= Solution
The stable fields are
$$
\boxed{\begin{array}{ll}
\text{corundum:}&y<\min(3.44-x,-7.59-3x),\\
\text{nepheline:}&x<-5.515,\quad y>3.44-x,\\
\text{albite:}&x>-5.515,\quad y>-7.59-3x.
\end{array}}
$$
Thus <corundum> occupies the low-$y$ field, <nepheline> the upper-left field and <albite> the upper-right field. Pairwise coexistence applies on the indicated stable boundaries, and all three coexist at $(-5.515,8.955)$. These are stability statements within the three-mineral chemical system, not a claim that other possible minerals have been excluded in nature.
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