On Phase Formation of MgSiO3 during Oxide Synthesis
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This study examines phase formation in MgSiO3 synthesized by firing equimolar mixtures of amorphous silica and magnesium carbonate-derived oxide, focusing on how heat-treatment temperature, repeated firing, and crystal size affect the stability of protoenstatite and its transformation to clinoenstatite. Microscopic and X-ray diffraction observations show that protoenstatite first forms near 1200°C, increases in amount with temperature and repeated firing, and becomes increasingly prone to transformation as crystal aggregates grow. The work argues that larger protoenstatite crystals generate internal stresses during cooling and transform more readily to clinoenstatite, with associated reductions in mechanical strength, while very fine-grained protoenstatite can persist metastably at room temperature. Addition of 1% MnO promotes a stable fine-grained protoenstatite structure resistant to heating, grinding, and storage.
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ON THE PHASE FORMATION OF MgSiO₃ DURING SYNTHESIS FROM OXIDES
P. G. Usov, V. N. Gurina
(Presented by the scientific seminar of the Department of Silicate Technology)
The system of composition MgSiO₃ is a physicochemical system with complex polymorphism. The results of studies [1, 2, 3] on the synthesis of magnesium metasilicate from oxides are highly contradictory.
In the present work, the nature and sequence of formation of the crystalline phases of metasilicate that occur during firing of a mixture of X-ray-amorphous silica, anhydrous, and MgCO₃ preparation (dissociation temperature 780°C), taken in a molecular ratio of 1 : 1, were studied as a function of the size of the forming crystals of magnesium metasilicate.
MgSiO₃ appears at a heat-treatment temperature of the mixture of 1200°C in the form of protoenstatite in an amount of about 30% of the total mass. Under the microscope, protoenstatite has the appearance of fine-grained aggregates with a refractive index of 1.620. The sizes of the MgSiO₃ formations do not exceed 8–10 microns.
The content of protoenstatite increases as the heat-treatment temperature rises; however, the sizes of the aggregates change almost not at all up to 1300°C. Repeated firing of ground and re-pressed specimens has a favorable effect on the yield of metasilicate. The yield of MgSiO₃ after repeated firing at 1350°C increases to 70%. The crystal sizes are 18–20 microns, compared with 10–12 microns after the first firing. As a result of repeated firing at 1350°C, the body has reduced strength. The compressive strength of specimens subjected to heat treatment at 1350°C in two cycles is 30% lower than the strength of the material after two firing cycles at 1300°C.
As follows from analysis of the X-ray diffraction patterns, the material fired at 1350°C in two heat-treatment cycles has a perfect crystalline structure represented by protoenstatite. The ground sample, however, when viewed under the microscope, consists predominantly of clinoenstatite. The X-ray diffraction pattern of the ground material likewise corresponds to clinoenstatite.
A similar phenomenon occurs in material fired at 1400°C in one cycle. The decrease in strength is also about 30% (Fig. 1). The mechanism of the decrease in the strength of the products consists in the appearance of large internal stresses at the grain boundaries in the structure of the protoenstatite body during cooling, caused by the tendency of the protoenstatite lattice to pass into the monoclinic form. Transfor-
...this is associated with volume changes. Because, when the material is ground, energy is released and supplied from outside, the transition of protoenstatite into clinoenstatite becomes possible at normal temperature. But such a transformation must be prepared by the prior thermal history of the specimen, since grinding of material fired at lower temperatures produces no clinoenstatite at all, or almost none, whereas after firing above 1400°C the clinoenstatite structure is already created during the heat treatment process, which includes the stages of firing and cooling of the material to normal temperature.
Fig. 1. Change in the mechanical strength of the synthesis products as a function of temperature and the number of heat-treatment cycles.
I — firing in one cycle, II — firing in two cycles.
As follows from an analysis of the data obtained, the probability of the transition of protoenstatite into clinoenstatite is related to the size of the crystalline formations of the metasilicate: the larger the crystals, the more readily the transformation takes place. During heat treatment at temperatures below 1350–1400°C, under the adopted regimes, protoenstatite grains not exceeding 10–12 microns are formed in the material. Such particles can exist for a long time in a metastable state. When the material is fired in the temperature range 1350–1400°C, or during repeated firings, the sizes of the protoenstatite crystals rapidly increase, owing to intensification of recrystallization processes, to 18–20 microns. Grains of this size do not transform into clinoenstatite on cooling, but transform into this form when additional energy is supplied to the system. As a result of heat treatment at a higher temperature, magnesium metasilicate occurs in the cooled products only in the form of clinoenstatite. The content of MgSiO₃ in the material is up to 85%. The crystal sizes exceed 40 microns. Study of the transformation of protoenstatite into clinoenstatite by high-temperature X-ray analysis shows that the rate of transition of the high-temperature form of MgSiO₃ into the monoclinic variety increases greatly in large crystals.
The introduction of 1% MnO into the batch as a mineralizer leads to the formation of a very fine-grained structure of the metasilicate in the form of protoenstatite. The structure obtained is resistant to prolonged exposure to elevated temperatures, to grinding, and to long-term storage.
Conclusion. The size of the protoenstatite crystals is one of the determining factors for the probability of existence of the protoenstatite structure in a metastable state at room temperature, or for its transition on cooling into clinoenstatite.
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