On Diffusion Layers formed around the Quartz Grains dissolving into Feldspathic Fusion

Abstract

In his previous papers, the writer has shown the presence of diffusion layers around the quartz grains which were under the course of dissolution into the feldspathic fusion. In order to make clear the actual mater of the layers, microscopical measurements were made as reported in this paper. On the glass formed around the remaining quartz grain, as shown in Fig. 1, the refractive index, n, and distance from the quartz surface, t, were measured microscopically, as many as possible, and then these data were plotted.From the diagrams (Figs. 2 and 3) thus obtained, the following results are concluded.(1) Gradients of refractive index are found around the quartz grains. In the quartz-feldspar system, refractive indices of glasses formed are functions mostly of the amounts of quartz dissolved, i.e., the concentration of SiO2, so that the presence of refractive index gradient correspond to the presence of concentration gradient of SiO2, suggesting the formation of the diffusion layers around the quartz grains.(2) In the fusion of potash feldspar, the diffusion layers are continuous and of linear gradient, while in the fusion of soda feldspar, there appeared temporarily two layers s howing a discontinuity; but in the course of prolonged soaking time this discontinuity is gradually diminished, and they become one layer.(3) The higher the holding temperature is, the thiner the diffusion layer becomes.(4) In the stationary state, the thickness of diffusion layers formed in the potash feldspar fusion is of the same order with the one formed in the soda feldspar fusion, only the former being somewhate thicker.(5) The temporary formation of the double diffusion layers in the fusion of soda feldspar, as described in (2), may due to the singular fusion of the soda feldspar (plagioclase) used, by which the formation of melt is fast in earlier stage and then much slower down at a certain temperature range.(6) Furthermore, it is inferred that the dissolution velocity of quartz grains into the feldspathic fusion is determined by the diffusion velocity of SiO2 in the diffusion layers.