Journal of Mineralogical and Petrological Sciences,Volume 95,page 145 161,2000 Phase equilibria in the system Fe Ni S at 500°C and 400°C Teiichi UENO,Shin ichiro ITO,Shinya NAKATSUKA, Kazuki NAKANO,Takayuki HARADA and Takeshi YAM AZAKI Department of Earth Sciences,Fukuoka University of Education, 729-1 Akama,Munakata,Fukuoka 811 4192,Japan Phase equilibria in the system Fe Ni S were investigated by dry synthesis at 500°C and 400°C using three elements(Fe,Ni and S)and two synthetic sulfides(FeS and NiS).Phase diagrams of this ternary system at these two temperatures were obtained.The extent of the monosulfide solid solution(mss)is complete at 500°C and 400°C.The d(102)values of the mss are increasing with the increasing Fe/(Fe＋Ni)ratio and are decreasing with the increasing sulfur content of the mss.Pentlandite has the solid solution areas ranging from(FeNi)Sto(FeNi)Sat 500°C and from(FeNi)Sto(FeNi)S at 400°C.Violarite has the solid solution area ranging from(FeNi)Sto(FeNi)Sat 400°C and does not exist at 500°C.Godlevskite is stable at 400°C and it transforms toα NiSphase at 500°C run.Tie lines of pentlandite to godlevskite at 400°C and pentlandite toα NiSphase at 500°C are stable.On the Fe Ni join,two kinds of Fe Ni alloys(α iron andγ phase)and pure nickel are stable. Alfa iron has a body centered cubic str ucture and pure nickel has a face centered cubic structure. Gamma phase has a primitive tetragonal structure and the FeNiphase is included in theγ phase solid solution. NiS solid solution(mss)at 600°C and 400°C.Nal- drett et al.(1967)investigated the central portion of the Introduction Fe Ni S system and applied for the interpretation of the The information on the phase equilibrium in the Fe pentlandite exsolution in iron nickel sulfide ores. Ni S system is very important for understanding of the They showed the relationship between d(102)values genesis of many ore deposits producing iron nickel and compositions of the mss.Craig et al.(1967) minerals,meteorites and core materials of the earth.A showed the 400°C isothermal diagram of this system,but large number of the phase equilibrium studies were quite different from the 400°C isothermal diagram performed by many authors and the phase diagrams of obtained by Kullerud(1963a).Main differences are(1) this ternary system were published from 1100°C to 300°C. the presence of the(Ni,Fe)Ssolid solution,(2)the Kullerud(1963a)showed many phase diagrams of this continuity of the mss,(3)the presence of the tie line system throughout the temperature range 400°C 1100°C. between pentlandite and NiSphase,(4)the solid Kullerud(1963b)showed the thermal stability of pent- solution range of pentlandite.Shewman and Clark landite.Clark and Kullerud(1963)studied the sulfur (1970)studied the pentlandite phase relations in the Fe rich portion of this system. Craig(1966)studied the Ni S system and showed the phase diagrams of the pyrite pentlandite relations and Craig(1967)showed central portion of this system at 600°C,500°C and 400°C. the violarite stability relations.Naldrett(1966),and Their diagrams are quite different from the diagrams Naldrett and Craig(1967)investigated the FeS shown by Kullerud(1963a)and are similar to the diagrams shown by Craig(1967),and Craig et al. T.Ueno，上野禎一，uenot＠fueipc.fukuoka edu.ac.jp Corre- (1967).Recently,Karup Mo/ller and M akovicky sponding author (1995)studied the phase equilibria in this system at S.Ito，伊東伸一郎 725°C,and Sugaki and Kitakaze(1998)investigated the S.Nakatsuka，中務慎也 K.Nakano，中野一樹 high form of pentlandite and its thermal stability.But, ，原田貴之 the more important phase relations at 500°C and 400°C T.Harada T.Yamazaki，山崎 武 in the study of ore deposits are not completely cleared. Teiichi Ueno,Shin ichiro Ito,Shinya Nakatsuka,Kazuki Nakano,Takayuki Harada and Takeshi Yamazaki Table 1. Experimental results for Phase equilibria in the system Fe Ni S at 500°C and 400°C of the Fe-Ni-S system at 500°C pn,pentlandite;mss,monosulfied solid solution;S,sulfur;py,pyrite;hz,heazlewoodite;vae,vaesite;γ,γ-phase; α-NiS,α-(Ni,Fe)S. The present study was performed in order to prepare the and by X ray powder diffractometry(Ni filtered CuKα detailed phase diagrams of this system at 500°C and radiation,40 kV,100 mA),with Si as an internal stan- 400°C,and to show the relationship between d(102) dard.The d(102)values of the mss were precisely values and compositions of the mss. measured and cell parameters were calculated for some phases by the least squares method.The chemical compositions of the phases were determined by a JEOL Experimental 50A electron microprobe.Operating conditions for the Elemental Fe(99.99%),Ni(99.99%)and S(99.999%)and microprobe were 25 kV and 20 nA(on MgO crystal). synthetic mono sulfides(FeS and NiS)were used for the Synthetic FeS,FeS,NiS,NiSand NiSand metallic phase equilibrium experiments.Tables 1 and 2 show Ni and Fe were used as standards.The analytical bulk compositions of the starting mixtures.They were results,each an average of several grains,are shown in sealed in evacuated pyrex tubes.The tubes were heated Tables 3 and 4.All analytical errors are within 0.1 in an electric furnace at 200°C for 1 day to transform wt.%.Chemical formulae were calculated for each starting mixtures into sulfides and were reheated at phase from normalized atomic proportions. 500°C or 400°C to obtain the equilibrium states.After reheating,the tubes were quenched in cold water and products were examined by reflected light microscopy Teiichi Ueno,Shin ichiro Ito,Shinya Nakatsuka,Kazuki Nakano,Takayuki Harada and Takeshi Yamazaki Table 2. Experimental results for Phase equilibria in the system Fe Ni S at 500°C and 400°C of the Fe-Ni-S system at 400°C pn,pentlandite;mss,monosulfied solid solution;S,sulfur;py,pyrite;hz,heazlewoodite;vae,vaesite;god,godlevskite;γ,γ-phase; α,α-iron;vio,violarite. Teiichi Ueno,Shin ichiro Ito,Shinya Nakatsuka,Kazuki Nakano,Takayuki Harada and Takeshi Yamazaki Table 3. Chemical compositions of Phase equilibria in the system Fe Ni S at 500°C and 400°C phases in the Fe-Ni-S system at 500°C Description of phases method.The upper line corresponds to the values of relatively low sulfur contents.The middle line corre- Many phases are synthesized in this experiment,and they sponds to the values of mss inside.The lower line are described as follows. corresponds to the values of relatively high sulfur con- tents.The d(102)values are increasing with the Monosulfide solid solution increasing Fe/(Fe＋Ni)mole ratio,reflecting the differ- ence in ionic radii of Fe and Ni.The d(102)values are As reported previously by Naldrett et al.(1967),the decreasing with the increasing sulfur content of the mss. existence of a complete FeS NiS solid solution Decrease of the metal content with regard to the amount (monosulfide solid solution＝mss)was first suggested by of sulfur means the increase of the vacancies of metal Hawley et al.(1943)and was demonstrated by Lund- sites in the mss structure.Unit cell of mss decreases qvist(1947).The crystal structure of the mss obtained with the increase of the vacancies.The results obtained in the present study is the simple hexagonal NiAs type. at 400°C were similar to those at 500°C. The solid solution is continuous between FeS and NiS at 500°C and 400°C isotherms.Microscopically, Pentlandite solid solution the Fe rich mss shows creamy pinkish brown and has very strong anisotropy from yellow gray to grayish blue, As reported previously by Shewman and Clark(1970), and the Ni rich mss shows creamy yellow and has and by Lindqvist et al.(1936)pentlandite was iso struc- strong anisotropy from lemon yellow to blue or violet. tural with synthetic CoS.They wrote its formula as Table 5 shows compositions and d(102)values of the (Fe,Ni)S.We obtained the pentlandite solid solu- mss at 500°C.The mss phases from run nos.P007 to tion areas ranging from(FeNi)S(P019)to P021 coexist with pentlandite orα NiSand they show (FeNi)S(P012)at 500°C and from relatively low sulfur contents.The mss phases from run (FeNi)S(HA008)to(FeNi)S nos.K011 to UT033 are mono phases.The mss (UT072)at 400°C.These solid solution ranges are phases from run nos.UT055 to UT063 coexist with larger than the areas shown by Kullerud(1963a)and pyrite or vaesite and show relatively high sulfur contents. similar to those of Shewman and Clark(1970),but with Figure 1 shows the relation between compositions and a little shift rich in iron compared with the areas shown d(102)values of the mss at 500°C.The horizontal axis by Craig et al.(1967).Table 6 shows the X ray pow- shows the Fe/(Fe＋Ni)mole ratio and the vertical axis der diffraction data for the pentlandite solid solution. shows the d(102)values(Å).Three lines are deter- According to this Table,the cell dimension,a,decreases mined as the quadratic curve by the least squares with the increasing nickel content.As for the cell Teiichi Ueno,Shin ichiro Ito,Shinya Nakatsuka,Kazuki Nakano,Takayuki Harada and Takeshi Yamazaki Table 4.