The under certain conditions magnesium can be reduce `SiO_(2)` and silicon can reduce MgO. What are those conditions?

Explanation

Below the melting point of silicon (1693 K), the `Delta_f G^@` curve for the formation of `SiO_2` lies above the `Delta_f G^@` curve for Mgo. Therefore, below 1693 K, Mg can reduce `SiO_2` to Si. However, above 1693 K, the `Delta_f G^@` curve for MgO lies above `Delta_f G^@` curve for `SiO_2` and therefore, at temperatures above 1693 K, Si can reduce MgO to Mg.