ- ated with undetermined Mn-Fe-Mo oxide minerals with hexagonal forms, and it occasionally coexists with 3 small amounts of powellite. Its Mohs hardness is 4-5, and its calculated density is 5.85 g/cm . The empirical formula of iseite is (Mn1.787Fe0.193)Σ1.980Mo3.010O8. Its simplified ideal formula is written as Mn2Mo3O8. The min- eral is isostructural with kamiokite (hexagonal, P63mc). The unit cell parameters are a = 5.8052 (3) Å, c = 10.2277 (8) Å, V = 298.50 (4) Å3, and Z = 2. The Rietveld refinement using synchrotron radiation (λ = 0.413 Å) powder XRD data converges to Rwp = 3.11%, and confirms two independent Mn sites—tetrahedral and octahe- IV VI dral—in the crystal structure of iseite, indicating the structure formula Mn Mn Mo3O8. Keywords: Iseite, Mn2Mo3O8, Kamiokite, Fe2Mo3O8, Ise city INTRODUCTION CNMNC). During our mineralogical survey in Ise City, Mie The reaction between the monoxides of certain metals (M Prefecture, Japan, we found the Mn analogue of kamiok- = Mg, Mn, Fe, Co, Ni, Zn, Cd) and molybdenum dioxide ite, Mn2Mo3O8. This new mineral is named iseite after the 2+ 4+ 2+ 4+ forms compounds of M 2 Mo3 O8 rather than M Mo O3. locality. The pronunciation is ‘iseait’. The mineral and The M2Mo3O8 compound has a unique crystal structure name have been approved by IMA-CNMNC (IMA2012- with a hexagonal P63mc symmetry (McCarroll et al., 020). The type specimen of iseite is now stored at the Na- 1957). The MO4 tetrahedron and MO6 octahedron are con- tional Museum of Nature and Science, Tokyo, Japan nected at the apical oxygen atoms to form a honeycomb- (NSM M-43652). Here, we describe the new mineral, is- like lattice sheet in the a-b plane. The sheet structure eite. stacks with the MoO6 octahedral sheet alternately along the c-axis. The M2Mo3O8 compounds have been drawing OCCURRENCE attention in the solid state physics owing to their wide va- riety of magnetic properties (e.g., McAlister and Strobel, Shima Peninsula of Japan is composed of accretionary 1983; Abe et al., 2010; Nakayama et al., 2011). complexes within the Sambagawa, Chichibu, and Shi- Among naturally occurring M2Mo3O8 compounds, manto Terranes. We found a stratiform ferromanganese only the Fe mineral species, kamiokite Fe2Mo3O8, is deposit in Chichibu Terrane, in the Shobu area of Ise City, widely recognized (Sasaki et al., 1985). Recently, the Mg Mie Prefecture. The Shobu ferromanganese deposit em- analogue of kamiokite, Mg2Mo3O8, was reported from the bedded in the chert is closely associated with limestone Allende meteorite (Ma et al., 2009). However, this does and greenstone. The matrix of the ore in this deposit con- not yet appear in the list of new minerals approved by the sists mainly of magnetite, hematite, and caryopilite. Mon- International Mineralogical Association, Commission on azite-(La), chalcopyrite, and Ni-Fe sulfides such as pent- New Minerals, Nomenclature and Classification (IMA- landite and heazlewoodite are also found in the ore. Three mineral veins randomly cross the ore—bementite, tephro- doi:10.2465/jmps.120621c ite, and rhodochrosite. Bementite and tephroite veins in- D. Hamane, [email protected] Corresponding author clude La-rich allanite group minerals. The rhodochrosite 38 D. Nishio-Hamane, N. Tomita, T. Minakawa and S. Inaba Figure 1. A photomicrograph (a) and back-scattered electron images (b)- (d) of iseite and associated miner- als. Iseite forms a single-phase ag- gregate up to about 1 mm in size (a), and it is occasionally associated with powellite (b), undetermined Mn-Fe-Mo oxide minerals (c), and molybdenite (d). Ise, iseite; Mol, molybdenite; Pwl, powellite; Rds, rhodochrosite. vein occasionally includes molybdenite. Iseite occurs in Table 1. The reflectance values of iseite the rhodochrosite vein, and its occurrence is quite rare. The occurrence mode of iseite is shown in Figure 1. Iseite occurs as aggregates up to about 1 mm in size of minute crystals, whilst the single crystals are anhedral and are several microns to 20 μm in size. Iseite has a zoned texture closely associated with undetermined Mn-Fe-Mo oxide minerals with hexagonal forms, and it occasionally coexists with small amounts of powellite. Iseite and mo- lybdenite rarely coexist (Fig. 1d), but their individual ag- gregates are common. PHYSICAL AND OPTICAL PROPERTIES Iseite is iron-black in color with black streak and subme- tallic luster. It appears light yellowish grey in color under reflected light with no internal reflections. Pleochroism is medium, ranging from pale grey to yellowish grey. An- isotropy is medium to strong, grey to pale grey. The re- flectance spectrum for iseite was measured in air relative to a Si standard using an OLYMPUS photometry system JSM-5600 electron microprobe analyzer equipped with with a JASCO CT-50 grating monochromator (Table 1). an Oxford Link ISIS energy-dispersive X-ray spectrome- No fluorescence was observed under long- or short-wave ter at the Institute for Solid State Physics, the University ultraviolet radiation. Mohs hardness is 4-5. Its density of Tokyo. The analytical conditions were 15 kV, 0.4 nA, 3 was calculated to be 5.85 g/cm on the basis of the empiri- and 1-μm beam diameter. The ZAF method was used for cal formula and unit cell volume. corrections. The chemical compositions of iseite and the undetermined Mn-Fe-Mo oxide minerals are listed in Ta- CHEMICAL COMPOSTION ble 2. Iseite is characterized by the dominance of Mn and Chemical analyses of iseite were carried out using a JEOL Mo, whilst a small amount of Fe is also included. Other Iseite, a new mineral 39 Table 2. Chemical composition of iseite. Averaged values are giv- for Pulse X-rays (PF-AR), The High Energy Accelerator en for individual aggregates with a variation in the Mn/Mn + Fe Research Organization (KEK), Japan. This beam line pro- vides a 30 μm diameter collimated beam of monochroma- tized X-ray radiation (λ = 0.413 Å). The XRD spectrum was collected using the Debye-Scherrer method and re- corded using an imaging plate detector. The XRD pattern was obtained using the homoge- neous micro sample (~ 100 μm) of the iseite aggregate, the Mn-richer sample (Table 2-1). Rietveld analysis was carried out using the RIETAN-FP program (Izumi and Momma, 2007), and peak profiles were fitted with pseu- do-Voigt functions. The initial structural model is from crystallographic data of the synthesized Mn2Mo3O8 com- pound, which has the kamiokite-type structure of hexago- - - The data for undetermined Mn Fe Mo oxide minerals are also nal P63mc symmetry (Abe et al., 2010). The occupancy given for comparison. parameters for the two Mn sites were fixed to 0.9Mn + 1, iseite in the pure aggregate of minute crystals; 2, iseite associat- 0.1Fe according to the chemical data. The unit cell param- ed with the undetermined Mn-Fe-Mo oxide minerals; 3 and 4, the undetermined Mn-Fe-Mo oxide minerals with dark and bright eters were refined using Miller indices and the observed rims (Figures 1-3), respectively. d-values, and then those were also fixed during Rietveld *Number of analyzed points. analysis. Since the refinement of the isotropic atomic dis- placement parameters failed owing to the low quality for elements such as Ca, Mg, Ti, and W are below the detec- the peak resolution of the XRD pattern, those were fixed tion limit. The compositional variations in iseite are rela- at 1.0 for all atoms throughout the refinement procedure. tively small within a single aggregate, while the composi- Figure 2 shows observed and calculated XRD patterns for tion varies slightly between separate aggregates. The iseite. Rietveld refinement based on the kamiokite-type variation is mainly attributed to Mn-Fe contents. Iseite in structure converged to Rwp = 3.11, Rp = 1.91, RR = 15.90, single-phase aggregates shows a high Mn content, Mn/ Re = 1.58, RB = 6.23, and RF = 2.98%, indicating that is- (Mn + Fe) = 0.90 (Table 2-1), whereas that coexisting with undetermined Mn-Fe-Mo oxide minerals shows a slightly lower Mn content, Mn/(Mn + Fe) = 0.75 (Table 2-2). In either case, Mn dominates Fe. The empirical for- mulae for samples of iseite existing as single-phase ag- gregates and coexisting with other aggregates are (Mn1.787Fe0.193)Σ1.980Mo3.010O8 and (Mn1.540Fe0.508)Σ2.048 Mo2.976O8 on the basis of O = 8, respectively, leading a simplified formula as (Mn,Fe)2Mo3O8. The ideal formula, Mn2Mo3O8, requires MnO 26.99 wt% and MoO2 73.01 wt%, for a total of 100.00 wt%. The chemical data of undetermined Mn-Fe-Mo ox- ide minerals can be divided into two compositions: (Fe,Mn)6Mo5O16 and (Mn,Fe)MoO3 (Table 2).