Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

Abstract

The phase stability in part of the P2O5‐bearing pseudoquaternary system CaO–SiO2–Al2O3–Fe2O3 has been studied by electron probe microanalysis, optical microscopy, and powder X‐ray diffractometry. At 1973–1653 K, the α‐Ca2SiO4 solid solution [α‐C2S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C2S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt‐differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca3Al2O6 solid solution [C3A(ss)]. When the α‐C2S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al2O3 with respect to Fe2O3. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C3A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.