${\mathrm{MnO}}_2$ is the most important oxide of maganese. It occursnaturally as the black coloured mineral pyrolusite. It is an oxidising agent, and decomposes to ${\mathrm{Mn}}_3{\mathrm{O}}_4$ on heating to530°C. It is used in the preparation of potassium permanganate and in the production of Cl2gas. Over half a million tonnesper year of ${\mathrm{MnO}}_2$ is used in dry batteries.