= Solution
Use the <pressure correction to a mineral reaction> for the solids, and the gas <thermodynamic activity> $f/P_0$ for water. These are separate contributions: do not also add a gas-volume pressure correction after including $RT\ln(f/P_0)$. For this reaction,
$$
\Delta_rG=\Delta H^\circ-T\Delta S^\circ+\Delta V_{\rm solids}(P-P_0)+RT\ln\left(\frac{a_{\rm san}a_{\rm cor}}{a_{\rm mus}}\frac f{P_0}\right).
$$
At $7000\ \mathrm{bar}$ with pure solids and an <ideal gas>, $f=P$ and the solid-activity ratio is one. Therefore
$$
0=98157-164T-0.61(7000-1)+8.3145T\ln7000,
$$
and
$$
\boxed{T_{\rm eq}=\frac{98157-0.61(6999)}{164-8.3145\ln7000}=1038.74\ \mathrm K=765.59\ ^\circ\mathrm C}.
$$
The pressure unit in the solid term is <bar>, matching the supplied $\mathrm{J\,bar^{-1}}$. The gas logarithm is dimensionless because $P/P_0=7000$. High water <chemical fugacity> opposes dehydration and raises the equilibrium <temperature> substantially.
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