STUDY OF THE CRYSTALLIZATION PRODUCTS OF TALC-COMPOSITION GLASS

P. G. Usov, V. I. Vereshchagin, N. V. Sobora

(Presented by the scientific seminar of the Department of Silicate Technology)

Glass of talc composition was obtained by melting Onot talc of the following chemical composition, in %: SiO₂ = 62.0; MgO = 32.22; Al₂O₃ = 0.20; Fe₂O₃ = 0.28; CaO = 0.30; loss on ignition = 5.00.

The melt of talc composition has a high crystallization capacity, and obtaining glass from it is very difficult. A melt obtained without holding at high temperature (about 1600°) crystallizes completely upon rapid cooling of the crucible in air. It crystallizes partially even when poured into water (Fig. 1), curve \(DT\) (b). Glass is obtained if the composition in the molten state has been held for a certain time at high temperature. Curve \(DT\) (a) (Fig. 1) characterizes the crystallization of glass of talc composition, melted with a 30-minute hold and cooled by immersing the crucible in water. Such glass visually appears homogeneous, but when examined under a microscope, although weakly, elements of anisotropy and nonconstancy of the refractive index appear in it; the index varies within the limits 1.561—1.570.

Fig. 1. Thermograms of glasses of talc composition: \(a\) — glass with a 30-minute hold at a temperature of 1600°; \(b\) — without holding at a temperature of 1600°.

Thermal analysis of powders from glasses of talc composition establishes two thermal effects with maxima at temperatures of 320 and 900°. The first effect is due to the crystallization of cristobalite from the glass; the second, at temperatures of 850—900°, to the crystallization of magnesium metasilicate. This effect is preceded by a precrystallization endothermic effect beginning at a temperature of 700°, the nature of which is described in [1], [2]. The crystallization of the glasses was studied by us as follows—

by the following method: first powders were prepared from the glasses, from which disks were then formed on a paraffin binder. The disks were fired at various temperatures with a 2-hour hold. Petrographic studies were carried out on powders, in immersion liquids. X-ray patterns were obtained from the disks, without destroying the structure. After firing at temperatures of 900–1100°, the predominant product of crystallization of the glass is protoenstatite in the form of fibrous and radially radiating aggregates with a mean refractive index \(N_{\mathrm{cp}} = 1.618\). The sizes of the aggregates reach 0.030 mm. The amorphous phase forms independent clusters up to 0.008 mm in size and fills the centers of grains with radially radiating aggregates of crystals. The presence of isotropic regions in the centers of the grains is explained by diffusion of magnesium ions into the zone of crystallization of metasilicate, which always proceeds in the direction from the periphery toward the center of the grain.

Fig. 2. X-ray patterns of crystallization products of the “glass” without holding at 1600°, treated at different temperatures

Clinoenstatite is found in an amount of 15–20% of the area of the thin section, with refractive index \(N_{\mathrm{cp}} = 1.658\), in the form of grains up to 0.015 mm in size. Clinoenstatite is also recorded by X-ray diffraction with indices \(d = 2.09\), \(d = 1.76\), \(d = 1.59\) (Figs. 2, 3). With increasing tempera-

during firing the shape of the crystals changes. Fibrous and radial-acicular aggregates become fine-grained through fragmentation of the fibers along the long axis. Each crystalline grain is enveloped by glass. At the same time, with increasing firing temperature the total refractive index of the fine-grained aggregates decreases.

Fig. 3. X-ray diffraction patterns of the products of glass crystallization with a 30-minute hold at 1600°, treated at different temperatures

In the firing products at temperatures below 1200° the mean refractive index is \(N_{\mathrm{av}} = 1.618 \pm 0.003\); at 1200° — \(N_{\mathrm{av}} = 1.612 \pm 0.003\); at 1300° — \(N_{\mathrm{av}} = 1.610 \pm 0.003\); at 1350° — \(N_{\mathrm{av}} = 1.608 \pm 0.003\), and at 1380° — \(N_{\mathrm{av}} = 1.603 \pm 0.003\). We explain the decrease in the total refractive index of the fine-grained crystalline aggregates by an increase in the fineness of mixing of the crystalline phase with the glass, and by an increase in the homogeneity of the structure. The product of glass crystallization at low firing temperatures is represented by coarse crystalline formations and accumulations of glass. At low temperatures crystallization is accompanied by differentiation of the composition, and the measured refractive indices correspond to a greater extent to the true values of the minerals of the crystalline phase and of the glass.

After firing at a temperature of 1350–1380°, the specimens have a uniform, extremely fine-grained submicrocrystalline struc-

structure. All grains of the crystalline phase are surrounded by a film of glass, and the refractive index of such a structure is the combined one of the glass and the crystalline phase. The protoenstatite composition of the crystalline phase is established by optical and X-ray methods.

Clinоenstatite, in single grains up to 0.003–0.005 mm in size, with a refractive index of 1.656, is encountered. The presence of clinоenstatite in firing products at high temperatures is not established by X-ray analysis.

Thus, the principal crystalline phase in the crystallization of glass of talc composition upon firing to temperatures up to 1380° is protoenstatite. Clinоenstatite and cristobalite are of subordinate importance. Clinоenstatite is established by the optical method as single grains.

In the crystallization products of glass melted with a 30-minute hold at a temperature of 1600°, X-ray analysis records forsterite with indices: \(d = 3.32\); \(d = 2.98\); \(d = 2.27\) (Fig. 3). There is less cristobalite phase in this specimen than in the “glass” without a hold.

The investigations show that at a temperature of 900° the crystallization of the glass is completed; magnesium metasilicate is separated quantitatively in accordance with the theoretical magnesium content.

The amount of amorphous phase over the entire range of firing temperatures (from 900 to 1300°) is about 15%, and its refractive index remains constant, equal to \(1.494 \pm 0.008\). In the firing products of the glass at temperatures of 1350 and 1380°, it is not possible to measure separately the refractive indices of the glass and crystalline phases owing to the glass-ceramic-like structure of the material.

Conclusions

1. From glass of talc composition at a temperature of 900°, magnesium metasilicate crystallizes in the form of protoenstatite completely in accordance with the magnesium content. The crystallization product at low temperatures is represented by comparatively large crystals of fibrous and radial-fibrous form. With increasing firing temperature, the fibrous form of the crystals changes to a fine-grained one, more favorable for stabilization of the structure.

2. From glass melted with a hold, forsterite also crystallizes along with the metasilicate.

References

1. I. I. Kitaigorodskii, R. Ya. Khodakovskaya, “The precrystallization period in glass and its significance.” In: The Glassy State, issue 1, Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1963.

2. I. I. Kitaigorodskii, R. Ya. Khodakovskaya, “Some regularities of glass-ceramization in the system \(\mathrm{SiO_2—Al_2O_3—MgO}\).” Proceedings of the Academy of Sciences of the USSR. Inorganic Materials, vol. 1, no. 5, 1965.