In order to study the solubility of quartz into the feldspathic melts, and to measure the refractive index of the glass thus formed, powders of fired specimens of potash-, or soda- feldspar, together with the specimens of feldspar with various amounts of quartz (Table 2), were microscopically observed.
The results obtained are summarized as follows:
(1) Soda feldspar, originally of low temperature form, does not invert into high temperature form until its dissolution temperature which is usually higher than 1150°C.
(2) The amount of quartz dissolved into feldspathic melt becomes larger in accordance with the increase of firing temperature, and the firing time. The solubility is shown in Table 3.
(3) The velocity of dissolution of quartz grains is faster into potash feldspathic melt than into soda feldspathic one, while the total amount of quartz dissolved into the melt is a little larger in the latter than in the former.
(4) The refractive indices of potash-, and soda-feldspar glasses melted at 1260-1280°C are 1.488 and 1.490 respectively.
(5) In the glass of feldspar-quartz system, the larger the quartz content is, the lower the refractive index becomes. The relationship between the refractive index of the glass and its quartz content is shown in Fig. 14.
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