Crystallization of sodium fluormica Na(Mg, Zn, Ca)2.5Si4O10F2 glasses

Na-fluorphlogopite glasses were prepared from natural and commercial raw materials. Substitutions of Ca or Zn for Mg in stoichiometric Na-fluorphlogopite decrease the transformation temperature and increase the crystallization range. In the parent glass, the fluormica grows at the early stage of crystallization which transforms into Mg-fluorrichterite at high temperature. Crystallization of the sample containing CaO/MgO in molar ratio (m.r.)=0.25 did not enhance Na-fluorphlogopite, but catalyzes fluorrichterite together with cristobalite. Devitrification of the glass takes place when CaO/MgO m.r. is >0.25. Substitution at the ZnO/MgO m.r.=0.25 in the base glass led to the crystallization of the Na-fluorphlogopite, whereas zinc silicate, Zn1.9SiO4 and Mg-fluorrichterite phases were formed at higher temperature or for long duration heat treatment. Crystallization at ZnO/MgO m.r.=0.66 begin with Na-fluorphlogopite and cristobalite, while willemite in addition to Mg-fluorrichterite were developed at higher temperatures. On increasing the ZnO/MgO m.r., devitrified opalescence glass was obtained. Improvement of the chemical durability of the investigated samples towards distilled water were achieved by crystallization of fluorrichterite and cristobalite or the development of the fine grain microstructures. The coefficients of thermal expansion (CTE) values of the crystalline glasses were increased by the substitution of MgO by CaO or ZnO.