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• Physical Properties

  Black tetragonal crystal; exhibits two allotropic modifications—a stable alpha phase, occurring in tetragonal crystalline form (as hausmannite) and an unstable beta modification; Moh’s hardness 5.5; insoluble in water; soluble in hydrochloric acid. ;

• Occurrence and Use

  Manganese(II,III) oxide occurs in nature as the mineral hausmannite [1309-55-3]. It is used to make ferrites and thermistors. The oxide also is used in the thermite process for producing manganese. ;

• Preparation

  Manganese(II,III) oxide is made by heating manganese(IV) oxide, MnO2, or manganese(III) oxide, Mn2O3, above 950°C. When Mn2O3 is heated in air, the temperature should be above 940°C, but if heated in oxygen, the temperature should be above 1,090°C. Also, heating manganese(III) oxide at 230°C in hydrogen yields Mn3O4. However, further heating above 300°C converts Mn3O4 formed to green manganese(II) oxide, MnO.
  Manganese(II,III) oxide also is obtained by heating the dioxide, MnO2, with carbon at 600 to 700°C. ;

• Reactions

  Manganese(II,III) oxide reacts with dilute acids forming the corresponding manganous salt and manganese(IV) oxide, MnO2:
  Mn3O4 + 2H2SO4 → 2MnSO4 + MnO2 + 2H2O
  The oxide, however, dissolves slowly in cold sulfuric acid forming a red solution that also contains manganic sulfate, Mn2(SO4)3:
  Mn3O4 + 4H2SO4 → MnSO4 + Mn2(SO4)3 + 4H2O
  Reaction with acetic acid yields manganese(II) acetate and manganese(III) oxide:
  Mn3O4 + 2CH3COOH → Mn(CH3COO)2 + Mn2O3 + H2O
  Manganese(II,III) oxide is reduced to manganese metal when heated with powdered aluminum (the Thermite process). The reaction is vigorous and exothermic:
  3Mn3O4 + 8Al → 4Al2O3 + 9Mn ;

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