= Solution
For pure phases at the reference <pressure> $P_0=1\ \mathrm{bar}$, the <Gibbs energy of reaction> is $\Delta_rG=\Delta H^\circ-T\Delta S^\circ$. <Chemical equilibrium> requires $\Delta_rG=0$, so
$$
\boxed{T_{\rm eq}=\frac{98157}{164.0}=598.52\ \mathrm K=325.37\ ^\circ\mathrm C}.
$$
The reaction is <muscovite> dehydration to <sanidine>, <corundum> and water; its positive <standard entropy of reaction> makes the product assemblage favored at high <temperature> under this constant-property approximation.
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