F. N. SHAKHOV and V. S. POPOV.

MAGNESIOFERRITE DEPOSIT IN THE LOWER TUNGUSKA AREA

1. Introduction.

In the summer of 1932, a party led by engineer V. S. Popov, working on assignment from the Komseveroput, surveyed in the Lower Tunguska area iron-ore deposits that were partly known earlier¹), as well as newly discovered by him (the Ayan deposits), which in their geological setting and composition very closely resemble the known deposits of the Angara–Ilim area, on whose ores the Nikolaevsk plant operated²). Optical examination established that, in diagnostic features, the magnetite of these deposits differs considerably from normal magnetites occurring in the ores of ordinary contact deposits. Therefore, on the one hand, we organized chemical analyses of these ores, and, on the other, carried out studies of the magnetic ores of the Igirma deposit, which had been brought to the ore-deposit laboratory by the student G. F. Kulakov, who in 1931 was doing practical work in the Angara–Ilim area. In all cases the analyses revealed, in the composition of the magnetic ironstones, a considerable amount of magnesia, which compelled us to assign the magnetite of these deposits to the magnesioferrite group, provisionally establishing the diagnostic properties of this still rare mineral.

The present article was compiled on the basis of materials of V. S. Popov by F. N. Shakhov; the latter also carried out the studies of magnesioferrite in reflected light. Chemical analyses of the mineral were made in the laboratory of the Siberian Chemical Institute by A. P. Kalishev and in the laboratory of the West Siberian Geological Prospecting Trust by A. I. Kutolin. The specific gravity of the minerals was determined under the direction of V. S. Popov by the foreman of the Ilimpei party of the Komseveroput, F. I. Kuznetsov.

2. Magnesioferrite deposits.

The veins of magnesioferrite from which the samples for study were taken are located at the following points. On the right bank of the Lower Tunguska River, 150 km below the village of Erbogachen, is located the Khoma-

¹) V. S. Sobolev. On the question of the distribution of magnetite deposits on the Siberian platform.—Izv. VGRO, 1931, p. 1551.

²) S. A. Doktorovich-Grebnitsky. Sketch of the iron deposits of the Nikolaevsk plant, Irkutsk Province.—Tr. GGRU, issue 33, 1931.
S. S. Smirnov. Iron-ore deposits of the East Siberian Territory.—Irkutsk, 1932, 13–31.

Kara deposit. Outcrops of the Davakit vein have been found on the left bank of the N. Tunguska River, 60 km above the mouth of the Ilimpeya River, while the Sverkaushchii Yar and Ayanskoe deposits are situated on the Ilimpeya River: the former approximately 240 km, and the latter 300 km, from its mouth.

In the character of their ores and in the structure of the ore bodies, the deposits are very similar to one another. Especially well exposed is the “vein” of the Davakit deposit in the high bank of the N. Tunguska, which makes it possible to characterize most fully the composition and structure of the ore body.

From the attached schematic section it is easy to see that the vein of the Davakit deposit is exposed by the river to a depth of 80 m, with an average thickness of about 8 m. This makes it possible, if one takes into account the dimensions of the deposits of the Angara–Ilim region, to assign it to the large deposits of this type¹. The enclosing rocks are gently dipping tuffites of Permian age, whose composition includes a significant quantity of trap fragments. The vein has a well-expressed banded texture. The peripheral portions consist of massive fine-, more rarely fine-grained and, in this case, fibrous aggregates of magnesioferrite. The central portions are composed of coarse—sometimes coarse-grained—aggregates of magnesioferrite of columnar texture; moreover, the peculiar elongated columnar individuals of the mineral are arranged perpendicular to the walls of the vein. The latest formation should be considered the red veinlets of iron-bearing carbonates, which cut the fine-grained varieties of ore and tend to be located at the walls and along the boundary between bands composed of coarse-columnar and fine-grained varieties of the ores described. In polished sections of the carbonate ores, only very small quantities of fine grains of magnesioferrite are noticeable; hematite is absent. The red coloration of the ore is apparently connected with the oxidation of iron-bearing carbonates.

3. Chemical composition of the ores studied.

Magnesioferrite is the principal ore-forming mineral in these deposits. Especially pure in composition are the ore varieties that have a coarse-grained columnar texture. Examination in reflected light of the material submitted for chemical analysis did not reveal significant amounts of extraneous mineral impurities in the ore, including hematite. The data of chemical analyses, carried out by the laboratory of the Siberian Chemical-Technological Institute under the direction of A. P. Kalishev, are presented in the table.

On the basis of the material presented in the table, the following idea may be formed concerning the chemical composition of the minerals:

a) MgO is contained in the ore in too large an amount for it to be associated with foreign mineral impurities in the ore mass;

b) the amount of FeO in the ore is too small for constructing the formula of magnetite¹, and only in sum with MgO can the figure characterizing the amount of these oxides replace FeO in this formula;

c) the amount of MgO varies in different samples, and it must be assumed that this variation cannot be ascribed to inaccuracy of the analysis, since with an increase in MgO the specific gravity of the mineral decreases, approaching that of magnesioferrite².

On the basis of the chemical analyses, we necessarily arrive at the conclusion that, in our ores, MgO partially substitutes for FeO in the magnetite molecule, and in the present case the magnitude of this substitution does not exceed 50% of the total amount of FeO in the magnetite molecule.

Thus, in chemical composition the mineral studied is magnetite in which part of the FeO is replaced by MgO. Of the spinel group, the only suitable mineral is magnesioferrite, encountered in small quantities in the fumarolic products of the 1855 eruption of Vesuvius. The essential distinguishing features of our mineral are its occurrence in large masses and the incomplete replacement of ferrous oxide by MgO. The latter circumstance would seem to justify distinguishing this mineral separately within the spinel group; however, taking into account the variation in the amount of MgO in the composition of this mineral and the usual presence of MgO in the composition of magnetites³, we are inclined to the opinion that magnetite and magnesioferrite are only end members of the isomorphous series magnetite–magnesioferrite, and that the mineral studied is one of the members of this series.

4. Principal diagnostic features of magnesioferrite from the Lower Tunguska deposits

The chemical composition and specific gravity of the minerals have already been noted above. Examination of the material available in the collection makes it possible to establish that, within these deposits, the most widespread crystallographic form in this mineral is

is the octahedron in combination with the rhombic dodecahedron, the faces of the latter in some cases predominating. The color of the mineral is steel-gray, with a stronger metallic luster than that of magnetite. The streak is blackish gray, lighter than that of magnetite, and in some varieties faint brownish shades may be detected in its color. It is strongly magnetic.

On examination in reflected light, diagnostic features were found which make it possible to distinguish this mineral readily from magnetite. The polishing hardness is unusually high. It polishes only with great difficulty, and only after preliminary treatment of the section on leather and on coarse and fine cloth can a tolerably polished surface be obtained by prolonged polishing (up to 0.5 hour) on felt with alumina.

The reflectivity is similar to that of magnetite and franklinite. The color is gray. In the fine depressions, which always remain on the polished surface, careful examination with high-magnification immersion objectives reveals an internal reflection of reddish color, resembling in character that of franklinite, but still weaker and with brownish-yellow shades. It is isotropic. In crossed nicols, almost complete darkness is observed, with very weak lightening when strong illumination is used.

50% HNO₃, 50% HCl, KCN, FeCl₃, KOH, and HgCl₂ have no effect whatever on the mineral over two minutes. The structure is not revealed by etching with HF and with a solution of SnCl₂ in HCl. It should be noted that, over 5 minutes, etching of the surface of the section with these reagents revealed only cracks in the mineral and scratches from polishing. It was not possible to obtain outlines of grains, the internal structure of individual individuals, or in general traces of any chemical action, apart from slight dissolution. Only in some sections, from the action of the solution of SnCl₂ in HCl, was slight browning noticeable here and there around pits and cracks.

5. Study of magnetite from the Igirma deposit (Angara-Ilim region).

The ores of the deposits of the Nikolaevsk plant were studied by E. E. Gutkina¹), who noted the special resistance to structural etching of the magnetite forming part of these ores. In addition, S. S. Smirnov, in analyzing chemical analyses of the ores, according to data from the laboratory of the Nikolaevsk plant, noted a discrepancy between the contents of ferric and ferrous iron in relation to the observed quantities of magnetite and hematite in the ores²). Finally, in three

¹) Doktorovich-Grebnitskii, cited above, p. 76.

²) S. S. Smirnov, cited above. On p. 23 we find the following note: “The ratio of ferric to ferrous iron has probably been determined incorrectly. According to the analyses, the ore consisted by half of magnetite and hematite, whereas there is in fact very little of the latter.”

The present article was written at the beginning of 1933, and its publication was delayed for a number of reasons. At the end of 1933 S. S. Smirnov published a paper (Probl. Sov. Geol., 1933, 10, pp. 97–121), in which he notes the presence of magnesium in the magnetites from the ores of the Angara-Ilim deposits.

of the five analyses cited by him, the amount of MgO in the ores exceeds 6%. All these data led us to pay attention to these ores, the genesis of which, incidentally, is identical to that of the deposits of the Lower Tunguska.

From the collections available in the ore-deposit cabinet of the Tomsk Industrial Institute, a specimen of massive ore from the Igirma deposit was submitted for analysis; as studies in reflected light showed, it consists of magnetite of radial-fibrous texture with an admixture, in an extremely insignificant amount, of fine needles (0.002 mm) of hematite. A chemical analysis, performed in the laboratory of the West Siberian Geological Prospecting Trust by A. I. Kutolin, showed the presence in the ore of 5.3% MgO. The specific gravity of the ore is 4.87. The external appearance and streak are identical with normal magnetites.

The magnetic iron ores of these deposits polish much more easily than the magnesioferrites of the Lower Tunguska and, like normal magnetites, make it possible without particular difficulty to obtain an evenly polished surface. The reflectivity and color are similar to normal magnetite. In comparison with magnesioferrite, it sometimes appears more brownish. HNO₃ (50%), HCl (50%), KCN, FeCl₃, KOH, HgCl₂ do not act within one minute. Etching with a solution of SnCl₂ in concentrated HCl for one minute reveals a comparatively weak structure of the mineral aggregates. When etched with HF (one minute), the grain boundaries and, in places, the zonal structure of individual individuals stand out quite clearly. In comparison with ordinary magnetites, the latter reaction proceeds slowly. Thus, in magnetites from the Temir-Tau and Abakan deposits, etching with HF reveals a clear structure after 20–30 seconds. Thus, in this case as well, the data of chemical analysis are combined with the properties of the mineral (specific gravity and diagnostic etching), which is why we are inclined to regard these magnetites too as MgO-containing spinels of the isomorphous series magnetite—magnesioferrite.

6. Conclusion.

1. In the Lower Tunguska region, the known and newly discovered iron-ore deposits are identical in genesis with those of the Angara-Ilim region and belong to the group of contact deposits genetically connected with the injection of traps.

2. The magnetic ores of the deposits consist essentially of a mineral of the magnesioferrite group; in the ores studied, this mineral should be regarded as magnetite in which FeO is almost 50% replaced by MgO. According to chemical analysis, the amount of MgO in the mineral ranges from 11% to 13%.

3. In external appearance the mineral is very similar to magnetite, but differs from the latter by the following characteristic features: a) the specific gravity varies from 4.56 to 4.64; b) a lighter steel-gray streak; c) higher polishing hardness; d) diagnostically and structurally it is not etched by ordinary reagents; for 5 minutes HF and a solution of SnCl₂ in concentrated HCl have absolutely no effect on it.

1. It may be assumed that the presence of MgO in magnetite is, in general, characteristic of all deposits of this type. A study of ores from the Igirma deposit of the Angara-Ilim region revealed the presence of MgO in magnetite in the amount of 5.3%. In its properties the mineral approaches magnetite: a) specific gravity—4.87; b) streak and hardness, as in magnetite; c) it etches after a comparatively long exposure (one minute) of an isolated surface to HF and a solution of SnCl₂ in concentrated HCl.

2. Variations of MgO in the composition of the magnetites considered, in combination with changes in the properties of these minerals, compel one to assume the existence of a continuous isomorphous series magnetite—magnesioferrite, of which the minerals we have studied are individual members.

3. The discovery of magnesioferrite is of considerable interest from the standpoint of descriptive and genetic mineralogy, since this mineral had previously been found only in small quantities, as a product of fumarole activity during the eruption of Vesuvius. But since, in the present case, accumulations of this mineral have been found in quantities deserving industrial interest, the detection of MgO in the composition of the magnetite molecule is also of serious significance from the standpoint of iron-ore technology. On the basis of the investigations carried out, it is already possible to speak of the chemical activity of the mineral studied as differing from that of normal magnetites. It is difficult to think that, in connection with the entry of MgO into the composition of magnetite, there would not also have been changes in such of its properties as the temperatures of melting and crystallization. Perhaps investigation of the mineral from this point of view will not only prove useful for the correct calculation of the technological process, but will also link the specificity of the composition of these ores with their specific texture, whose oolite-like character remains an unresolved riddle to this day.

4. Finally, it must be noted that until now very little attention has been paid to the chemical composition of magnetites from contact deposits in general; meanwhile, as the material presented above shows, the composition of these minerals is apparently much more complex than it seems at first glance. In addition to MgO, MnO, TiO₂, Al₂O₃, Cr₂O₃ may enter into the composition of the magnetite molecule, which in many cases can evidently be connected with the specificity of the composition of the parent magma. Moreover, in our opinion, analyses of magnetites, especially from deposits important for industry, can provide a series of valuable data of interest not only for solving genetic questions, but also for metallurgical calculations.

SUMMARY.

1. The explored as well as the discovered iron-ore deposits in the district of the riv. L. Tungooska are identical by their genesis to those of the Angaro-Ilimsky Region and belong to the group of contact ones being genetically dependent on the intrusion of traps.

2. The magnetic ores of these deposits consist essentially of a mineral belonging to the group of magnesium-ferrite. In the studied ores this mineral is to be considered as magnetite, in which FeO is partly replaced by MgO. The amount of MgO in the mineral according to the data of a chemical analysis ranges from 11 to 13 p. c.

3. By its outward appearance this mineral is very similar to magnetite, but it differs from the last by the following characteristic features: a) the specific gravity varies from 4.56 to 4.64; b) it has a lighter steel-gray streak; c) it is harder in polishing; d) it cannot be etched diagnostically or with regard its structure by means of common reagents: HF and the solution of SnCl₂ in the concent. HCl have no affect on it during 5 minutes.

4. It is possible to suggest that the presence of MgO in the magnetite is characteristic for all the deposits of this type. The study of ores from the Iggirminsky deposit of the Angaro-Ilimsky district discovered the presence of MgO in the magnetite amounting to 5.3 p.c. The mineral approaches to magnetite by its properties: a) its specific gravity is 4.87; b) its streak and hardness are identical to those of magnetite; c) it is ecthed after a long (one minute) action of HF and of SnCl₂ in the concent. HCl on its polished surface.

5. The variations of the content of MgO in the composition of the examined magnetites together with the alteration of properties of the given minerals make us suggest the existence of an unbroken isomorphous range magnetite—magnesium-ferrite, the minerals studied being separate members of this row.

6. The finding of magnesium-ferrite presents a certain interest from the point of view of the descriptive and genetic mineralogy, for the mentioned mineral was formerly discovered only in small amounts as a product of the activity of fumaroles at the eruption of Vesuvium. But, since in this case the accumulations of this mineral were found in quantities worth of economic interest, the discovery of MgO in the composition of the molecules of magnetite is of a considerable importance even from the point of view of the technology of iron-ores only.

7. It is to be mentioned, at last, that in general very little attention has been as yet paid upon the chemical composition of magnetite of contact deposits, whilst the above given material of study proves that the composition of these minerals is apparently much more complicated than it seems to be from the first sight. Other oxides too as MnO, TiO₂, Al₂O₃ and Cr₂O₃ may enter into the composition of a molecule of magnetite, besides MgO.

Translated by B. Gootovskaya.

1. Especially if one takes into account the absence of hematite in the ore. ↩↩

2. E. Dana. The System of Mineralogy.—New York, 1900, p. 226. Specific gravity—4.568—4.654. ↩

3. Rosenbusch-Mügge. Mikroskopische Physiographie, I Bd., 2 Hälfte. Stuttgart, 1927, S. 16. ↩