Solution
= Solution
The three stable fields are
$$
\boxed{\begin{aligned}
\text{nepheline: }&x<-5.515,\quad y>3.44-x,\\
\text{albite: }&x>-5.515,\quad y>-7.59-3x,\\
\text{corundum: }&y<\min(3.44-x,-7.59-3x).
\end{aligned}}
$$
The bounding equalities give the stable two-mineral coexistences. In particular, the stable <corundum> region lies below the lower of the two oblique lines; treating every line extension as stable would mislabel parts of the <mineral activity diagram>.