MnO₂ is the most important oxide of manganese. MnO₂ occurs naturally as the black cultured mineral pyrolusite. It is an oxidising agent, and decomposes to  ${\mathrm{Mn}}_3{\mathrm{O}}_4$ on heating to 5300 C. It is used in the preparation of potassium permanaga-nate and in the production of Cl₂ gas. Over half a million tonnes per year of MnO₂ is used in dry batteries