STUDY OF THE STABILITY CONDITIONS OF PROTOENSTATITE IN THE SYNTHESIS OF MAGNESIUM METASILICATE

P. G. Usov, V. N. Gurina, N. V. Sobora

(Presented at the scientific seminar of the inorganic departments)

The study of the interaction of MgO and SiO₂ in a molecular ratio equal to 1 : 1 is of interest for investigating the phase relationships of MgSiO₃ modifications and the conditions of stability of protoenstatite. The latter polymorphic form, metastable under normal conditions, has wide practical application. Protoenstatite is the principal crystalline constituent of the structure of steatite materials. The widely known phenomenon of aging in talc-based ceramics is associated with the transformation of protoenstatite into clinoenstatite in the steatite body [3].

According to studies [1], the reaction in the system under investigation begins with the formation of forsterite, which then reacts with an excess of silica and is converted into magnesium metasilicate. In the temperature range 1200—1400°C, protoenstatite is formed. The product of higher-temperature firings is clinoenstatite. The question of whether transformation into clinoenstatite occurs at the maximum temperature, or whether it forms during cooling of protoenstatite, remains controversial. Studies using a high-temperature X-ray camera show that protoenstatite is stable above 1000°C and, upon cooling, transforms into clinoenstatite. The rate of transformation is sensitive to particle size: with an increase in crystal size, the reaction proceeds more rapidly [2].

This article presents part of the work on investigating the influence of crystal size on the characteristics of phase transitions of magnesium metasilicate during synthesis in the solid phase. The starting materials were anhydrous silicic acid and basic magnesium carbonate of chemically pure grade. The use of the carbonate salt as the magnesium-containing component makes it possible to obtain the most active form of magnesium oxide for the reaction. In order to ensure optimal synthesis conditions, the components were ground and mixed in a laboratory porcelain ball mill in the presence of water. Samples for investigation were pressed on a laboratory hydraulic press under a pressure of 750 kg/cm² in the form of disks 20 mm in diameter and 2 mm thick (for studying phase composition and microstructure), and in the form of cylinders 14 mm in diameter with equal height (for determining the compressive strength).

Firing of the material was carried out in an electric silicon-carbide furnace at a temperature of 1100—1450°C with an interval of 50°. The holding time at the final temperature was 2 hours. Cooling after firing was under free—

of furnace cooling. Temperature control was carried out with a platinum–platinum-rhodium thermocouple. The heat-treatment cycle was repeated twice. Before the second firing, the material was dispersed and repressed. The phase composition and microstructure were monitored by the optical method in polished sections and in immersion liquids, and also by X-ray diffraction on a URS-50I unit.

Study of the phase composition of the fired products shows that appreciable formation of magnesium metasilicate is observed only beginning at 1200°C. The processes occurring in the temperature interval 1000–1200°C lead to the formation of periclase (lines \(d = 2.43;\ 2.11;\ 2.48\ \text{Å}\)); crystallization of quartz (\(d = 3.34;\ 1.81;\ 1.54\ \text{Å}\)) and to the synthesis of forsterite (\(d = 4.03;\ 2.49;\ 2.44;\ 2.25;\ 1.93;\ 1.74;\ 1.61;\ 1.49\ \text{Å}\)). Petrographically, in the immersion preparation only quartz and protoenstatite are determined. The latter phase has the appearance of monomineral fine-grained aggregates with an average refractive index of 1.620. X-ray diffraction records protoenstatite by the lines \(d = 3.16;\ d = 2.89\ \text{Å}\). The other crystalline phases have a very fine-grained structure and are not differentiated under an ordinary microscope. The fired products have the appearance of isotropic accumulations ranging in size from 1–2 to 12–15 microns with refractive indices from 1.520 to 1.490. The change in the sizes of MgSiO\(_3\) grains during the firing process (according to microscopic data) is given in Table 1.

Table 1

Change in the sizes of MgSiO\(_3\) crystals and the compressive strength of heat-treatment products

From an analysis of X-ray patterns taken from disks fired at different temperatures, it follows that clinoenstatite appears after firing at 1400°C. However, microscopically the presence of this phase is detected in material fired at 1350°C twice. The habit of clinoenstatite crystals is marked by imperfection: the grains have an uncharacteristic, isometric rounded form. The refractive index is 1.656. The average crystal sizes are 15–20 µm; in the X-ray reflection spectrum of the powdered sample, diffraction lines of clinoenstatite also appear (\(d = 3.28;\ 2.98;\ 2.86\ \text{Å}\), etc.). The phenomenon described repeats that observed by Bussem, Schusterius, Stuttgart [3], and others: the transformation of protoenstatite into clinoenstatite under mechanical action. An analogous phenomenon occurs in material fired at 1400°C in one cycle. The transformation of protoenstatite into clinoenstatite during grinding is observed after those firings as a result of which metasilicate particles with an average size of 15–20 microns are formed. The nonequilibrium state of the structure determines the reduced mechanical strength of the material (Table 1) after firings at 1350°C twice and at 1400°C in one cycle. With an increase in the heat-treatment temperature, the sizes of cry-

crystals of protoenstatite increase to 30 microns and more. During cooling, a transition to clinoenstatite takes place. The transition was recorded by means of high-temperature X-ray phase analysis and DTA. The transformation temperature is about 800°C. The transformation of clinoenstatite into protoenstatite on heating occurs at 1100°C. Apparently, the relationship between the probability of the transformation during cooling and the grain size of protoenstatite is ensured by the existence, in MgSiO$_3$ crystals, of stresses arising as a result of nonuniform cooling of protoenstatite grains: whereas at technical cooling rates small crystals cool uniformly throughout the particle volume, in large crystals there may be a considerable discrepancy between the temperatures of the outer and inner zones of the crystal. As a result, thermal stresses will arise inside large crystals, which will lead to a shift of equilibrium and ensure the occurrence of the transformation with a change in volume.

The introduction of 1% MnO additive into the batch, under analogous heat treatment, leads to the formation of a very fine-grained product (grain sizes less than 1 μm). X-ray phase analysis shows that magnesium metasilicate is represented by protoenstatite with a perfect internal structure. The structure of the material is resistant to the action of elevated temperatures, grinding, and long-term storage.

Conclusions

The first metasilicate phase during the firing of a stoichiometric mixture of silica and MgCO$_3$ is protoenstatite. The probability of transition of high-temperature protoenstatite to clinoenstatite during cooling is a function of the crystal size. The critical crystal size at which the transformation in the material studied is observed as a result of grinding is 15–20 μm. When the crystals are larger than 20 μm, a transition to clinoenstatite occurs during cooling. In order to prevent the aging of ceramics with a crystalline phase in the form of magnesium metasilicate, it is necessary to obtain a fine-grained structure of the body with a protoenstatite composition stabilized by the finely dispersed state of the crystals.

Literature

1. I. G. Bubenin. Construction Materials, No. 3, 3, 1937.
2. Sarwerl F., F. A. Hummel, Journ Amer Ceram Soc, 45, No. 4, 152, 1964.
3. W. Bussen, C. Schusterius, K. Stuckardt, Wiessenschafte, Veröffend d Siemens-Werke, 17, 59, 1938.