NOTES
3361
and give rise to an invariant point involving the five phases: AIgCO3,MgO, K&Oa, liquid, and vapor. At temperatures above 755", the system is definitely ternary and the probable phase relations are shown in Figure 2e. At 755", there is an isobaric invariant point involving 1fgC03,MgO, vapor, and liquid (Figure 2d). This represents one point on a univariant pressuretemperature curve for the dissociation of RIgC03 by the reaction MgC03 --t MgO vapor, and as this temperature falls near the value for the dissociation curve determined by Harker and T ~ t t l e ,it~ is asassumed that there is very little, if any, solid solution of I
