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

Mullite-glass Gibbs energy of formation (ΔGeff), 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 (<s>), 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 <s> and, in general, monotonically increase upon firing temperature. ΔGeff has been modeled by α(<s>)×(F/K)2+ß(<s>)×F/K+γ(<s>)(α,ß and γ are linear functions of <s>). The parameters of such a function have been determined by fitting it to the experimental ΔGeffs, inferred from quantitative phase analysis of X-ray diffraction patterns. We have gathered that (i) F/K affects energetics and mullite content more markedly than <s> does, and (ii) the mullite formation is energetically favored by decreasing F/K and increasing <s>.