STUDY OF BARIUM FELDSPAR POLYMORPHISM AS A FUNCTION OF TEMPERATURE AND CALCIUM CONTENT

A family of new glass-ceramic materials, of the general formula (25-x) CaO.xBaO.yMgO. zAl(2)O(3).5OSiO(2), where x=1, 2, 5, 10, 15, 20, 25, y =20 or 14 and z=5 or 11 (mol%), has been prepared by melting raw mater ials in two parent glasses and performing heat treatments. The systema tic substitution of BaO for CaO in the base glasses allows the effect of feldspars' isomorphism and polymorphism to be studied in a series o f glass-ceramics where the structural environment around the bivalent cations, Ca2+ and Ba2+, is systematically altered. Ba2+ has a large ef fect on the glass transition temperature and dilatometric softening po int, causing a decrease with increasing BaO. The crystalline phases ha ve been identified and found to be dependent on the preparation condit ions, which are the BaO and Al2O3 contents, the heating rate and the s oaking temperature adopted for the crystallization treatments. The inf rared spectroscopy technique helped to identify the different polymorp hs of barium feldspar, that were not clearly distinguishable by X-ray powder diffractometry due to preferred orientations. The kinetic param eters for the formation of the different crystals have also been deter mined and correlated with their thermal stability resumed in the well- known time-temperature-transformation curves.