On the Polymorphism of Magnesium Metasilicate in Steatite Products at Low Temperatures

P. G. Usov, V. I. Vereshchagin

(Presented at the scientific seminar of the Department of Technology of Silicates, Inorganic Substances, and Electrochemical Production)

In steatite ceramic products, X-ray and petrographic analyses identify magnesium metasilicate in two forms—protoenstatite and clinoenstatite. In the opinion of many researchers [1, 2, 3, 5], protoenstatite is the stable high-temperature modification of \(\mathrm{MgSiO_3}\), while clinoenstatite is the low-temperature modification. The transition point from one modification to the other is given as lying in the temperature range \(1000\)—\(1200^\circ\mathrm{C}\). The thermogram that we obtained for clinoenstatite, prepared from talc from the Alguy deposit by melting and subsequent crystallization, gives a temperature for the transition of clinoenstatite into protoenstatite of \(1100^\circ\mathrm{C}\) and a temperature for the reverse transition of \(1060^\circ\mathrm{C}\) (Fig. 1). Consequently, the protoenstatite detected at ordinary temperatures is in a metastable state, and there is a thermodynamic possibility of its transition into clinoenstatite. It should be noted, however, that protoenstatite is stabilized to one degree or another by the glass phase of the ceramic; therefore, the follow-

Fig. 1. DTA curves (1—heating, 2—cooling) of clinoenstatite obtained by crystallization of a melt.

one must assume a slow transition of protoenstatite into clinoenstatite. In individual cases, for example, when grinding the fired bodies of compositions P-5 and B-17, a rapid transition of the bulk of the protoenstatite into clinoenstatite is observed; this is confirmed by the appearance of new diffraction maxima on the X-ray pattern of the ground fired bodies and by a decrease in the intensity of the maximum for \(d = 1.95\), characteristic of protoenstatite (Table 1, Fig. 2). This is explained by insufficient stabilization of the protoenstatite by the glass phase formed during the firing of the bodies.

Table 1

X-ray analysis data for fired steatite bodies before and after grinding

bodies. When the products are ground, no appreciable change in the composition of the crystalline phase of the ceramic is observed. Changes in the ratio of the MgSiO\(_3\) modifications are shown by X-ray patterns of the ground products before heat treatment and after heating to 500°C in the direction of decreasing protoenstatite (Fig. 3, curves 1, 4). The DTA curve of this same material, recorded in the temperature interval 20–500°C, has an exothermic effect at 280–350°C, with a maximum at 320° (Fig. 4). The effect is rather weak and, in magnitude, is 3–4 times smaller than the effect of the polymorphic transformation of cristobalite. To determine whether the transition of protoenstatite to clinoenstatite takes place gradually over the entire temperature interval 20–500°C, or whether this transition occurs in the narrow temperature region 280–350°C, i.e., whether the detected DTA effect corresponds to this transition, X-ray patterns were taken of powders of the same material treated to 280 and 350°C. Analysis of the X-ray patterns (Figs. 4 and 3) showed that up to 280° a change in the magnitude of the diffraction maximum \(d = 1.95\) does occur, but it is slight, i.e., a slow transformation of protoenstatite into clinoenstatite takes place. In the temperature interval 280–350°C, a sharp change in it occurs; consequently, the ratio of the modifications of the crystalline phase changes toward a decrease in protoenstatite; the maximum \(d = 2.09\), characteristic of clinoenstatite, becomes clearly defined. With a further increase in the treatment temperature to 500°C, the ratio of protoenstatite and clinoenstatite in the specimens does not change. X-ray patterns of ground ceramic products heated to 350 and 500°C do not have

Fig. 2. X-ray diffraction patterns of steatite bodies: a—B-17, b—P-5
(1—taken from the disk, 2—taken from the powder)

Fig. 3. X-ray diffraction patterns of ground products of body P-5: 1—initial specimen;
2—after heating to 280°; 3—after heating to 350°; 4—after heating to 500°.

different (Fig. 3, curves 3, 4). This is also confirmed by other data: the effect at 320° on the DTA curve of specimens after preliminary temperature treatments up to 100°, 200°, and 300°, with holding for 2 to 6 h at the indicated temperatures, decreases as the temperature and holding time increase; the effect on the DTA curve disappears completely in specimens treated at 300°C (Fig. 5). The molecular volume of protoenstatite is 32.4 cm³/g-mole; that of clinoenstatite is 31.5 cm³/g-mole [4]; consequently, the transition of protoenstatite into clinoen-

Fig. 4. DTA curve of an article made of steatite ceramic P-5 (heating rate 5°/min) and change in the diffraction maximum \(d = 1.95\) on the radiographs of Fig. 3.

Fig. 5. DTA curves (heating rate 15°/min): \(a\)—initial sample and after preliminary thermal treatment (100, 200, 300°—treatment temperature; 2, 4, 6—holding time in hours at the final temperature).

enstatite proceeds with a decrease in volume, which entails a loss of the article’s vacuum density, its strength, and its dielectric characteristics. It should be noted that in reality the processes described by us proceed much more slowly. All our studies were carried out on samples ground so as to pass through sieve No. 0060, and not on monolithic specimens.

Conclusions

1. Owing to the presence in the crystalline phase of finished steatite-ceramic articles of the metastable modification—protoenstatite—at the temperatures of their storage and service, relaxation processes occur that are associated with the transition of protoenstatite into clinoenstatite and that are the main cause of aging of the articles.

2. The above-mentioned processes are almost imperceptible at room temperature; with increasing temperature their intensity rises, and they proceed most intensively in the temperature range 280–350°C.

Literature

1. V. Eitel. Physical Chemistry of Silicates. Vol. II, 97, 98. Inostrannaya Literatura Publishing House, Moscow, 1962.

2. A. I. Paltsev. Technical Ceramics. Gosenergoizdat, Moscow–Leningrad, 1959.

3. N. A. Toropov, V. P. Barzakovskii et al. Phase Diagrams of Silicate Systems, 41–47. Nauka Publishing House, Moscow–Leningrad, 1965.

4. A. S. Berezhnoi, P. A. Kordyuk. Reports of the Academy of Sciences of the Ukrainian SSR, No. 10, 1417–1420, 1960.

5. W. R. Foster. High-Temperature X-Ray Diffraction Study of Polymorphism of \(MgSiO_3\). Amer. Ceram. Society V. 34, No. 9, 255–259, 1951.