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Tephroite and Knebelite from Hijikuzu Mine, Iwate Prefecture, Japan

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Tephroite and Knebelite from Hijikuzu Mine, Iwate Prefecture, Japan j, Japan. Assoc. Min. Petr.Econ. Geol. 81, 341-347, 1986 Tephroite and knebelite from Hijikuzu Mine , Iwate Prefecture, Japan WATCHARAPORNKEANKEO*, TsuYosHI KITAMURA** and KATSUTOSHITANIDA** *Department of Geotechnology, Facultyof Technology, KhanKaen University,Khon Kaen, 40002. Thailand ** ResearchInstitute of MineralDressing and Metallurgy, TohokuUniversity, Katahira, Sendai 980. Japan. Tephroite and knebelite samples from the Hijikuzu mine were chemically analyzed by an elec tron microprobe analysis (EPMA) and examined by X-ray diffraction, optical and DTA methods . The minerals occur as aggregates of small prismatic and/or granular crystals in the bedded manganese ore deposits of the Hijikuzu mine in the highly metamorphosed Jurassic quartzite caused by the Miyako granitic mass. EPMA leads to formulas (Mn,.97Mg0.03Ca0.02)2.02Si0.9904.00 for tephroite and (Mn1.34Fe2+0.61Mg0.07)2.02Si0.9904.00for knebelite. typical bedded manganese ore which occurs in Introduction the highly metamorphosed Jurassic quartzite The fayalite-tephroite series occur mainly (Iwaizumi group) caused by the intrusion of the in iron-manganese ore deposits and their as Cretaceous Miyako granitic mass (Nambu et sociatedskarns, and rarely in pegmatitic rocks. al., 1975). The geologic outline of mining area Although tephroite forms an important constit is shown in Fig. 1. uent in the metamorphosed bedded manganese Rhodonite and pyroxmangite are predomi ore deposits in Japan, but knebelite has been nant in the ore and bustamite, tephroite, recognised from only four localities: Kaso knebelite, spessartine, rhodochrosite and mine (Yoshimura, 1939) and Kyurazawa mine jacobsite are common as well. Also, the ore (Watanabe and Kato, 1957), Tochigi Prefecture, includes some minerals such as manganoan lizaka, Fukushima City, Fukushima Prefecture aegerine, manganoan actinolite, manganoan (Oman et al., 1950) and Hakozaki mine, Iwate winchite, manganoan biotite, orthoericssonite, Prefecture (Nambu et al ., 1966). The nomen yoshimuraite, quartz, albite, potash feldspars clatures fayalite-tephroite series (Henriques, and calcite (Nambu et al., 1973; Matsubara et 1956),fayalite (Fa 100-90 mol percent), iron at., 1975; Nambu et al., 1980 a, b). Following knebelite (Fa 90-70) , knebelite (Fa 70-30), man- minerals are really found in the ore: magnet ganknebelite (Fa 30-10) and tephroite (Fa 10-0) ite, hematite, chalcopyrite, pyrrhotite, pyrite, are used in this paper . cryptomelane, nsutite, amorphous manganese dioxide, penwithite and manganoan hedenberg- Modeof occurrence ite. Deposits of the Hijikuzu mine consist of Tephroite and knebelite occur as aggre- (Manuscript received, March 20, 1986; accepted for publication, May 8, 1986) 342 Watcharaporm Keankeo, Tsuyoshi Kitamura and Katsutoshi Tanida Fig. 1. Map showing the location and geologic outline of the Hijikuzu mine, Iwate Prefecture. After Nambu et al. (1973). 1, Miyako granite; 2, slate hornfels; 3, quartzite; 4, limestone. gates of small prismatic and/or granular crys •@cleavage is (101) moderate. Some knebelite tals up to 1.5•~1.0mm in the bedded mangenese crystals are twinned on (100) and show a ore deposits. Tephroite is associated mainly subparallel alignment. The Vickers hardness with pink rhodonite, while knebelite is associat (HV) is 4.70 (load 100gm) and specific gravity is ed with brown-red rhodonite and ferroan bus 4.11. The refrective indices are ƒ¿a 1.780, ƒÀ tamite. 1.816, ƒÁ 1.825 (all•}0.003). Physical and optical data for tephroite and Physical properties knebelite from the Hijikuzu mine are shown in Tephroite is dark gray with vitreous luster. Table 1. Its streak is light ash gray. In the thin section, Chemical compositions it shows non-pleochroic and very pale green. Cleavage is (010) moderate. The Vickers hard- Chemical analyses of tephroite and ness (HV) is 478 (load 100gm). Specific gravity knebelite were carried out by means of an measured by Bermann balance is 4.07. The electron microprobe (Hitachi X-650S+Kevex) refractive indices measured by the immersion using the following standards: synthetic method are ƒ¿ 1.766, ƒÀ 1.802, ƒÁ 1.815 (all •}0.003). clinopyroxene, Fe2O3 and metallic Mn. The Knebelite is dark grayish green with vitre results are shown in Table 2. ous luster. Streak is light greenish gray. On the basis of 4 oxygens, the chemical Under the microscope the mineral is non- analyses of tephroite and knebelite yield pleochroic and very pale greenish yellow. The the empirical formulas as follows: Tephroite and knebelite from Hijikuzu Mine 343 Table 1. Physical and optical properties of tephroite and knebelite from the Hijikuzu mine, Iwate Prefecture Table 2. Electron microprobe analyses of te (Mn1.97Mg0.03Cao.o2)2.02Si0.99O4.00for tephroite phroite and knebelite from the Hijikuzu and (Mn,.34Fe2+0.6iMg0.07)2.02Si0.9904.00 for mine, Iwate Prefecture knebelite. Both formulas are in agreement with the general formula X2SiO4 for the olivine group. The tephroite shows a little replace ment of manganese by other elements, but it close in composition to the theoretical end member Mn2SiO4. The knebelite contains over 30 mol percent Fe2SiO4 component. This value falls within the compositional range of knebelite described by the Henriques (1956) nomenclature. The tephroite from Hijikuzu mine is as sociated mainly with iron poor pink rhodonite (Nambu et al., 1976), whereas the knebelite is associated with iron rich brown-red rhodonite (Nambu et al., 1976) and ferroan bustamite (Nambu et al., 1977). The molecular percent age composition of tephroite, knebelite, rhodonite and bustamite from Hijikuzu mine is plotted in Fig. 2, in terms of MnO, FeO and CaO. 344 Watcharaporm Keankeo, Tsuyoshi Kitamura and Katsutoshi Tanida Fig. 2. Molecular percentage composition of analysed manganese silicates from the Hijikuzu mine, Iwate Prefecture. T, tephroite; K, knebelite; R, rhodonite; B, bustamite. Fig. 3. DTA and TG curves of tephroite and knebelite from the Hijikuzu mine , Iwate Prefecture. Tephroite and knebelite from Hijikuzu Mine 345 Table 3. X-ray diffraction patterns and unit cell dimensions of tephroite and knebelite X-ray studies from the Hijikuzu mine, Iwate Prefecture. The materials used for X-ray and thermal (Fe/Mn radiation) studies were purified by the Frantz isodynamic separator. The X-ray diffraction patterns of tephroite and knebelite were obtained with a Rigaku- Denki X-ray diffractometer using Fe radiation with Mn filter. The patterns are shown in Table 3. The lattice dimensions of tephroite and knebelite from the Hijikuzu mine were calculated from the diffraction data as follows: a0 4.87, b0 10.60, c0 6.25•ð for tephroite and a0 4,85, b0 10.61, c0 6.17•ð for knebelite. Thermal studies DTA and TG of tephroite and knebelite were carried out simultaneously in air by means of a Rigaku-Denki Thermoflex unit using about 500mg of pure materials. Heating rate was 10•Ž per minute. The DTA and TG curves are shown in Fig. 3. The DTA curve of tephroite shows a sharp exothermic peak at 1,050•Ž, whereas that of knebelite shows a broad exothermic reaction from 800•Ž to 1,000•Ž. The TG curves of both materials show one step of weight increase. This occurs between 8000 and 1,1000 for tephroite and between 450•Ž and 1,100•Ž for knebelite, corresponding to the exothermic reaction in the DTA curves. The weight increases are as follows: 2.5% for tephroite and 5.2% for knebelite. The X-ray diffraction patterns obtained from samples of tephroite and knebelite heated where (Keankeo et al., in preparation). at 1,100•Ž are compared with those of the unheated materials in Table 4. The decompo Acknowledgements: One of the authors sition products of tephroite after heating at (W.K.) would like to acknowledge the staff of 1,100•Ž are rhodonite, braunite and tridymite, the Japan Ministries of Education, Science and whereas those of knebelite are rhodonite, Culture and also Japan International Coopera braunite, jacobsite and hausmannite. Details tion Agency (J. I.C. A.) of whom are concerned of the thermal decomposition of the fayalite with the Group Training Course on Mineral tephroite series olivine will be published else- Processing and Metallurgy at Tohoku Univer- 346 Watcharaporm Keankeo, Tsuyoshi Kitamura and Katsutoshi Tanida Table 4. X-ray diffraction patterns of heated samples of tephroite and knebelite from the Hijikuzu mine, Iwate Prefecture T: Tephroite or Knebelite, R: Rhodonite, B: Braunite, J: Jacobsite, H: Hausmannite, S: Tridymite sity and provision of facility for this study. We are grateful for the encouragement of Prof. A. Yazawa, Tohoku University. References Henriques, A. (1956),Optical physical properties of knebelite, Arkiv Min. Geology,2, 225-270. Matsubara, S. and Nagashima, K. (1975),Orthoer icssonite Japan.Mineralogicalin northeastern Inst. Min.Mineral.from Dress. Hijikuzustudy J.,Metallur.Japan of7, mine,513-525.Nambu, manganeseM., (I).Tanida, K. TohokuIwateand KnebeliteKitamura, silicatePrefecture, T.Univ., (1966), fromores mine,22,Iwate(in Hakozaki Prefecture.63-75Japanese Bull. withRes.English abstract). Nambu , M., Tanida, K., Oikawa, S., Kumagai, S. Tephroite and knebelite from Hijikuzu Mine 347 and Nasukawa, S. (1973), Manganese Ore Chemical composition of manganese bearing Deposits of Kitakami Mountainland, II. 85p., amphibole in Japan and its classification. J. Iwate Prefecture (in Japanese). Mineral. Soc. Japan, 14, Spec. Issue 3, 98-116 (in Nambu, M., Tanida, K. and Kitamura, T. (1975), Japanese with English abstract). Mineralogical study of manganese silicate ores Nambu, M., Tanida, K. and Kitamura, T. (1980 b), in northeastern Japan (XVIII). Tephroite Chemical composition of manganese bearing from Hijikuzu mine, Iwate Prefecture. Bull. pyroxene in Japan and its classification. J. Res. Inst. Min. Dress. Metallur. Tohoku Univ., Mineral. Soc. Japan, 14, Spec. Issue 3, 117-128 31, 59-70 (in Japanese with English abstract). (in Japanese with English abstract). Nambu, M., Tanida, K. and Kitamura, T. (1976), Omori, K., Hasegawa, S. and Ishii, M. (1950), A new Mineralogical study of manganese silicate ores occurrence of iron knebelite from a pegmatite in northeastern Japan (XX).
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