Influence of Natural Organic Matter on the Magnetic Properties of Ilmenite in Placers
Abstract Generated abstract
This paper examines why ilmenite grains from a Western Siberian titanium placer show differing magnetic properties, with particular attention to clayey organo-mineral films deposited on grain surfaces. The authors compare altered ilmenite, leucoxene, and authigenic rutile by chemical composition, specific gravity, magnetic susceptibility, organic matter content, and electromagnetic separation before and after alkaline removal of surface films. The results indicate that ilmenite bearing more organic carbon enters magnetic fractions more readily, while removal of the film lowers and equalizes magnetic susceptibility. The study attributes the enhanced magnetism chiefly to iron-bearing organo-mineral coatings, possibly involving strongly magnetic iron-organic compounds formed under reducing conditions, rather than to detected maghemite or hematite.
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Proceedings
of the Tomsk Order of the Red Banner of Labor
Polytechnic Institute
named after S. M. Kirov
Volume 163 1970
Influence of Natural Organic Matter on the Magnetic Properties of Ilmenite in Placers
S. A. Babenko, V. A. Yankovsky
(Presented by the scientific seminar of the departments of CTF)
While studying the physicochemical properties of ilmenite separated from a titanium placer of Western Siberia, we encountered a clearly expressed dependence of the magnetic properties of the minerals on the presence on their surface of clay films impregnated with organic matter. It has been established that ilmenite grains containing elevated amounts of organic matter on their surface are more magnetic.
The accumulation of ilmenite in this placer, confined to the Kuskovskaya suite (Eocene), is due to prolonged reworking of the underlying sandy-clayey deposits of the Simonovskaya suite (Cr₂), as well as the weathering crust of ancient Paleozoic sedimentary rocks containing ilmenite. In turn, ilmenite entered these sedimentary rocks during the destruction of many magmatic rocks of different ages and genesis. As a result, the ilmenite accumulated in the sands of the Kuskovskaya suite in the form of an ancient placer has different source areas and different physicochemical properties owing to unequal conditions of crystallization in the magmatic stage and weathering in the hypergene stage.
The ilmenite-bearing sands of the Kuskovskaya suite in some areas contain, on the surface of the grains, organic matter that was deposited from groundwater as a result of the washing of lignites occurring higher in the section; some portion of the organic matter was deposited at the time of formation of the placer.
The ilmenites studied constitute the predominant part of the heavy fraction of the sands. Their characteristic forms are angular-rounded grains of isometric shape, less commonly well-rounded spherical grains, and still more rarely hexagonal tabular crystals. The grain size varies from finely dispersed particles to 0.15 mm. Grains of 0.07–0.10 mm predominate.
The alteration of ilmenite during weathering is expressed externally in constantly changing colors: from black through brown and yellow to gray. Black ilmenites with a brownish tint are more commonly distributed; the most altered, light-colored varieties occur in subordinate amounts. Completely unaltered ilmenites are almost absent.
By color, luster, optical properties, specific gravity, magnetic susceptibility, and chemical composition, three varieties of ilmenite alteration products may be distinguished.
The first variety is black ilmenite with a brownish tint, brownish in powder, with a strong metallic luster grading into adamantine. The fracture is granular to earthy, the hardness is reduced, the specific gravity is within the range 4.2–4.56; it is opaque or slightly translucent at the edges of the grains with a reddish-brown color, and is anisotropic.
The second variety is deeply altered ilmenite, having the appearance of the mineral usually called leucoxene.
The third variety of ilmenite alteration products is authigenic rutile; it is formed at the expense of leucoxene and is a new formation in sedimentary rocks. Authigenic rutile is closely associated with leucoxene: gradual transitions are observed from opaque leucoxene to transparent, already recrystallized rutile.
The magnetic properties of the titanium minerals studied are not the same. The different magnetic susceptibility is due not only to the unequal degree of alteration of ilmenite, but also to the content of organic matter forming discontinuous shells around the grains, the so-called organo-mineral films [4]. The film is a finely dispersed mixture of kaolinite minerals, opaloid silica, leucoxene, and iron hydroxides, cemented by organic matter; in addition, there is undoubtedly some amount of organic compounds of iron and titanium formed under conditions of hypergenesis through the action of organic substances on ilmenite and other mineral sands.
To establish the primary magnetic properties of ilmenite, we removed the organo-mineral film by stirring (rubbing) ilmenite grains in an alkaline medium. The “clean” ilmenite obtained in this way has the following characteristics (Table 1).
Table 1
Chemical composition and some physical properties of ilmenite and products of its alteration
| Ilmenite and products of its alteration | Average content, %, TiO₂ | Average content, %, FeO | Average content, %, Fe₂O₃ | Average content, %, Al₂O₃ + SiO₂ | Specific gravity | Magnetic susceptibility, cm³/g |
|---|---|---|---|---|---|---|
| Slightly altered ilmenite with organo-mineral film | 60 | 3 | 32 | 5 | 4.05 | \(310 \times 10^{-6}\) |
| Slightly altered ilmenite without film | 65 | 1 | 33 | 1 | 4.20 | \(256 \times 10^{-6}\) |
| Leucoxene | 80 | 0.5 | 14.5 | 5 | 3.40 | \(43 \times 10^{-6}\) |
| Authigenic rutile | 95 | — | 4 | 1 | 4.10 | — |
From the table presented it is evident that the transition of ilmenite into leucoxene and authigenic rutile proceeds with the loss of ferrous iron, and then also ferric iron, and with a decrease in magnetic susceptibility and specific gravity.
The initial ilmenite, unlike ilmenite deprived of the organo-mineral film, has increased magnetic susceptibility and, for slightly altered ilmenite (the 1st variety), is \(310 \times 10^{-6}\ \text{cm}^3/\text{g}\). Since ilmenite of the first variety constitutes 90% of all titanium minerals of the placer, further investigations were carried out only with it.
Chemical analysis of the organo-mineral film from ilmenite established that it consists predominantly of
SiO₂ — 30–40%, Al₂O₃ — 20–30%; ferrous iron — 10%, and minor admixtures of TiO₂, CaO, and MgO. Organic substances are present in variable amounts in the form of bitumens and humic acids.
We studied two samples of ilmenite concentrate, which contained different amounts of organic matter, as can be seen from Table 2.
Table 2
Group composition of the organic matter isolated from the surface of ilmenite
| Sample designation | \(C_{\text{org}}\), % | Bitumen c/b, % of total \(C_{\text{org}}\) | Humic acids, % of total \(C_{\text{org}}\) | Insoluble residue, % of total \(C_{\text{org}}\) |
|---|---|---|---|---|
| I | 0.12 | 41.0 | 7.7 | 51.3 |
| II | 0.73 | 8.3 | 66.0 | 25.7 |
In the group composition of the organic matter isolated from the surface of ilmenite, the presence of bitumen, humic acid, and an insoluble residue was established. The organic matter was isolated according to the procedure adopted in bituminological studies [1].
Before removal of the organo-mineral film from the ilmenite, both samples were subjected to electromagnetic separation on a single-roll laboratory separator. The separation results are given in Table 3.
It follows from Table 3 that ilmenite containing more organic carbon on its surface (sample II), during electromagnetic separation already at a current of 0.75 ampere, passes by 71.9% into the magnetic fraction, whereas ilmenite having 0.12% organic carbon passes only by 21.1%.
After removal of the organo-mineral film from both ilmenite samples, the magnetic properties became equalized, with a simultaneous decrease in their magnetic susceptibility, as can be seen from Table 4.
Table 3
Results of electromagnetic separation of ilmenites
| Designation of electromagnetic fractions | Yield, % — sample I | Yield, % — sample II |
|---|---|---|
| MF 0.5 a | 4.6 | 36.3 |
| MF 0.75 a | 16.5 | 35.6 |
| MF 2.5 a | 73.4 | 16.9 |
| MF 5.0 a | 2.3 | 1.6 |
| NMF 5.0 a | 3.2 | 9.6 |
| Total | 100.0 | 100.0 |
Table 4
Magnetic properties of ilmenite (sample I) before and after removal of the film
| Current at which the fraction was isolated, amperes | Specific magnetic susceptibility, cm³/g — original ilmenite | Specific magnetic susceptibility, cm³/g — ilmenite after removal of the film |
|---|---|---|
| 0.5 | \(615 \times 10^{-6}\) | \(464 \times 10^{-6}\) |
| 0.75 | \(202 \times 10^{-6}\) | \(142 \times 10^{-6}\) |
Thus, the experiment carried out established that the increase in the magnetic properties of ilmenite depends on the presence of an organo-mineral film containing up to 10% ferrous iron.
An increase in the magnetic properties of the outer part of grains of titanium minerals under the conditions of a reducing environment has been reported in the literature.
and earlier. Thus, E. M. Bradinskaya [2] establishes the strongly magnetic properties of the outer, most weathered and decomposed part of ilmenite grains, containing magnetic maghemite formed as a result of hypergene alteration of ilmenite in a reducing environment. V. M. Yakshin [3] points to an increase in the magnetic susceptibility of rutile in the presence of organic matter under floodplain conditions.
In all cases, the increase in magnetic properties is associated with the transition of iron to a lower valence state and with the formation of the magnetic authigenic mineral maghemite, which is unstable and rapidly passes into hematite. Both minerals possess greater magnetism than ilmenite, especially leucoxenized ilmenite.
A careful study of ilmenite and of the organo-mineral film by microscopy and X-ray structural analysis failed to establish the presence of either maghemite or hematite. The increased magnetic susceptibility of the organo-mineral film is evidently due to the formation, under reducing conditions, of certain strongly magnetic compounds of iron and organic matter.
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