Roasting of an ore of a metal usually results in the conversion of the metal sulphide into oxide. Why does the roasting of cinnabar (HgS), produce metallic mercury and not its oxide ?

Explanation

HgS upon roasting with excess of oxygen forms HgO. Since the `DeltaG^(@)` for the formation of HgO is positive (See Ellingham diagram 6), it is unstable and decomposes of its own under the reaction conditions to form metallic mercury.
` 2HgS +3O_(2) overset("Heat") to 2HgO+2SO_(2)`
`2HgO overset("Heat") to 2Hg+O_(2)`