Upon consideration of the SiO2-Al2O3 phase diagram in Figure 12.25, for each pair of the following list of compositions, which would you judge to be the more desirable refractory? Justify your choices.

(a) 99.8 wt% SiO2-0.2 wt% Al2O3 and 99.0 wt% SiO2-1.0 wt% Al2O3

(b) 70 wt% Al2O3-30 wt% SiO2 and 74 wt% Al2O3-26 wt% SiO2

(c) 90 wt% Al2O3-10 wt% SiO2 and 95 wt% Al2O3-5 wt% SiO2