Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) which also occurs naturally as the mineral hematite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙ Fe2O3. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black pigment. For this purpose, it is synthesized rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production. Preparation
Heated iron metal interacts with steam to form iron oxide and hydrogen gas.
This process is described by the Schikorr reaction:

  
    
      
        
          
            
              
                3
                
                Fe
                
                  
                    (
                    OH
                    )
                  
                  
                    2
                  
                  
                    
                  
                
              
              
                ferrous
                 
                hydroxide
              
            
          
          ⟶
          
            
              
                
                  Fe
                  
                    3
                  
                  
                    
                  
                
                
                  O
                  
                    4
                  
                  
                    
                  
                
              
              magnetite
            
          
          +
          
            
              
                H
                
                  2
                
                
                  
                
              
              hydrogen
            
          
          +
          
            
              
                2
                
                
                  H
                  
                    2
                  
                  
                    
                  
                
                O
              
              water
            
          
        
      
    
    {\displaystyle {\ce {{\underset {ferrous\ hydroxide}{3Fe(OH)2}}->{\underset {magnetite}{Fe3O4}}+{\underset {hydrogen}{H2}}+{\underset {water}{2H2O}}}}}
  This works because crystalline magnetite (Fe3O4) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH)2 ).The Massart method of preparation of magnetite as a ferrofluid, is convenient in the laboratory: mix iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl3·6H2O and FeCl2·4H2O with vigorous stirring at about 2000 rpm.