Magnetic Properties of Cubanite (CuFe2S3)

By M. Sawada, M. Ozima Geophysical Institute, University of Tokyo

and

Y. Fujiki Mining Department, University of Tokyo (Read May 25, 1962; Received June 1, 1962)

Abstract

A study has been made of the magnetic properties of cubanite (CuFe2S3). It was found that the saturation magnetization of orthorhombic cuaanite at room temperature was 0.87emu/gram. An irreversible change revealed in a thermo-

magnetic curve at about 270℃ was identified as a ponymorphic transition from

an orthorhombic to a cubic structure by X-ray analyses, chemical analysis and microscopic observation. It is further suggested from the result on differential thermal analysis that the polymorphic transition is of an order-disorder type, which results in a marked decrease of the magnetization at this temperature.

Introduction.

Owing to their economic importance and to their potentiality as geological ther- mometer, sulphide minerals have attracted the attention of both theoretical petro- logists and economical geologists. Recently, systematic studies of synthetic sulphide systems have yielded information about the stability of minerals as well as of solid solutions between the sulphide minerals (Kullerud, 1959). Cubanite is one of the naturally occuring sulphide minerals and is often accom- panied by other sulphide minerals such as chalcopyrite, pyrrhotite, valleriite and pentlandite. Cubanite is orthorhombic and weakly ferromagnetic (Burger, 1945), but its magnetic properties are little known. The principal purpose of this study is to furnish more data on the magnetic properties of this mineral. The study of synthetic sulphide minerals is difficult because of their instability. It is hoped that this work on cubanite will furnish more data of the magnetic properties and also provide an alternative approach to understand the phase relationships of this mineral.

1. sample Preparation.

Cubanite grains separated from cubanite ores from Komori mine, Kyoto prefecture and Mihara mine, Okayama prefecture were used for the experimental studies. Cubanite s usually found with pyrrhotite, chalcopyrite and valleriite, and often shows exsolutioni

(107) 108 M. SAWAI)A M. OZIMA and Y. FUJIKI textures. Pyrrhotite is ferrimagnetic and extreme care was taken to separate cubanite from pyrrhotite, as a very small amount of contamination will mask the magnetic properties of cubanite. First, cubanite grains were crushed and sieved into 325-400 mesh. Secondly, a heavy liquid method was employed, in which the sieved sample was treated in acetylene tetrabromide to remove non-metallic minerals. Thirdly, an isodyna- mic separator was used to separate cubanite from non-magnetic minerals such as chalcopyrite, valleriite and pentlandite. Because of the large difference between the value of the saturation magnetization of cubanite and that of pyrrhotite, the isodynamic separator was also employed to separate the latter from cubanite. Cubanite thus separated was examined under the reflection microscope. The magnetic separation were repeated until more than 99% of concentration of cubanite was ensured micro- scopically. Finally X-ray analysis was made to check the purity of the sample and only peaks corresponding to cubanite were observed.

2. Experimental Results.

Saturation magnetization This was measured at room temperature with a magne- tic balance in which balancing was obtained by automatically feeded electric current passing through a pair of coaxial solenoids mounted on one of the balance arms. Relative values of the force exerted on the specimens by the magnetic field were thus measured in terms of the current which gave the intensity of the total magnetization . The detailed description of this magnetic balance was given by Hirone , Maeda and Tsuya (Hirone, Maeda and Tsuya, 1954). Calibration was obtained using a known amount of nickel. Fig. (1) shows that the magnetization is almost saturated at about 1,000 Oe. The value of the saturation magnetization at room temperature was deter-

Fig. (1) Magnetization curve for cubanite at room temperature Magnetic Properties of Cubanite (CuFe2S3) 109 mined to be 0.87emu/gram from the magnetization curve. The intensity is about one- twentieth of weakly ferrimagnetic pyrrhotite (14emu/gram). Thermo-magnetic analysis Thermo-magnetic analyses were carried out using a quartz spring magnetic balance as described by Akimoto (Akimoto, 1954). As cubanite is very susceptible to oxidation the apparatus was evacuated and the pressure kept less than 10-4mmHg throughout the Fig. (2) Thermo-mognelic curve for cubonite (H=1800Oe) experiment. In Fig. (2) a thermo- magnetic curve obtained for H= 1,800Oe is shown. The intensity of magnetization shows little change for temperature up to

250℃. The magnetization almost

disappears at about 270℃ and in cooling from this temperature to room temperature only one- tenth of the original magnetiza- tion is observed. The marked decrease of magnetization was first considered to be due to thermal decomposition such as into chalcopyrite and pyrrhotite as this appeared to be most likely from the phase relationships of Cu-Fe-S system. However, later experiments showed that the irrever- sible change at about 270℃ was due to polymorphic transition and not to thermal decomposition.(*) X-ray analysis X-ray analyses were done with a Norelco X-ray spectrometer, in which all analyses were calibrated with pure silicon powder. Lattice parameters were calculated by means of a least square method adopting the lattice indices of the A. S.T. M. index (1960). The analysis of original cubanite yielded the same orthorhombic lattice parameters as those obtained by Buerger (Buerger, 1945). The data are shown in table

(1). X-ray analyses were done on the sample which had been heated to 300℃ in vacuum in order to examine the nature of the irreversible change in the thermo- magnetic curve at about 270℃. Analyses with iron target and with Cu target using Ni filter showed that the heat-treated cubanite was cubic. Microscopic observation Polished surfaces of the original and heat-treated cubanite were examined under a reflection microscope. Anisotropism was not recognized in the heat-treated sample. No indication of the presence of chalcopyrite or pyrrhotite was found in the heat-treated sample. The microscopic observations eliminate the possibility that cubanite was decomposed into chalcopyrite and pyrrhotite.

(*) Note: During the preparation of this manuscript we received a copy of 'Annual Report, Carnegie Institution of Washington,1960-61', in which Yund and Kullerud reported

three stable polymorphs of cubanite, orthorhombic below 200℃, tetragonal (?) between 200℃ and 260℃ and cubic above 260℃. 110 M. SAwADA, M. OZIMA and Y. FuJIKI

Table 1 (a) X-ray analysis on original cubanite with Fe-target, no filter

(b) X-ray analysis on heat-treated cubanite (i) with Fe-target, no filter

(ii) with Cu-target and Ni-filter

(*1) Indices are taken from "Index to the X-ray powder Data File, 1960, A. S. T. M. " (*2) K-β peak, not used for the calculation of the lattice parameter.

Chemical analysis Chemical analysis was carried out to confirm whether the

irreversible change at about 270℃ is due to a polymorphic transition or to chemical reaction. Because of the difficulty of preparing large amount of pure cubanite, about a half gram of sample were used for the chemical analysis which was done by Mr. T. Uozumi of the Mining Department, University of Tokyo. As seen in the table (2) Cu, Fe and S atoms constitute only 80% of the sample analysed. Mr. Uozumi points out that this is due to the smallness of the sample used rather than to impurities. As- suming that Cu, Fe and S atoms constitute 100 percent of the sample, the relative abundances of these atoms were re-calculated. These values are compared in table (2) with those of orthorhombic cubanite. The latter value is the average of the analyses on cubanites from seven different localities by Dana (Dana, 1946). The similarity of the analyses supports the view that the irreversible change is due to polymorphic transition and not to chemical alteration. Differential thermal analysis In order to understand the nature of the polymor- phic transition, differential thermal analysis was undertaken. The apparatus used here Magnetic Properties of Cubanite (CuFe2S3) 111

Table 2 Results of Chemical analysis

(*1) The percentage values were re-calculated assuming that Cu, Fe and S atoms con- stitute 100 percent. (*2) The data were taken from a book "The system of mineralogy" by E. S. Dana (1946) John Wiley & Sons, N. Y. is DA-IA type of Shimazu Seisakusho Ltd. Cubanite was first sieved into 250 mesh. The sieved sample was mixed with five times its weight of alumina powder, the latter being employed as an inert material for the differential thermal analysis. The experi- ment was performed with a heating rate of 12℃/min. in vacuum.

As seen in figure(3), there is a remarkable negative peak at about 270℃ which is endothermic. It is obvious that the endothermic peak corresponds to the phase tran-

5ition, at 270℃. Other mess conspicuous peaks of both types(endothermic and exothermic) are also observed at about 520℃, 800℃ and 900℃. No attempt, however, was made in this study to exploit the further nature of those peaks.

3. Discussion.

The results of all the above experiments strongly suggest that cubanite undergoes a polymorphic transition at about 270℃. The crystal structure changes from orthor- hombic to cubic at this transition temperature. The most remarkable feature of this transition is observed in the thereto-magnetic curve by the marked decrease of magneti- zation at the transition temperature. It has been shown by Buerger (Buerger, 1945) that in orthorhombic cubanite the copper atoms and one-third of the sulphur atoms occupy the equipoint 4C, while the iron

fig. (3) Differential thermal analysis curve for Cubanite 112 M. SAWADA, M. OZIMA and Y. FUJIKI atoms and the remaining sulphur atoms are in the ground position, 8d. Buerger further suggested that the above configuration favoured the idea that the iron atoms were bonded to one another in pairs to give rise to ferromagnetism. The marked decrease of magneti- zation associated with the transition may be understood as a random re-distribution of iron and copper atoms in 4c and 8d positions, or disordering of iron and copper atoms. The argument is strengthened from the result of the differential thermal analysis, as an endothermic peak is characteristic to disordering transition, which is characterized by an increase of entropy or heat absorption. The results of the X-ray analyses also support the argument, as an orthorhombic structure has a lower degree of symmetry corresponding to an ordered state, while a cubic structure has a higher one in accord with a disordered state. It is noted that ferric and copper ions have a similar ionic radius and have the same electrical charge. Both these facts favour mutual exchange of the iron and copper atoms at relatively low temperatures.

4.Conclusion.

In order to investigate the magnetic properties of cuba :life, thermo-magnetic studies as well as X-ray analyses, microscopic observation, chemical analysis and differential thermal analysis were undertaken. From the thermo-magnetic analysis of cubanite an irreversible transition at about 270℃ was inferred. Comparative studies on X-ray analyses, chemical analysis and microscopic observation on the original and heat-treated samples confirmed that the transition was polymorphic. Differential thermal analysis strongly supports that the polymorphic transition is of an order-disorder type in which the marked decrease of magnetization is caused by the disordering of ferric and copper ions.

Acknowledgement.

The authors wish to thank professors T. Nagata and H. Imai for their continuous encouragement throughout this study. Thanks are also due to Dr. A. Kato and Miss M. Yama-ai for their help in the X-ray analyses. The authors are indebted to Professor S. Akimoto, Dr. K. Kobayashi and other colleagues in Geophysical Institute, University of Tokyo for their informative suggestions.

References

Akimoto, S. (1954) Jour. Geomag. Geoelect., Kyoto 6, 1. Buerger, M. J. (1945) Jour. Amer. Chem. Soc., 67, 2056. Dana, E. S. (1946) The System of Mineralogy, Vol, 1 pp. 245, John Wiley and Sons, Inc. New York. Hirone, T., Maeda, S. and Tsuya, T. (1954) Rev. Sci. Instr., 25, 516. G. Kullerud, (1959) Research in Geochemistry, pp. 301-335, edited by Abelson, A. H., John Wiley and Sons, Inc. New York,