Forsterite Crystals Research Articles

The instability of plagioclase in peridotite at high pressure

Experimental crystallization of forsterite + anorthite (2.2: 1 molar ratio) in a piston-cylinder solid-media apparatus shows that forsterite and anorthite are stable at less than 8.5 ± 0.5 kb at 1250°C, but forsterite + orthopyroxene + clinopyroxene + spinel is the stable assemblage at higher pressures. The fayalite + anorthite assemblage is stable to 7–7.5 kb at 1050°C and to 6–6.5 kb at 900°C, and at higher pressure is replaced by almandine-grossular garnet. Reactions between magnesian olivine (Fo 92) and labradorite (An 59), and between olivine and plagioclase in a complex peridotite composition (pyrolite), show that there is a 5-phase field of olivine + orthopyroxene + clinopyroxene + plagioclase + aluminous spinel between 9 kb and 11–14 kb (depending on composition) at 1200°C. At higher pressures plagioclase is absent and at lower pressures pyroxenes are absent or present in smaller amounts, and alumina-rich spinel is absent. The experimental data are applied in discussions of corona formation and of the stability of plagioclase in peridorite compositions.

The Relation Between the Microstructure and the Thermal Expansion of Forsterite Porcelain

The thermal expansion of forsterite porcelain, which is one of the most important properties for use in electron tube, and the effect of different firing temperature, and of repeated heating were investigated. Based on the experimental results the relation between the microstructure and the thermal properties were discussed.The coefficients of linear expansion along each axis of synthesized forsterite were measured by X-ray with a result; a-axis 10.47, b-axis 12.50, c-axis 11.05×10-6, in the temperatures 20°-800°C. These values correspond to that of the forsterite porcelain containing much forsterite.Referring to the data of Navias (J. Am. Ceram. Soc., 37 [8] 329 (1954)) it was pointed out that the elevation of the firing temperature of 25°C would accompany with the increase of the coefficient by 1.0×10-6, and this is the figure being intorelably large for the vacuum sealing with metal.To study this phenomenon many batches of forsterite porcelain were prepared, and fired at different temperatures. The specimen made from talc, sea-water magnesia and BaCO3 showed the marked decrease of the coefficient (about 1×10-6) with the elevation of firing temperature from 1400° to 1500°C. Under microscope was observed the increase of glassy phase with increasing firing temperature, which probably leads to the reduction of the expansion coefficient.The coefficient of the specimens containing MgO and SiO2 in various ratios have also beem measured. Forsterite porcelain having excess of SiO2 (SiO2 47%) showed the increase of the coefficient of about 1.2×10-6 by the elevation of firing temperature from 1400° to 1500°C, which, however, decreased with increasing MgO. The specimens containing excess MgO showed only a very small variation.By microscopic observation of the specimens containing excess SiO2, the forsterite crystal showed a marked aggregation by the elevation of firing temperature, causing the decrease of the coefficient, while in the forsterite containg excess of MgO the aggregation was scarecely recognized, and consequently gave the constant coefficient.Moreover, for many kinds of forsterite porcelain, the variations of the coefficients of linear thermal expansion by the repeated heating were measured. They could be classified into two groups, i.e., one coefficients increased, and the other coefficient did not nearly or entirely increased with repeated heating. The former has more glassy substance than the latter, and it was concluded that such a glassy substance decreased the coefficient by the repeated heating.The authors have confirmed that the composition of forsterite porcelain for use in electron tube should be in excess of MgO and the glassy substance should be reduced as far as possible.

The Relation Between the Microstructure and the Electrical and Mechanical Properties of Forsterite Porcelain

Three kinds of forsterite batches of molar ratios, MgO:SiO2=2.00:0.82 (specimen M), 2.00:1.00 (specimen F), and 2.00:1.05 (specimen S) were calcined at 1300°C, added 5% of Kaolin and fired at various temperatures.Firing shrinkage, apparent specific gravity, water adsorption, dielectric loss, insulation resistance and bending strength of the fired specimens were measured to discuss the relation between the properties and the microstructure.Above physical properties gave the maximum or the minimum values at certain firing temperatures. The specimen M fired at 1550°C, F at 1390°C, and S at 1370°C gave the maximum values of firing shrinkage, apparent specific gravity, Te values of volume resistivity, and bending strength, and the minimum values of water adsorption and dielectric loss.At these maximum or minimum points each specimen showed a similar microstructure being composed of the forsterite crystals of about 10μ. The texture was dense but not yet the perfect mosaic structure which came out by firing at higher temperature through the rapid aggregation of forsterite crystals.In such a specimen cracks were observed in crystal and matrix which leads to the lowering of many properties.Comparing the three specimens the aggregation of forsterite crystals by overfiring and concequently the fall of properties was the most remarkable in the specimen S.The electrical properties of the specimen M were the best, but the proper firing temperature were considerably high.It was suggested that the best composition for forsterite porcelain should lie between M and F.

Influence of Reducing Agents on the Structure of Electro-molten Dunite

The refractories made from dunite shows usually the complicated change of microstructure during the furnace campaign due to the change of temperature and atmosphere. This may lead to the complete collaps of the whole structure, and is the very reason which has checked its practical use.R. L. Stone and also Ikenoue suggest that the iron, existing in the state of solid solution in the mineral, would be the main source of the occurence of such damage. The author has carried out the investigations for the purpose of improving the instability by taking away the iron through the electric melting under reducing condition.The present paper concerns the observation of the author on the process of remove of iron and silica by the electromelting of dunite from Fukushima prefecture. Especially on the influence of the reducing agent (carbon powder and the powder of pig iron) on the structure of electro-molten dunite is discussed. It was confirmed that the size of forsterite crystals in the mass obtained by electric melting reduced their size with the increasing amount of the reducing agent.Above all it would be worthy of noted that the separation of periclase crystals was observed in the mass reduced by iron, and the probable existence of close relation between the compositions of the dust and the ingot obtained by electro-melting was also noticed.

CORROSION RESISTANCE TEST OF CRUCIBLES FOR MICA SYNTHESIS, (II)

Test pieces of the compositions corresponding to several points of the MgO-Al2O3-SiO2 system were prepared and tested for corrosion on the phlogopite melt.The compositions of the test pieces are as follows:SiO2 Al2O3 MgO R2O Ig. loss% % % % %SFA 13.9 52.8 32.1 0.4 1.5SFB 10.7 59.0 30.1 0.3 1.4SP 8.6 45.0 44.7 0.2 2.9PF 24.7 16.8 55.1 0.4 2.5FP 31.0 12.8 53.1 0.6 3.4CM 30.6 63.2 7.0 0.8 0.4Test pieces were moulded by the extrusion method and heated to 1600-1800°C in a cryptol furnace. They were dipped in the synthetic phlogopite melt and maintained at 1400°C for 5 hours. When the specimens were cooled, they were observed with unaided eye and under a petrographic microscope.Specimens of SF composition were composed of granu lar spinel crystals and poikilitic forsterite crystals. They were attacked by the melt, when their degrees of water absorption were greater than 2-3%. But spinel crystals suffered any change and separated into the melt.Specimens of SP composition were composed of granular crystals of spinel and periclase and poikilitic forsterite crystals. They were not attacked appreciably even when the degree of water absorption was 8%. Periclase crystals reacted with the melt to separate forsterite needles, i.e. the melt near the crucible wall was saturated with forsterite, so the forsterite crystals of the wall were attacked no more.Specimens of CM composition were composed of corundum crystals with a small amount of glassy matter. Corundum reacted with the melt to separate spinel crystals. A dense layer of the precipitated spinel crystals was found along the wall of the specimens. The degree of water absorption of the specimen should be smaller than 1-2% to stand for the melt corrosion.Specimens of FP composition were composed of a large amount of granular crystals of forsterite and a little amount of granular crystals of periclase. The matrix of the specimen was attacked very easily. The forsterite crystals became separately and dispersed into the melt finally. Periclase was solved in the melt and forsterite crystals grew larger. The degree of water absorption of the specimen should be less than 4-5% to stand for the melt.