Effects of particle size distribution and starting phase composition in Na-feldspar/kaolinite system at high temperature

Mullite-glass Gibbs energy of formation (Geff), micro-texture and phase composition evolution are investigated in the Na-feldspar (F) and kaolinite (K) system, over the 1240-1320 °C interval, as a function of the starting F/K ratio by weight and particle size distribution of F (), using scanning electron microscopy, X-ray diffraction and thermodynamic modeling. Electron microscopy images show that size and aspect ratio of primary and secondary mullite have their largest figures for the smallest and, in general, monotonically increase upon firing temperature. Geff has been modeled by a()×(F/K)2+b()×F/K+g() (a, b and ? are linear functions of ). The parameters of such a function have been determined by fitting it to the experimental Geffs, inferred from quantitative phase analysis of X-ray diffraction patterns. Note that we used average Geff-values over the T-range explored because of the modest dependence on temperature shown. We have gathered that (i) F/K affects energetics and mullite content more markedly than does, and (ii) the mullite formation is energetically favored by decreasing F/K and increasing .