Study of Phase Formation Processes of Magnesium Metasilicate During Crystallization of a Melt of Composition MgSiO3
Abstract Generated abstract
This study examines phase formation in magnesium metasilicate during crystallization of a stoichiometric MgSiO3 melt, focusing on the sequence of polymorphic transformations under different cooling conditions. Using quenching at varying degrees of supercooling, along with optical, density, and X-ray diffraction observations, the authors compare products formed in water, air, a furnace, and during slow crystallization. The results indicate that rapid cooling preserves metastable, less dense protoenstatite-like structures, while slower cooling promotes progressive densification and transformation toward clinoenstatite. The work argues that MgSiO3 crystallization proceeds stepwise in accordance with Ostwald’s rule, from a high-temperature, less dense form toward the low-temperature clinoenstatite phase.
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Study of Phase-Formation Processes of Magnesium Metasilicate during Crystallization of a Melt of Composition MgSiO₃
P. G. Usov, V. N. Gurina
(Presented by the scientific seminar of the Department of Silicate Technology)
From an analysis of experimental data it follows that the processes of formation of modifications of MgSiO₃ during crystallization of a melt of stoichiometric composition proceed in accordance with Ostwald’s rule [1].
Transformations in the system under study occur at a very high rate; therefore it is difficult to preserve, in pure form up to normal conditions, the structure of the material that forms on cooling to a given temperature, under which the phase composition of the products is studied. In order to determine the sequence of formation of the polymorphic forms of MgSiO₃, the method of quenching melts with different degrees of supercooling at the moment of crystallization was used.
At high cooling rates, quenching in water and in air, the phase-formation processes of MgSiO₃ do not reach the equilibrium low-temperature state, and a large number of intermediate metastable phases are found in the crystallization products. The crystalline form of MgSiO₃ closest to the molten state, possessing the lowest density, has a fibrous structure; its refractive index is \(N_g' = 1.620\).
Its amount increases in small pieces and droplets of the crystallized melt and decreases, down to complete disappearance, as the size of the pieces of material increases, that is, under conditions of the least intense removal of heat from the system. In large pieces of metasilicate, the predominant phase becomes one which under the microscope has a fibrous, acicular, coarse-fibrous and, finally, bar-like and tabular structure, with a refractive index from 1.620 to 1.654—1.660. Material cooled under milder conditions undergoes deeper changes in structure and is farther removed from the molten state; it has a higher density and a higher specific gravity. Table 1 gives some properties of MgSiO₃ as a function of the degree of supercooling of the melt at the moment of crystallization. Comparison of X-ray diffraction patterns shows that the material with the least dense structure has many features characteristic of the diffraction pattern of protoenstatite: \(d = 3.16;\ I = 10;\ d = 2.72;\ d = 1.97;\ d = 1.71;\ d = 1.49;\)
\(d = 1.31\ \text{Å}\). With increasing density of the structure, the lines of clinoenstatite become stronger in the X-ray diffraction patterns: \(d = 2.86;\ d = 2.10;\ d = 1.78;\)
\(d = 1.60;\ d = 1.52;\ d = 1.37\ \text{Å}\). The product of slow crystallization of the melt is pure clinoenstatite. This form of MgSiO₃ possesses na-
Properties of magnesium metasilicate as a function of the degree of supercooling of the melt
Table 1
| Glass | Crystallization in water | Crystallization in air | Crystallization in a furnace at 700°C | Slow crystallization of the melt | |
|---|---|---|---|---|---|
| Refractive index | 1.58 | predominates \(N_g' = 1.620\), rises to \(N_g' = 1.653\) | \(N_g' = 1.620\text{–}1.660\), predominates \(N_g' = 1.653\) | \(N_g' = 1.640\text{–}1.660\), predominates \(N = 1.660\) | \(N_g = 1.660\), \(N_p = 1.651\) |
| Specific gravity, g/cm³ | 2.76 | 3.13 | 3.15 | 3.19 | 3.20 |
| Crystal habit | fibrous aggregates, fine | fine and coarse-fibrous aggregates | tabular, bar-like crystals | prismatic tabular crystals, polysynthetic twins |
with a high refractive index (Ng = 1.660; Np = 1.651) and specific gravity; it has lower symmetry than protoenstatite and exhibits polysynthetic twinning. This indicates [2] that clinoenstatite is the product of a first-order polymorphic transformation from a higher-temperature form.
Consequently, in the initially precipitating, least dense structure of protoenstatite—which can be fixed by applying rapid supercooling of the melt at the moment of crystallization—more or less profound changes toward densification occur, depending on the degree of supercooling; complete transition to the low-temperature form takes place only upon slow cooling of the melt. The existence of nonequilibrium intermediate structures of MgSiO₃ is allowed on the basis of the lattice features of magnesium metasilicate [3, 4].
A crystallization sequence analogous to that described above can also be observed during heat treatment of glass of MgSiO₃ composition. In accordance with Ostwald’s rule, from glass of the composition studied, the phase that should separate first is one close to that stable at high temperature. But the product of low-temperature crystallization of the glass, at 920°C, is a phase with a high refractive index (1.653). It should be assumed that this phase undergoes transformations toward densification already at the moment of separation, since the process occurs at the temperature of the stable state of clinoenstatite. By stabilizing in a certain way the phase that separates during heat treatment of the glass (by introducing into the batch 30% barium aluminosilicate glass), one can obtain MgSiO₃ crystals of analogous habit, but with a lower refractive index (1.612). Upon firing above 1200°C, the structure of the newly formed phase passes into the stable rhombic form—protoenstatite—which has a fine-grained structure and, for this reason, is not subject to transformations upon cooling.
Conclusion: Phase formation of MgSiO₃ during crystallization of a melt of stoichiometric composition proceeds stepwise, from the less dense structure of protoenstatite to clinoenstatite.
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