ON THE POLYMORPHISM OF MgSiO₃ IN STEATITE CERAMICS

P. G. USOV, V. I. GURINA, I. V. SOBOL

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

The crystalline phase of the ceramic under study has a purely metasilicate composition, represented by two modifications of MgSiO₃—clinoenstatite and protoenstatite. It is known that steatite ceramics, during storage and service of products, are subject to aging; the aging mechanism is explained by the spontaneous transformation of protoenstatite into clinoenstatite, accompanied by a change in the parameters of the unit cell.

One of the factors considered [1] to exert a substantial influence on the structural stability of a steatite body is the particle size of the crystalline phase.

When the material composing a steatite-ceramic body is examined under a microscope, an interesting regularity is observed in the distribution of the crystalline forms of MgSiO₃ according to the size of their grains: protoenstatite is always recorded in the form of very fine-crystalline aggregates, with individual grains no larger than two or three microns. The refractive index of such aggregates in a mixture with glass is 1.612 and lower, down to 1.605. Clinoenstatite is usually encountered in the form of large, idiomorphic crystalline formations ranging in size from 5 to 100–150 microns, with refractive index \(N_g' = 1.658—1.660\). In the ceramic composition the metasilicate is present in an overwhelming amount in the form of protoenstatite. The clinoenstatite grains are isolated in character.

At present, the works of Atlas [3], Foster [2], and others have shown that under normal conditions metasilicate is stable in the form of clinoenstatite. Upon heating, it transforms into the high-temperature form—protoenstatite. On the derivatogram of clinoenstatite (Fig. 1), this is indicated by an endoeffect at \(1100^\circ\text{C}\). This transformation is reversible (an exoeffect on the cooling curve). The degree of reversibility is determined by a number of conditions, in particular:

1) particle size;
2) cooling rate;
3) degree of overheating of the high-temperature phase;
4) presence of a glass phase.

In the present work the task was set to study how the sizes of the particles of the crystalline phase affect the course of structure formation during firing and the structural strength of the body during storage of products. The study was carried out on steatite bodies synthesized from separately obtained crystalline and glassy components.

Fig. 1. Derivatogram of clinoenstatite, fraction finer than 5 microns: 1 — heating curve, 2 — cooling curve, 3 — temperature curve.

…ing. The crystalline phase, 70% of the total weight of the batch, was introduced in the form of clinoenstatite having a definite particle-size composition. The glass phase, in an amount of 30%, was represented by a barium aluminosilicate glass corresponding in chemical composition to the amorphous component of the ceramic. The grain composition of the clinoenstatite for each batch is given in Table 1. The glass was ground until it passed completely through sieve No. 100. The components were thoroughly mixed; specimens for X-ray and optical investigations were formed as disks 25 mm in diameter, using a binder consisting of a solution of paraffin in kerosene. Firing was carried out at 1150°, 1200°, and 1250°C. The holding time at the final temperature was 3 hours. The specimens were cooled in air. This “quenching” was intended to fix the high-temperature structure.

After firing, protoenstatite is found in all specimens. The transition from low-temperature clinoenstatite to the high-temperature form is associated with a partial rearrangement of the structural elements in the lattice, which is reflected in the X-ray diffraction patterns of the material by shifts of certain diffraction maxima ($d = 2.11$ Å, $1.52$ Å for clinoenstatite and, correspondingly, $d = 1.97$ Å and $1.49$ Å for protoenstatite). Indexing of the metasilicate phases was carried out mainly from these maxima.

Under the microscope, this rearrangement is recorded by the following changes in structure: the clinoenstatite grain breaks up within

Table 1

Grain composition of the steatite batches studied

of their boundaries on the finer parts, forming aggregates of fine-crystalline grains. The refractive indices in this case decrease from \(N_g' = 1.658\text{—}1.660\) to \(N_p = 1.610\text{—}1.616\). The X-ray diffraction pattern of the new structure is represented by protoenstatite lines. With an increase in the firing temperature, the protoenstatite content in the specimens increases.

Thus, the body of mass 1 (maximum grain size of the crystalline phase 5 microns), after firing at \(1200^\circ\)C, is represented by an almost monomineral protoenstatite structure. In specimens made from coarser fractions, the amount of protoenstatite decreases as the initial grain size of the crystalline phase increases. After firing at \(1250^\circ\), protoenstatite predominates in all specimens. The body made from mass 1 is characterized by a homogeneous fine-crystalline structure of protoenstatite cemented by a uniformly distributed glass phase. The body made from mass 2 consists predominantly of fine-grained protoenstatite with individual inclusions of larger (20—50 µm) grains of clinoenstatite. The distribution of the glass phase is uniform.

Bodies of masses 4 and 5 after firing have a mixed structure; protoenstatite is recorded in the form of individual fine-grained aggregates, clinoenstatite in the form of large grains; the glass forms accumulations and is distributed nonuniformly.

The structural stability of the ceramic during storage is determined entirely by the initial grain size of the crystalline phase. The phase composition of the specimens was studied after specified storage periods (Table 2).

Table 2

Phase composition of the crystalline constituent of ceramic masses after firing and storage under normal conditions

Ceramics made from mass 1 (maximum grain size 5 µm) undergo no structural changes at all. The X-ray diffraction patterns taken from the specimen immediately after firing and after storage for 1 year are completely identical.

The structural strength of the bodies decreases as the initial grain size increases. Thus, ceramics made from mass 4 (crystalline-phase grain size 100—250 µm) acquired an exclusively clinoenstatite composition already after two weeks of storage. Under the microscope, the transformation of protoenstatite into clinoenstatite appears as follows: within the boundaries of large protoenstatite aggregates, individual grains of which were not surrounded and stabilized by glass—

Fig. 2, 3. Change in the intensity of diffraction maxima on X-ray patterns: 1—clinoenstatite \((d = 2.11\,\text{Å})\); 2—protoenstatite \((d = 1.97\,\text{Å})\) after firing and storage of the specimens. — mass 1, —×— mass 2, —●— mass 3, —▲— mass 4.

Visible labels in the plots: \(J_{\text{rel}}\); \(T^\circ C\); 1150, 1200, 1250; “1 year of storage”; panels 1 and 2.

, \(10M\), clinoenstatite crystals are formed, corresponding to the initial grain sizes.

Graphs 2–3 present curves of the change in the intensity of the maxima \(d=2.11\) for clinoenstatite and \(d=1.97\) for protoenstatite as a function of firing temperature and the time of subsequent storage of the products. It is evident from the graphs that formation of the structure of the steatite body does not end at the firing stage, but continues for a very long time during storage and service of the products. This consists in the protoenstatite present in the ceramic body in a metastable state gradually transforming into the stable low-temperature form—clinoenstatite.

Barium glass, provided it is uniformly distributed among the protoenstatite grains, stabilizes the structure well, ensuring that the ceramic does not age, at least for 1 year. Glass not stabilized by external action, containing aggregate accumulations of protoenstatite in bodies with a coarser initial fractional composition of the crystalline phase, transforms within a short period of time into clinoenstatite; the ceramic ages.

Thus, the mechanism of aging consists in the transformation, under normal conditions, of metastable protoenstatite with crystal sizes greater than \(5\,\mu\) into clinoenstatite, the low-temperature form of \(\mathrm{MgSiO_3}\).

LITERATURE

1. Büssem W., Schusterius C., Stuckardt K. “Über die Konstitution des Steatits,” wiss Veröffen Siemens werken, 17 [1] 77—89 (1938).

2. W. R. Foster. “High-Temperature X-Ray Diffraction Study of the Polymorphism of MgSiO3.” Jour. Amer. Ceram. Sos. 34 [9], 255—59 (1951).

3. L. Atlas. Polymorphism of MgSiO3 and Solid-State Equilibria in the System MgSiO3—CaMgSi2O6, Jour. Geol., 60 [2], 125—47 (1952).