Dependence of the Thermo-EMF of Pyrites on the Heating Temperature of the Electrode

A. Ya. Pshenichkin, A. F. Korobeinikov, A. F. Komoedov

(Presented by Professor A. M. Kuzmin)

The nature of the conductivity of pyrites sometimes changes substantially depending on the magnitude of the temperature difference between the electrodes [1, 3, 4]. To determine the dependence of the thermo-EMF on the heating temperature of the hot electrode, we studied pyrite crystals from a number of gold-ore deposits (Balachino, Berezovskoe, Darasun) and polymetallic deposits (Yuliya, Sora), as well as pyrites from metamorphosed shales and low-temperature carbonate veinlets (Kulyumbe, northeastern part of the Siberian Platform). The thermo-EMF was measured separately for the faces of the cube (100), octahedron (111), and pentagonal dodecahedron (210) at intervals of 50°, beginning from 50° up to 350—400°C. Above this temperature, measurements were not carried out because of oxidation of the pyrite and its transformation into hematite.

As a result of the investigations carried out, the following was established.

1. With an increase in the heating temperature of the hot electrode, the thermo-EMF of pyrites increases.

2. The character of the change in the thermo-EMF curves as a function of the temperature gradient \((\Delta T)\) for pyrites from deposits of different genesis is not the same, but is constant for each deposit (Fig. 1, A, curves 3, 4, 6, 7), which may be a typomorphic feature for pyrites of different genesis.

3. For different faces of one and the same pyrite crystal, the curves of change in thermo-EMF as a function of \(\Delta T\) are not the same; this is especially characteristic of pyrites from the metamorphosed shales of Kulyumbe (Fig. 1, curves 1, 2), whose crystals occur in a combination of cube and octahedron. The thermo-EMF for these faces differs sharply (especially in the temperature interval 200—300°C) and is greater on the octahedron face (111). For pyrites of the Darasun deposit, which occur in the combination (100), (210), (111), the thermo-EMF values for different faces are also different (Fig. 1, B, curves 9, 10, 11), but close to one another. All this indicates anisotropy of the thermo-EMF for different crystallographic directions of pyrite crystals.

4. The angle of inclination of the thermo-EMF curves as a function of \(\Delta T\) apparently depends on the crystallization temperature of the pyrites, and it is the greater, the higher the temperature of their crystallization. Thus, the crystallization temperature of pyrites from the Darasun, Berezovskoe, Balachino, and Sora deposits, determined by the decrepitation method, ranges within 200—360°C, and the curves have a larger angle of inclination than the curves for pyrites from metamorphosed shales and carbonate veinlets (Kulyumbe), whose decrepitation temperature is 100—120°C.

Fig. 1. Dependence of the thermoelectric power of pyrites on the magnitude of the temperature gradient \((\Delta T)\).
1, 2—pyrites from metamorphosed shales of Kulyumbe; deposits: Balakhchino (3—Oktyabrskaya vein, 4—Maiskaya vein), 5—Yuliya, Berezovskoe (6—from beresites, 7—from quartz veins), 8—Sora, 9, 10, 11—Darasun, 12—pyrites from carbonate veinlets of Kulyumbe.

1. The fact that the curves of the dependence \(E=f(T^\circ)\) are close for pyrites of ore veins from one deposit (Balakhchino) and close for pyrites from quartz-ore veins and the altered wall rocks enclosing them (Berezovskoe), as well as for pyrites from different deposits (Berezovskoe—Sora, Kulyumbe—Yuliya), apparently indicates similar (close) conditions of their formation.

When analyzing the curves of the dependence \(E=f(T^\circ)\) for pyrites from the deposits studied, two types of change in thermoelectric power with \(\Delta T\) can be distinguished:

a) a direct dependence (the Berezovskoe, Sora, Balakhchino, Yuliya deposits, and pyrites from carbonate veinlets of Kulyumbe), when the ther-

the thermoelectric power increases almost linearly with increasing temperature of the hot electrode (Fig. 1, A).

b) a more complex dependence (Darasun, pyrites from the metamorphosed shales of Kulombe, Fig. 1, B).

The magnitude of the thermoelectric power of semiconductors (and, in particular, of pyrite) is influenced by the concentration in them of charge carriers—electrons and holes—and by their ratio. This, in turn, depends on the quantity and character of impurity elements isomorphically incorporated into the mineral lattice [5]. The mobility of electrons supplied by different impurities and associated with an excess or deficiency of charges in the pyrite lattice itself is different; their energy of emergence into the conduction band is also different [4]. These differences are recorded as the temperature of the hot electrode is increased. Thus, in the case of a linear dependence of the thermoelectric power on $\Delta T$ in low temperature intervals, conduction is effected by those impurity elements whose ionization energy is insignificant. Gradually, as the temperature of the hot electrode is raised, elements with higher ionization energies begin to take part in conduction, and subsequently the electrons supplied by the lattice of the mineral itself enter into conduction. But there may also be a case in which groups of elements with sharply differing ionization energies enter isomorphically into the crystal lattice of pyrite, and in which some impurity elements have already given up their electrons to the conduction band, while for the ionization of other impurity elements and the ionization of the crystal lattice itself there is still insufficient energy; therefore the magnitude of the thermoelectric power may remain constant for some time (the appearance of horizontal plateaus), or it may decrease as the current carriers are exhausted. Then, when the temperature becomes sufficient to ionize impurity elements with a higher ionization energy, the curve will again conform to a linear dependence (Fig. 1, B).

Thus, the character of the curves of the dependence of the thermoelectric power of pyrites of different genesis on $\Delta T$ can provide valuable genetic information about the conditions of formation of the mineral.

REFERENCES

1. G. V. Voitkevich, V. G. Prokhorov, I. A. Khairetdinov. On the question of the nature of the thermoelectric effect in minerals. DAN SSSR, vol. 162, no. 1, 1965.

2. G. I. Gorbatov. Thermoelectric properties of pyrite and galena and their possible relation to the temperature of mineral formation. In: Methods for the Study of Mineral Raw Materials. Moscow, 1957.

3. G. I. Knyazev, V. K. Kudelya. Semiconductor properties of galenas and pyrites as a criterion of ore-formation conditions. Kiev, 1969.

4. V. G. Prokhorov. Pyrite. Krasnoyarsk, 1970.

5. M. O. Sominskii. Semiconductors. Moscow, Fizmatgiz, 1961.