| Abstract: | On the Modification Change of SiO2 in Presence of Alkali Fluorides and Silicates The influence of NaF and Na2SiF6 on SiO2 and sodium silicates was investigated. The SiF4, developed in this process, influences the reaction as shown by a comparison of experiments in open and closed systems. With low Pressures of SiF4 the reactions of NaF or Na2SiF6 with certain SiO2 modifications lead to sodium silicates, in presence of a perceptible pressure of SiF4 to cristobalite and tridymite and also to low-quartz. Sodium silicates react with NaF mainly to Na2SiO3. Na6Si8O19 could be obtained from Na2SiO3 only when NaF was replaced by Na2SiF6. In this case additional low-quartz was formed. The formation of tridymite, only possible in the presence of certain foreign ions, is attainable at high temperatures (1150°C) if HF is added together with an alkali fluoride (A = Na, K, Rb, Cs). The amount of fluoride added was decisive for the size of the crystals of tridymite, which were prepared by chemical transport. The impurities of the used low-quartz (Furka) are not sufficient for the formation of tridymite. |
