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ANHYDROUS SYNTHESIS of GREIGITE. Fe3s4

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ANHYDROUS SYNTHESIS of GREIGITE. Fe3s4 MINERALOGICAL JOURNAL, VOL. 6, No. 6, pp. 458-463, APRIL 1972 ANHYDROUS SYNTHESIS OF GREIGITE. Fe3S4 HIROMOTO NAKAZAWA and KouSUKE SAKAGUCHI National Institute for Researches in Inorganic Materials, Kurakake, Sakura-mura, Niihari-gun, Ibaragi, Japan ABSTRACT The vacuum evaporation method has been applied for preparation of low-temperature phases of iron sulfides. Thin films were synthesized from natural pyrrhotite crystals on cleaved halite crystals heated at different temperatures. The films have been identified by the electron diffraction method to be greigite, Fe3S4. Presence of a small amount of marcasite has been also confirmed in the films prepared on halite crystals at room tem perature. Introduction The phase relations of the iron-sulfur system were recently studied below 320•Ž in detail (Morimoto et al., 1970; Nakazawa & Morimoto, 1971). However, the relations have not been completely settled at relatively low temperatures, because the reaction rate was very slow in the usual dry synthesis of the iron sulfides stable in nature. In order to study the phase relations in more details, it is indispensable to produce low temperature phases under known conditions and to examine their properties. For the preparation of the low-temperature phases in dry condition, the vacuum evapora tion method was considered to be useful, because the evaporated atoms or molecules were considered to deposit on the substrate of low temperatures and crystallize into low-temperature phases. In this investigation, greigite and marcasite have been success- fully synthesized by vacuum evaporation technique as examples of H. NAKAZAWA and K . SAKAGUCHI 459 dry synthesis of low-temperature phases of the iron -sulfur system . Experimental Crystals of natural pyrrhotite of the 4C type (Fe ,S,) from the Chichibu Mine, Saitama Pref ., Japan, were used as the starting material. The crystals were crushed into coarse powder and were set on a helical cone basket made of tungsten wire (Fig . 1). Small Fig. 1. Vacuum evaporation device in a bell glass. a, halite; b, glass plate; c. tantalum ribbon heater; d, glass plate; e, mercury thermometer; f, silica-glass fibre; g, tungsten coil. halite plates of 7•~7mm2 in dimensions and 1 mm or less in thickness were mounted on a glass plate as substrate. After evaporation, the vertical difference between the surface of a deposited film on the glass plate and that of the glass plate was measured after the halite substrate being taken off under the interferometric microscope. The temperature of the substrate was measured by a mercury thermo meter which was set under the tantalum-ribbon heater separated by a glass plate and covered by silica- and alumina-glass fibres. The measured temperatures might be slightly higher than that at the 460 Anhydrous Synthesis of Greigite, Fe3S4 surface of the substrate. These devices were all set in a bell glass and evacuated to about 10-b mm. Hg. The substrates were kept at constant temperatures for about two hours before evaporation. When the evaporation source was kept at slightly higher than its melting point, the rate of deposition was in the range of 800-1200 A/min. The films produced on the substrate were black and had metallic luster on the cleaved surface. Their thickness measured by interferometric microscopy was about 1000 A. After the halite substrate having been dissolved in water, the thin films were examined by the selected area diffraction under the electron microscope (HU-11D) operated at 100kV. They were identified to be greigite by comparing their d-values with those previously reported (Skinner, Erd & Grimaldi, 1964). Though the electron microscope was operated carefully under the same condition for all observations, the deviations in d-values are considered to be in the range of +0.02A without internal standard. In order to examine the compositional change of the source material during the evaporation, some residual of it was examined by the X-ray powder method after an evaporation for several seconds. Only troilite with stoichiometric composition of FeS was detected. Metallic iron was, however, considered to exist in the residual source because the samples were attracted by a hand magnet (troilite is anti-ferromagnetic). Thus sulfur evaporates more easily than iron from source crystals of pyrrhotite (Fe,S3), and the iron content in the residual source increases with the progress of eva poration. Results and discussions Thin films were obtained on halite at room temperature , 83•Ž, 131•Ž and 179•Ž. They were all homogeneous under the bright-field images of the electron microscope. An electron diffraction pattern obtained from the thin film deposited on halite at 131•Ž (Fig . 2, a) H. NAKAZAWA and K. SAKAGUCHI 461 Fig. 2. Electron diffraction patterns from the thin films obtained on halite (a) at 131•Ž and (b) at room temperature. indicated that the film is composed of greigite, though a slightly high background between 311 and 400 of greigite in the pattern suggests a possibility of contamination by some other phase. In the diffraction patterns from the films obtained on halite at room temperature and 83•Ž, some additional reflections were ob- served (Fig. 2, b). These additional reflections can be explained by marcasite and pyrite, both of which have strong reflections with the similar d-values. However, the presence of a small amount of marcasite was more likely because the relative intensities of reflec tions were more reasonably explained by marcasite. These additional reflections from marcasite disappeared when the films were deposited on halite at higher temperatures, 131•Ž and 179•Ž, with the evapo ration source of Fe7S8 crystals. Because the sulfur content of the films decreases with the rise of temperature of the substrate, the 462 Anhydrous Synthesis of Greigite, Fe3S4 re-evaporation of sulfur from the films is likely to occur at high temperatures. The reflections from marcasite also decreased when troilite (FeS) was used for the evaporation source at room temper ature. Thus the chemical composition of films can be controlled not only by the temperature of the substrate but also by the source material. Greigite has been known to be stable at low temperatures from its modes of natural occurrence (Skinner et al., 1964). Many synthe tic experiments have been carried out of greigite: wet chemical (Berner, 1964; Horiuchi et al., 1970), hydrothermal (Yamaguchi & Katsurai, 1960; Uda, 1966; Yamaguchi et al., 1969), organic (Flaig- Baumann et al., 1970), and microbial (Bubela et al., 1969). Piggott and Wilman (1968) reported a new iron sulfide phase with a struc ture of the ƒÁ-Fe2O3-type by reaction of sulfur on mild steel surface, which was later considered to be greigite. However, greigite has not been formed from pure iron and sulfur by the usual dry method, and its stability in the iron-sulfur system has not been confirmed. Marcasite has also not been synthesized in anhydrous conditions. The polymorphic relations between pyrite and marcasite was recently doubted (Kullerud, 1967), but the stability field of marcasite has not been determined. In this investigation, the low temperature phases, greigite and marcasite, were successfully synthesized in the pure iron and sulfur system. These results indicate that their stability fields exist in the Fe-S system. This vacuum evaporation method is expected to be used for synthesis of not only low-temperature iron sulfides but also other metal sulfides stable at low temperatures . Some of the thin films were re-examined under the electron microscope after leaving them in air at room temperature for about a month. No detectable change was observed in the diffraction patterns and the bright- and dark-field images. Rapid oxidation of greigite obtained by hydrothermal method was reported by Yama guchi, Wada and Noguchi (1969). This seems to indicate that H. NAKAZAWA and K. SAKAGUCHI 463 chemical purity of the products is important in controlling the rate of oxidation. The authors are grateful to Drs . M. Nakahira and S. Yamaguchi, National Institute for Researches in Inorganic Materials , for their encouragements and interests . They also wish to thank Prof. N. Morimoto, Institute of Scientific and Industrial Research , Osaka University, for his illuminating discussions and improving the manuscript. REFERENCES BERNER, R.A. (1964). Journ. Geol., 72, 293. BUBELA, B. & MCDONALD, J.A. (1969). Nature , 221, 465. FLAIG-BAUMANN, R., Mou, G.H. & NUBER, B. (1970). Naturwiss., 57, 192. HORIUCHI, S., WADA, H. & NOGUCHI, T. (1970). Naturwiss., 57, 670. KULLERUD, G. (1967). Researches in Geochemistry (P.H. Abelson, Ed.), 2, 286. MORIMOTO, N., NAKAZAWA, H., NISHIGUCHI, K. & TOKONAMI, M. (1970). Science, 168, 964. NAKAZAWA, H. & MORIMOTO, N. (1971). Mat. Res. Bull., 6, 345. PIGGOTT, M.R. & WILMAN, H. (1958). Acta Cryst., 11, 93. SKINNER, B.J., ERD, R.C. & GRIMALDI, F.S. (1964). Amer. Min., 49, 543. UDA, M. (1965). Koll. Zeits., 170, 147. YAMAGUCHI, S., WADA, H. & NOGUCHI,T. (1969). Koll. Zeits. u. Polymar., 232, 813. Manuscript received 6 January 1972..
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