Cinnabar (HgS) is unusual because mercury can be obtained from it simply by

1. Roasting some ores directly gives the free metal without a separate reducing agent. 2. For cinnabar: $HgS + O_2 \rightarrow Hg + SO_2$; the sulphide itself is oxidised while the metal is set free. 3. This self-reduction on roasting is called **auto-reduction**. _Source: Samacheer Kalvi Class 12 Chemistry Vol.1, Unit 1 'Metallurgy', §1.3.2 (Auto-reduction)_