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.
The reaction conditions are critical to the process and determine the particle size.Iron(II) carbonate can also be thermally decomposed into Iron(II,III):
3FeCO3 → Fe3O4 + 2CO2 + CO

Reactions
Reduction of magnetite ore by CO in a blast furnace is used to produce iron as part of steel production process:

  
    
      
        
          
            
              Fe
              
                3
              
              
                
              
            
            
              O
              
                4
              
              
                
              
            
          
          +
          4
          
          CO
          ⟶
          
            3
            
            Fe
          
          +
          4
          
          
            CO
            
              2
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {{Fe3O4}+ 4CO -> {3Fe}+ 4CO2}}}
  Controlled oxidation of Fe3O4 is used to produce brown pigment quality γ-Fe2O3 (maghemite):

  
    
      
        
          
            
              
                
                  2
                  
                  
                    Fe
                    
                      3
                    
                    
                      
                    
                  
                  
                    O
                    
                      4
                    
                    
                      
                    
                  
                
                ⏟
              
            
            
              magnetite
            
            
              
            
          
          +
          
            
              
                1
                2
              
            
            
              O
              
                2
              
              
                
              
            
          
          ⟶
        
         
        
          
            
              
                
                  
                    
                      3
                      
                      
                        (
                        γ
                        
                          −
                        
                        
                          Fe
                          
                            2
                          
                          
                            
                          
                        
                        
                          O
                          
                            3
                          
                          
                            
                          
                        
                        )
                      
                    
                    ⏟
                  
                
                
                  maghemite
                
                
                  
                
              
            
          
        
      
    
    {\displaystyle {\ce {\underbrace{2Fe3O4}_{magnetite}+ {1/2O2}->}}\ {\color {Brown}{\ce {\underbrace{3(\gamma-Fe2O3)}_{maghemite}}}}}
  More vigorous calcining (roasting in air) gives red pigment quality α-Fe2O3 (hematite):

  
    
      
        
          
            
              
                
                  2
                  
                  
                    Fe
                    
                      3
                    
                    
                      
                    
                  
                  
                    O
                    
                      4
                    
                    
                      
                    
                  
                
                ⏟
              
            
            
              magnetite
            
            
              
            
          
          +
          
            
              
                1
                2
              
            
            
              O
              
                2
              
              
                
              
            
          
          ⟶
        
         
        
          
            
              
                
                  
                    
                      3
                      
                      
                        (
                        α
                        
                          −
                        
                        
                          Fe
                          
                            2
                          
                          
                            
                          
                        
                        
                          O
                          
                            3
                          
                          
                            
                          
                        
                        )
                      
                    
                    ⏟
                  
                
                
                  hematite
                
                
                  
                
              
            
          
        
      
    
    {\displaystyle {\ce {\underbrace{2Fe3O4}_{magnetite}+ {1/2O2}->}}\ {\color {BrickRed}{\ce {\underbrace{3(\alpha-Fe2O3)}_{hematite}}}}}

Structure
Fe3O4 has a cubic inverse spinel group structure which consists of a cubic close packed array of oxide ions where all of the Fe2+ ions occupy half of the octahedral sites and the Fe3+ are split evenly across the remaining octahedral sites and the tetrahedral sites.