SOME FEATURES OF ALMANDINE FROM THE TAKYR FORMATION OF THE IRTYSH SHEAR ZONE

(northwestern part of Rudny Altai)

A. M. KUZMIN, T. I. POLUEKTOVA

The almandine studied was found in intensely metamorphosed biotite schists of the Takyr Formation. Its crystals form abundant disseminations in the form of lenticular nests elongated along the schistosity of the rock. Their habit is usually rhombododecahedral {110} (Fig. 1, a). Along with regular isometric forms, individuals elongated along one of the axes of symmetry of the third order (Fig. 1, b) or fourth order (Fig. 1, c) are encountered. In the first case the crystals represent a form with developed faces of the hexagonal prism {1120} and rhombohedron {1011}; in the second, the polyhedra acquire a pseudotetragonal habit and are characterized by a combination of the square prism {100} and square bipyramid {111}. The size of the crystals is 1–1.5 cm; rare ones reach 2–2.5 cm.

Fig. 1. Almandine crystals: a — isometric crystal; b — elongated along L₃; c — elongated along L₄.

The surfaces of the crystal faces bear a distinctive sculptural pattern (Fig. 2, a, b, c, d). Such sculpture of the faces is evidently due in origin to the uneven deposition on its faces of microcrystallites in the form of platy formations with a thickness of 0.01—

Fig. 2. Sculpture of the face (a–d) and edge (e).

0.001 mm. In their habit the plates approach a rhombododecahedron compressed in the \([110]\) direction. The sculpture of the faces often has the character of steps descending from the periphery of the face toward its center (Fig. 2, c). In addition, the central part of the crystal face often forms a projection with more or less rectangular outlines (Fig. 2, d).

The edges of the garnet crystals are sharp or are blunted by narrow faces \(\{211\}\), which, as a rule, bear fine stepped striation corresponding to the edges \((110):(101)\) (Fig. 2, e).

Among the aggregate formations, the mineral described is characterized by intergrowths of 3–5 grains. Intergrowth takes place parallel to an edge of the rhombododecahedron along a complex surface perpendicular to an axis of the third or fourth order. Study of the intergrowth surface shows that crystal growth occurs by sectorial blocks (Fig. 3, a, b, c). The intergrowth surfaces are uneven and represent, as it were, an alternation of projections and depressions of different size, which in a simple image diverge from the center to the periphery (Fig. 3, d, e). On the intergrowth surface, growth striation parallel to the crystal faces is quite clearly visible; it indicates zonally discontinuous growth of the crystals (Fig. 3, a, b). The unit-cell parameters of almandine are \(a_0 = 11.531 \pm 0.007\) Å.

The garnet crystals contain a large number of very fine (0.5–0.002 mm) poikilitic inclusions of quartz grains, sericite flakes, biotite, and pyrite. The poikilitic inclusions are distributed, as a rule, in a streaky manner and in their relative position repeat the platy-schistose structure of the garnet-bearing schists. A similar distribution of mechanical inclusions in garnets has been described in the works of many investigators [1, 2, 3, 4, 5, 6].

Fig. 3. a, b — examples of sectorial and zonal growth of crystals; c — sectorial-block intergrowth of garnet crystals; d and e — sectorially complex intergrowth surface of two garnet individuals.

Of the physical properties of almandine, its color is noteworthy. In crystals it is black, black-brown; on edges and near cracks it is red-brown; in thin fragments it is reddish, pink. In transmitted light almandine is pale pink. It is optically isotropic; zoning is not noticeable. The specific gravity of the mineral is somewhat lower than reference data and is \(4.22 \pm 0.004\). Hardness is \(990.3 \pm 10\) kg/cm², in areas

crystals with a thin and complex platy structure (Fig. 2) decreases to 941–927, and in individual cases to 800 kg/cm².

The chemical composition of the garnets* is as follows, wt. %: SiO₂ 36.68; TiO₂ 1.65; Al₂O₃ 20.06; Fe₂O₃ 0.60; FeO 37.80; CaO 0.67; MgO 0.51; MnO 1.78; Na₂O 0.5; K₂O 0.1; loss on ignition 0.3. According to recalculation of the first eight oxides, the chemical composition of the garnet in its principal mass corresponds to almandine containing an isomorphous admixture of elements that correspond to other garnets: almandine—91%, spessartine—4%, pyrope—3%, andradite—2%.

With regard to the genesis of the almandine, as a result of the study it appears possible to draw the following conclusion. The Takyr Formation, in the schists of which the mineral studied occurs, is the host sequence for intrusive rocks of the Kalba type and small intrusive bodies of basic composition. The rocks of the formation are subjected to contact metamorphism and to injection into the schist mass of a network of aplite–pegmatite veinlets. Thus, rocks that had initially undergone regional metamorphism, as a result of the intrusion of basic and acid magmas, received additional portions of emanations. The latter favored the appearance of individual almandine crystals. Their crystallization took place under quiet conditions, as metacrysts with the capture of minerals of the rocks of the formation and with preservation of the structure of the schists.

REFERENCES

1. G. M. Drugova. Regional metamorphism of Archean formations. Proceedings of the Laboratory of Precambrian Geology, issue 8, 1959.

2. N. A. Eliseev. Metamorphism, Nedra, Moscow, 1964.

3. N. A. Eliseev. Metamorphism, Nedra, Moscow, 1967.

4. N. G. Sudovikov. Tectonics, metamorphism, migmatization, and granitization of rocks of the Ladoga Formation. Proceedings of the Laboratory of Precambrian Geology, issue 4, Academy of Sciences of the USSR, 1954.

5. F. Turner, J. Verhoogen. Petrology of Igneous and Metamorphic Rocks, Foreign Literature Publishing House, 1961.

6. G. W. Tyrrell. Principles of Petrology, Gosgorgeolnefteizdat, Moscow, 1935.

* The chemical analysis was performed by the chemical laboratory of the Novokuznetsk Geological Administration.