Phase Equilibrium and Chromium Partitioning in the Al ₂ O ₃ ‐SiO ₂ ‐MgO‐CaO‐CrO _x System Under a Reducing Condition

ABSTRACT The equilibrium phase relations and chromium partitioning in the Al 2 O 3 ‐SiO 2 ‐MgO‐CaO‐CrO x system were investigated at 1600°C and p O 2 of 10 −10 atm. The high‐temperature isothermal equilibration technique, followed by rapid quenching and direct phase composition analysis using electron probe microanalysis, was employed. Two‐phase equilibria of liquid‐corundum and liquid‐spinel, as well as three‐phase equilibria of liquid‐cristobalite‐corundum and liquid‐spinel‐corundum, were determined. To quantify the effects of Al 2 O 3 level and (MgO+CaO) on the primary phase fields of Cr‐rich components, the CrO x ‐SiO 2 ‐(MgO+CaO) pseudo‐ternary phase diagrams were constructed at Al 2 O 3 = 7, 11, and 17 wt% based on the experimentally determined liquid and solid compositions. It was found that the Al 2 O 3 content had no significant effect on either the spinel primary phase field or the liquid‐spinel‐corundum three‐phase equilibrium region. However, with increasing Al 2 O 3 content from 7 wt% to 17 wt%, the isotherm for the corundum primary phase field shifted toward the CrO x ‐SiO 2 side at higher CrO x contents, thereby enlarging the liquid region. Chromium incorporation in the spinel decreased as (MgO+CaO), and the Al 2 O 3 content of the liquid increased. Overall, chromium partitioned preferentially into spinel and corundum relative to the liquid, whereas the spinel‐liquid partitioning was more sensitive to Al 2 O 3 level and (MgO+CaO) in the liquid, consistent with decreasing chromium incorporation in Mg(Cr,Al) 2 O 4 spinel as the melt became more basic and Al‐rich, due to the enhanced Al for Cr substitution coupled with reduced dissolved Cr availability.