On the Matrix Phase and Pores in the Bodies Consisting of Feldspar and Kaolin

Abstract

In succession to the investigation on the crystallization of mullite needles in large feldspar particles this paper concerns on the microstructure of the bodies consisting of coarse feldspar grains embedded in kaolinite powder.Complicated Becke's lines were observed at the boundary between the molten feldspar grain and the matrix phase suggesting that there was diffusion between the two phases.The photomicrograph (photo 2) indicates that the molten feldspar showed the trend of approaching each other by forming the veins in the matrix, or by the deformation of the grains.During these processes the pores included in the grains joined together to become larger (photo 2, 3). At higher temperatures the reduction of viscosity made the reaction more and more rapid until the bubbles suddenly open. This may be a mechanism of the formation of bubbling in over-fired bodies.Feldspar grains locating next to cracks indicate that the molten mass pushed into the cracks through the veins by the pressure of expansion of feldspar with melting and pores in the grains.It was thus observed that the pores near the cracks are larger than ordinary size, suggesting that the bubbles in the molten feldspar had the potential power of further expansion puhsing out the matrix, changing the shape of the grains to promote the intrusion of molten feldspar through the veins.If the body was heated further after the cracks had closed the expanding gases found their way in the weak points to form large pores in the feldspar grains locating near the cracks. It is highly probable that a part of pores in porcelain bodies were formed in this way.The needle crystals came out earlier and far greater in number in feldspar melts being sandwiched in the crack.In a few specimens fired at as high as 1450°C it was observed that the needle crystals were concentrating at the surface of large bubbles showing that they were pushed back by expanding gases after the deposition of the crystals. It was also observed that the crystals were accumlating at the surface which was pushed back by expanding gases enclosed in the crack. This is the positive proof of the author's conclusion that the needle crystals in feldspar appears in the heating up period.Moreover, it was concluded that the deposition of the crystals in molten feldspar begins in the heating up period when the expansion of gases and the penetration of feldspar into the crack have just come to an end.