ON THE GENESIS OF ROUNDED TITANOMAGNETITE FORMS FROM THE GRANITES OF THE MOKHNATUKHINSKY MASSIF

(RUDNY ALTAI)

T. I. POLUEKTOVA

(Presented by Prof. A. M. Kuzmin)

In the literature, descriptions of the morphology of crystals of accessory magnetite in intrusive formations often note sculptured faces of its crystals, as well as rounded forms [2, 3]. However, information on the nature of magnetites of such appearance is still insufficient. V. V. Lyakhovich [3], for example, explains the smoothing of edges and the sculpture of faces by the process of dissolution of the crystal; B. K. Lvov [2] associates the sculpture of faces with the mechanism of crystal growth.

In order to clarify the genesis of rounded segregations of titanomagnetite, the author studied the material composition of the granites of the Mokhnatukhinsky massif, as well as the features of the morphology and distribution of titanomagnetite within it.

The Mokhnatukhinsky granite intrusion is located in the northwestern part of Rudny Altai, on the right bank of the Uba River near its confluence with the Irtysh River. The intrusive body is a large asymmetric laccolith and occurs in intensely dislocated sandy-shaly rocks of the Takyr Formation \((D_3—C_1)\), which in the exocontact zone underwent strong thermal metamorphism and were transformed into micaceous hornfelses. The age of the pluton is Permian.

The granites are a coarse-grained porphyritic rock. The porphyritic phenocrysts contain microcline with its characteristic lattice structure and plagioclase of oligoclase-andesine composition. Plagioclase phenocrysts in the marginal zone of the intrusion are intensely sericitized. The groundmass of the granites consists of plagioclase (20%), alkali-sodium feldspars (30%), quartz (44%), biotite (5%), accessory minerals (0.2%), and secondary minerals (0.7%). Plagioclase of oligoclase composition occurs as short-prismatic, tabular individuals 2–5 mm in size. Polysynthetic twins are common. Alkali-sodium feldspars occur as lattice and weakly lattice modifications of microcline. Mineral grains are allotriomorphic in form, 1.5–5.5 mm in size. Quartz occurs as rounded, xenomorphic grains 0.2 mm–1 cm in size. It has weakly wavy extinction. In contact with the enclosing rocks, two distinctly expressed generations of quartz appear in the rock. Biotite is present as platy grains of brown color. Pleochroism is strong: from straw-yellow along \(N_p\) to black-brown tones along \(N_g\). The total ferruginosity of the mineral is 65%.

For the study of accessory minerals, in particular titanomagnetite, samples were taken from the granites along specified geochemical profiles at intervals of 500 m; in the exo- and endocontact parts of the massif—at intervals of 100 m. The rock was crushed by hand, followed by threefold

Fig. 1. Forms of titanomagnetite crystals:
a—octahedral crystal; b—octahedral crystal with a weakly expressed sculptured surface of the faces; v—fragment of an octahedral crystal with triangular overgrowths on the faces; g—rounded-octahedral grain; d—spherical grain of titanomagnetite.

washing until a gray concentrate was obtained. Further processing of the samples was carried out according to the generally accepted scheme.

Titanomagnetite is distributed extremely unevenly within the massif and was not encountered in all crushings. The mineral described is most widely distributed in the granites of the endo- and exocontact ...

parts of the intrusion, where its content reaches, respectively, 1172 g/t and 1870 g/t.

Titanomagnetite is present in three morphological forms: octahedral, octahedral with a sculptured surface of the faces, rounded-octahedral, and spherical.

The octahedral crystals (Fig. 1a) are steel-gray in color; the {111} faces are rough. The size of the crystals is 0.22 mm. In the sample from the contact with the enclosing rocks, moreover, crystals of titanomagnetite with even, shiny faces and rectilinear edges were encountered. The size of these crystals is 0.14 mm; they probably belong to a second, later generation.

Crystals with a sculptured surface of the faces (Fig. 1b) have been noted both in the granites of the central part of the massif and in its endocontact. It is interesting to note that at different points of the massif the sculpture of the titanomagnetite faces is not the same: in some it is weakly expressed; in others, the number of triangular layers reaches 6 (Fig. 1c). The color of the sculptured crystals ranges from steel-gray to iron-black. The sizes of individuals of the form described are 0.16–0.18 mm.

Rounded-octahedral and spherical forms of titanomagnetite (Fig. 1d, e) were encountered in two samples of granites from the central part of the intrusion; the main mass of rounded grains of the mineral described is present in the rocks of the exocontact aureole of the intrusion—micaceous hornfelses. Statistical analysis of the frequency of occurrence of spherical segregations of titanomagnetite in the enclosing rocks shows that the number of spherical grains of the mineral increases with approach to the contact with the granite pluton (Fig. 2). The surface

Fig. 2. Variation diagram of the distribution of spherical individuals of titanomagnetite in micaceous hornfelses.

Vertical axis: Frequency of occurrence of spherical titanomagnetite crystals, %
Horizontal axis: Distance from the contact with the granites, m

the grains is rough and pitted. At high magnification it is not difficult to see that the depressions on the surface have triangular outlines. The color of the grains is iron-black. Their size is 0.15–0.17 mm.

Spectral semiquantitative analysis*) performed separately for spherical and octahedral crystals of titanomagnetite established a high content in them ($>1\%$) of iron and titanium.

Conclusions

Summarizing the material presented, it appears possible to conclude that the sculptural surfaces of the faces, as well as spherical grains of titanomagnetite, formed as a result of superimposed metasomatic processes, the presence of which is indicated by:

1) a change in the material composition of the granites in the endocontact zone of the intrusion;

2) an increase in the amount of titanomagnetite, as well as the appearance of two of its generations in contact with the enclosing rocks.

The process of dissolution of titanomagnetite crystals is conceived as follows: initially the crystal had an octahedral habit without noticeable surface sculpture; at the first stages of action of the dissolving agent, sculptural surfaces appear on the $\{111\}$ faces, the edges become smoothed, and the vertices—blunted; in the subsequent stages of dissolution, rounded-octahedral and rounded individuals are formed, the number of which increases as the intensity of the dissolution process increases. In the course of dissolution, the size of titanomagnetite crystals decreases.

A number of authors [1, 4, 5], using the example of diamonds crystallizing, like magnetite, in the cubic modification, have shown quite convincingly that the octahedral form is unstable during dissolution processes and, through blunting of the edges and vertices of the polyhedron, gradually approaches rounded forms.

LITERATURE

1. A. A. Kukharenko, V. M. Titov, New data on the dissolution of diamond crystals. Scientific Notes of Leningrad State University, No. 215, 1957.

2. B. K. Lvov. Petrology, Mineralogy, and Geochemistry of the Granitoids of the Kochkar District (Southern Urals). Leningrad State University Press, 1965.

3. V. V. Lyakhovich. Some data on the composition of accessory magnetite. Proceedings of the Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, issue 3, 1959.

4. I. I. Shafranovskii. On the genesis of rounded diamonds. In: Crystallography of Rounded Diamonds. Leningrad State University Press, 1948.

*) The spectral semiquantitative analysis was performed in the laboratory of TPI by L. V. Simakhina.