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

Na-fluorphlogopite glasses were prepared from natural and commercial raw ma terials. Substitutions of Ca or Zn for Mg in stoichiometric Na-fluorphlogop ite decrease the transformation temperature and increase the crystallizatio n range. In the parent glass, the fluormica grows at the early stage of cry stallization 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 togeth er 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 l ed 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 additio n to Mg-fluorrichterite were developed at higher temperatures. On increasin g the ZnO/MgO m.r., devitrified opalescence glass was obtained. Improvement of the chemical durability of the investigated samples towards distilled w ater were achieved by crystallization of fluorrichterite and cristobalite o r the development of the fine grain microstructures. The coefficients of th ermal expansion (CTE) values of the crystalline glasses were increased by t he substitution of MgO by CaO or ZnO. (C) 2001 Elsevier Science B.V. All ri ghts reserved.