STUDY OF THE CRYSTALLIZATION PRODUCTS OF MOLTEN TALC

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

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

In recent years a new method for manufacturing silicate products has emerged and is developing—crystallization of glasses of a specified composition. Talc is the basic raw material for many special-purpose ceramic materials; therefore, the study of the processes and products of crystallization of a melt of talc composition is of practical and theoretical interest. The crystallization of talc from the Onot deposit of the following chemical composition was investigated (Table 1).

Table 1

Chemical composition of talc from the Onot deposit

The theoretical composition of talc is expressed by the formula \(3\mathrm{MgO}4\mathrm{SiO}_2\mathrm{H}_2\mathrm{O}\), or, in percent, MgO—31.74; SiO₂—63.5; and H₂O—4.8. During firing, talc decomposes according to the reaction
\(3\mathrm{MgO}4\mathrm{SiO}_2\mathrm{H}_2\mathrm{O} \rightarrow 3(\mathrm{MgSiO}_3) + \mathrm{SiO}_2 + \mathrm{H}_2\mathrm{O}\), with the formation of 83.5% magnesium metasilicate and 16.5% silica. In the binary system MgO—SiO₂, talc is located near the eutectic of composition MgO = 35.1% and SiO₂ = 64.9%, with a melting temperature of 1543°. The initial melting temperature of talc is 1543°, with formation of the primary melt in an amount of 95.7%. Melting of talc ends at a temperature of about 1590°. According to Bowen and Andersen [1], cristobalite and clinoenstatite are in equilibrium with the talc melt. The literature contains indications that the crystallization ability of melts of talc and magnesium metasilicate composition is high [2]. Weil writes that, in general, pure substances or systems with a melting temperature above 1400° are difficult to obtain in the form of glass.

Melting of talc was carried out in a kryptol furnace in corundum and zirconia crucibles with a capacity of 30 g at a temperature of 1600°. Samples I, II-a, and II-b were melted and, without holding in the molten state, were cooled: sample I—in air for 30 minutes, and samples II-a and II-b—together with the furnace for 12–18 hours, respectively. Sample III was held in the molten state at a temperature of 1600° for 30 minutes and cooled together with the furnace. Samples IV and V were melted in zirconia crucibles with holding in the molten state at a temperature of 1600° for 2 hours. The melt of sample IV—

cooled together with the furnace, and specimen V—rapidly in air. In ingot specimen 1, two zones formed along the height—the upper 1-a and the lower 1-b.

Fig. 1. X-ray patterns of clinoenstatite and protoenstatite standards

The upper zone (1-a) consists mainly of protoenstatite with a crystal size of up to 0.012–0.015 mm. Clinoenstatite is of subordinate

Fig. 2. X-ray patterns of the crystallization products of molten talc during rapid cooling of the melt in air: a—upper zone of the ingot; b—lower zone of the ingot

importance. According to optical analysis, glass accounts for about 20%. The results of optical studies are also confirmed by X-ray analysis data (Fig. 2, curve a). Along with protoenstatite and cli-

In this sample, X-ray analysis identifies cristobalite with \(d = 4.03\); the latter was not noted by the petrographic method. The lower part of the crucible (I-b) is composed of crystals of irregular shape, platy and slightly elongated, up to \(0.011\) mm in size, with refractive index \(N_g = 1.660\). Protoenstatite is recorded in insignificant amounts in the form of fibrous aggregates with a mean refractive index \(N = 1.616\). Glass amounts to 15–20%. The clinoenstatite form of magnesium metasilicate is also established by X-ray analysis (Fig. 2, curve b). The protoenstatite indices are less pronounced. X-ray analysis of the crystallized phase of this zone reveals cristobalite.

Fig. 3. X-ray diffraction patterns of the crystallization products of molten talc upon slow cooling of the melt: a—over 12 hours; b—over 18 hours.

In sample II-a, protoenstatite and clinoenstatite are found under the microscope in approximately equal amounts. Protoenstatite occurs as fibrous aggregates \(0.012—0.015\) mm to \(0.056\) mm long, with refractive index \(N'_g = 1.618\). Clinoenstatite is present as platy and elongate-prismatic crystals ranging in size from \(0.030 \times 0.030\) to \(0.130 \times 0.050\) mm, with refractive indices \(N_g = 1.661\) and \(N_p = 1.652\). The amount of glass is about 15–20%.

The predominant form of magnesium metasilicate in the crystalline phase is identified as protoenstatite. Clinoenstatite is of subordinate importance. Cristobalite is recorded in small amounts (Fig. 3, curve a).

In sample II-b, cooled over 18 hours, clinoenstatite predominates in the form of well-formed crystals ranging in size from \(0.025\) to \(0.050\) mm, with polysynthetic twins. The refractive index of the crystals is \(N_g = 1.660\).

There is little protoenstatite; it occurs as fibrous aggregates up to \(0.015\) mm in size, with refractive index \(N_{\mathrm{cp}} = 1.616\). Grains of cristobalite ranging in size from \(0.007\) to \(0.012\) mm are clearly visible. The amount of glass is about 15%.

The data from the optical studies agree well with the X-ray analysis (Fig. 3, curve c). The clinoenstatite composition of the crystalline phase is expressed very clearly here. Cristobalite is also recorded quite distinctly.

Sample III—the melt after a 30-minute holding period, cooled together with the furnace—is composed of clinoenstatite (curve a, Fig. 4). Protoenstatite is of subordinate importance. Cristobalite is not established either by X-ray or by petrographic analysis.

Fig. 4. X-ray patterns of the crystallization products of molten talc, cooled together with the furnace—slowly: a—the melt was held at 1600° for 30 minutes; b—the melt was held at 1600° for 2 hours; c—X-ray pattern of forsterite standard.

Sample IV—with the melt held for two hours and cooled together with the furnace—has a more complex phase composition. The predominant crystalline phase is protoenstatite. It forms platy crystals and fairly large fibrous aggregates up to 0.110 mm in size, with refractive index \(N_{\mathrm{cp}} = 1.616\). Second in abundance is clinoenstatite. Cristobalite is not established by either optical or X-ray methods of analysis. Forsterite is clearly recorded by X-ray analysis. Its indices are shown on X-ray pattern b in Fig. 4, namely: \(d = 3.82\); \(d = 2.75\); \(d = 2.50\); \(d = 2.45\); \(d = 2.25\); \(d = 1.74\), \(d = 1.49\), and \(d = 1.47\). In the same figure, an X-ray pattern of forsterite is given for comparison. In addition to the phases listed, zirconium dioxide \((\mathrm{ZrO_2})\), which passed from the crucible into the melt as a result of dissolution, is established in the crystallization products. The amount of glass is 15–20%.

Sample V, melted with a two-hour holding period and cooled in air, has three sharply differentiated zones along the height of the crucible. The upper zone (V-a) consists mainly of glass, approximately 50–60%. The crystalline phase is represented by protoenstatite in the form of fine-grained aggregates.

with dimensions smaller than 0.001 mm, with a total refractive index \(N = 1.616\).

The X-ray diffraction pattern of this zone does not reveal crystalline phases, owing to the high glass content (Fig. 5, curve \(a\)) and the imperfection of the crystal lattices.

The middle zone (V-b) contains 25–30% glass. The crystalline phase is represented by fibrous aggregates of protoenstatite up to 0.040 mm in size, with refractive index \(N'g = 1.618\). Because of large defects in the crystal lattices, the peaks of the X-ray diffraction patterns are weakly expressed (Fig. 5, curve — \(b\)).

Fig. 5. X-ray diffraction patterns of the crystallization products of molten talc, obtained with holding at \(1600^\circ\) for 2 hours and quenched rapidly in air: \(a\) — upper zone of the crucible, \(b\) — middle zone of the crucible, \(c\) — lower zone of the crucible.

The lower zone (V-c) contains about 20% glass. The crystalline phase is composed mainly of clinoenstatite and protoenstatite. Forsterite is weakly manifested—\(d = 1.48\)—and \( \mathrm{ZrO_2} \) is recorded by indices \(d = 1.79\) and \(d = 1.53\).

Conclusions on the crystallization of a melt of talc composition:

1. Holding in the molten state impairs crystallization ability: a melt without holding, upon crystallization, yields more perfect crystalline phases with a smaller number of defects in the crystal lattices (compare the X-ray diffraction patterns in Figs. 3—without holding—and 5—with holding for 2 hours).

2. At technical cooling rates (together with the furnace), magnesium metasilicate crystallizes completely from the melt. The amount of glass in the crystallized product is 15–20%, with a theoretical content of 16.5%. This also follows from the constancy of the refractive index of the glass, which is about \(1.494 \pm 0.008\).

3. The siliceous material of the crystallized melt is present largely in the amorphous phase. It crystallizes partially in the form of cristobalite only from melts obtained without holding.

4. Under the applied melting and cooling regimes of the melts, it was not possible to obtain a monomineralic composition of magnesium metasilicate. Protoenstatite and clinoenstatite are found in all samples.

With more rapid cooling of the melt, the predominant form of metasilicate in the crystallization products is protoenstatite. The clinoenstatite form is obtained more readily with slow cooling of the melt.

1. The crystals of protoenstatite are more often fibrous and fine-grained, with absolute crystal sizes always smaller than those of clinoenstatite. Clinoenstatite is almost always tabular or in the form of elongated prisms, with crystal sizes larger than those of protoenstatite.

2. When talc is melted at a temperature of 1600°, forsterite forms relatively slowly; it is not observed in the crystallization products of a melt obtained without holding and with a holding time of 30 min, but it is established during crystallization of a melt obtained with a two-hour holding time.

REFERENCES

1. W. Eitel. Physical Chemistry of Silicates. Foreign Literature Publishing House, Moscow, 1962.

2. W. Weyl. Formation of crystallization centers, crystallization, and glass formation. In: Glass, a collection of translations from foreign periodical literature. Foreign Literature Publishing House, Moscow, 1963.