Microstructural Behaviour of Dissolution of Quartz Grains into Feldspathic Fusions

Abstract

In his previous paper, the writer has reported a quantitative investigation on the behaviours of dissolution of coarse quartz gains into feldspathic fusions, one of the most important reactions occurring in porcelain bodies during firing (this Journal 63 [712] 432 (1955)). The present one gives the results of a study on the same subject from a microstructural point of view.By firing the small tablets consisting of 97% fine ground potash-, or soda-feldspar and 3% coarse quartz grains (Table 1), one fired at 1150°-1450°C, with heating rates of 60°, 120°C, and 180°C per hr., and the other with 0.5-4 hrs. soaking at 1300°, 1350°, or 1400°C. They were studied with aid of polarization microscope, and occasionally by means of X-ray.The results obtained were summarized as follows:(1) The reaction between quartz and feldspar started from a little lower temperatures than 1150°C, with the formation of very thin reaction layers between them.(2) Refractive index distributions of glasses formed in the fired bodies were measured. From the results obtained, it is inferred that the uniformity of the melting phases formed in the bodies of quartz-potash-feldspar system were raised with the increase of firing temperatures and holding times (Fig. 4), and that in the case of the bodies of quartz-soda-feldspar system, unhomogeneity of the phases was, on the contrary, rather increased with their SiO2 content being also gradually increased with temperature and time (Fig. 5). These differences were mostly due to the difference of diffusion velocity of SiO2 which entered into the melting phases accompanied with dissolution of quartz grains.(3) In soda-feldspar fusions, the diffusion layers formed around the quartz grains temporarily showed double construction layers (Fig. 1-9), and, with soaking time, these layers gradually changed into continuous ones with the increase of their thickness. The temporary formation of these double diffusion layers was due to the fusion of soda-feldspar itself, whose fusion behaviour showed a marked change at a certain temperature range.(4) Quartz grains kept their phases metastablly until they dissolved at 1150°-1450°C., and converted to neither cristobalite nor tridymite, though a weak pseudo biaxial interferrence figures which were perhaps owing to the stresses formed with rapid cooling, were observed (Table 2). In the diffusin layers and the glassy phases, cristobalite or tridymite were also not found.