Phase transformations of talc upon heating
Abstract Generated abstract
This study examines phase transformations in high-purity Onot and Algui talcs during heating from 800 to 1500°C, addressing conflicting views on the decomposition of talc and the stability of resulting magnesium metasilicate phases. Using X-ray diffraction, high-temperature X-ray analysis, petrographic observation, and gravithermometric methods, the authors trace the breakdown of talc beginning near 900°C and its restructuring into protoenstatite with separation of amorphous silica, later crystallizing as cristobalite. Protoenstatite is described as developing through fibrous or lamellar defective forms, then fine-grained aggregates stabilized at ordinary temperatures by small crystal size, and finally larger crystals. At higher temperatures, especially near 1500°C, recrystallization and melt-assisted growth favor transformation on cooling to clinoenstatite, which becomes the dominant crystalline phase.
Full Text
Phase Transformations of Talc upon Heating
P. G. Usov, N. V. Sobora, E. N. Belomestnova
(Presented by the scientific seminar of the Department of Silicate Technology)
Steatite ceramics, possessing high mechanical strength and a low dielectric-loss angle, are widely used in the manufacture of high-voltage, high-frequency dielectrics.
Talc is a component which, as a result of heat treatment, forms the crystalline phase of steatite ceramics in the form of magnesium metasilicate. The results of numerous investigations of the nature of the changes occurring in talc upon heating are often contradictory. To date there is no unified view of the phase transformation and the temperature of the stable state of the newly formed phases [1, 2, 3]. The need for a clearer understanding of the temperature changes in this mineral, which is widely used in industry, prompted further study of this question.
High-quality beneficiated talcs (Onot and Algui) were taken as the object of investigation; their chemical composition is close to the theoretical composition of the mineral, but they have different crystal morphology. The Algui powdered talc is lamellar-scaly; the Onot talc is massive, finely scaly, and fine-crystalline. The studies were carried out in the temperature range 800–1500°C. Samples of talc were subjected to X-ray, high-temperature X-ray, petrographic, gravithermometric, and other methods of analysis.
The results of the studies show that after firing at 800°C the talc structure is retained. The first noticeable signs of decomposition occur at 900°C, and in Onot talc they appear more sharply than in Algui talc. According to microscopic examination, the decomposition is accompanied by a decrease in the strength of birefringence from 0.040 to 0.013 and by an increase in the refractive index from 1.582 to 1.596; the newly formed phases produced during heat treatment of Algui talc acquire a fibrous structure, while those of Onot talc often have a lamellar appearance. The X-ray pattern of Onot talc fired at 900°C shows that all the lines characteristic of talc have disappeared; Algui talc retains all the diffraction maxima of talc, but they have all diminished and lost their sharpness. Upon further heating, restructuring of the talc structure into the pyroxene structure continues, and the structure of the latter becomes more perfect. On X-ray patterns taken from samples fired at 1000° and 1100°C, along with lines corresponding to both modifications of magnesium metasilicate, there are diffraction maxima with \(d = 2.3\) Å, \(d = 1.97\) Å, \(d = 1.49\) Å, belonging to protoenstatite. High-temperature X-ray analysis also indicates that the magnesium metasilicate being formed is protoenstatite. The newly formed fibrous and lamellar phases observed
under the microscope—these are formations of protoenstatite, but carrying a mass of defects and stresses. This is indicated by the diffuseness of the diffraction maxima, whose interplanar spacings correspond to protoenstatite.
Beginning at 1200°C, the initial fibrous aggregates of protoenstatite are transformed into fine-grained ones, consisting of the smallest grains (the size of individual crystalline individuals is no more than 2 microns). Their total refractive index is 1.612±0.003. Externally, under the microscope, this is manifested in the rupture of fibers across the elongated axis and in the formation of microcrystals. The cause of fiber rupture, in our view, lies in the defective structure of the initial protoenstatite. The fibers that form have a somewhat greater length than is required for a stable crystal. This, in turn, is connected with the fact that the rate of crystal growth is greater than the rate of formation of crystallization centers during the decomposition of talc. One of the causes of the anisotropy of crystal growth is the constitution of its lattice—the elongation of the anionic chains. Fibrous crystals, whose length is 60–70 microns (with a width of 1–2 microns), experience rupture stresses and tear over time. In this process the orientation of individual microcrystals is preserved. Therefore, despite their extremely small size, in crossed nicols they stand out quite distinctly.
At the same time, inclusions of amorphous silica are separated, which then crystallizes in the form of cristobalite.
At 1400°C, in the products of the talcs studied, the fibrous aggregates completely disappear and fine-grained ones predominate. The latter do not undergo a polymorphic transformation and are stable at ordinary temperatures. This stabilization of protoenstatite is due to the small size of the crystals [4].
The processes that occur during the firing of talc do not end with the formation of fine-grained protoenstatite. Beginning at 1200°C, crystals up to 10 microns in size of another polymorphic form of magnesium metasilicate—clinoenstatite—are recorded in the firing products of talc. The appearance of clinoenstatite is connected with recrystallization growth of protoenstatite grains. The presence of melt at 1500°C promotes growth of protoenstatite grains, and upon cooling the latter readily transform into the monoclinic form of magnesium metasilicate. Clinoenstatite is the dominant crystalline phase at 1500°C. Its segregations have a tabular and short-prismatic habit without distinct faces, reaching 40–50 microns. Individual grains show polysynthetic twinning. X-ray diffraction patterns confirm the clinoenstatite composition.
Conclusions
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Upon heating, the structure of talc is transformed into the structure of protoenstatite with separation of amorphous silica.
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In its development, protoenstatite passes through 3 stages: platy and fibrous varieties, fine-grained aggregates, and large crystals. The latter, upon cooling, transform into clinoenstatite.
REFERENCES
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A. I. Avgustinik. Changes in talc upon heating. In: Talc, Promstroiizdat, 1952.
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D. S. Belyankin, B. V. Ivanov, V. V. Lapin. Petrography of Technical Stone. Publishing House of the Academy of Sciences of the USSR, 1952.
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M. Koltermann. Ton-Industrie-Zeitung, 6, 7/8, 1964.
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P. G. Usov, V. N. Gurina. The influence of the dispersion of magnesium metasilicate crystals on stabilization of protoenstatite at low temperature. Proceedings of the XI Regional Conference, Novosibirsk, 1968.