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Magnetic Minerals

The main magnetic minerals in Earth rocks are: 

The main magnetic parameters that characterized the minerals listed above and that allow their identification in rocks are:
  • the Curie temperature;
  • the temperature-dependent phase transition ;
  • the hysteresis parameters. They include:
    •  the saturation magnetization (that is the magnetization measured while the saturating magnetic field is applied: by unit mass Js, by unit volume Ms)
    • the saturation magnetic remanence (that is the remanent magnetization left, measured in zero field, after the saturation magnetic field is removed: by unit volume Mr)
    • the magnetic induction needed to saturate the magnetization (Bsat)
    • the coercive force (Bc)
    • the coercivity of remanence (Bcr)

Magnetite - Fe3O4

 
Figure 1 
 
Figure 2 
Magnetite is a cubic mineral with an inverse spinel crystal structure. It is ferrimagnetic and it undergoes a crystallographic phase transition (from cubic to monoclinic, fig.1, 2) at 110-120 K, called Verwey transition.
 
Curie temperature: 580o C
Js = 90 - 93 Am2/kg
Ms = 480 kA/m
Bsat = 0.1 - 0.3 T
Bc = (30) 20 - 0.1 mT
Bcr = (50) 35 - 10 mT
 
The hysteresis parameters vary with grain size and magnetite state (SD=single domain, PSD=pseudo single domain, MD=multi domain, Fig. 3):
 
Critical grain size (for equidimensional magnetite):
SPM --> SD : » 0.035 microns
SD --> PSD : 0.05 - 0.08 microns
PSD --> MD : 15 - 20 microns
Figure 3 

 

Hematite - α Fe2O3

 
Figura 4

Trigonal (rhombohedral) crystal structure, like corundum, fig. 4,5.
Canted antiferromagnetic.
Phase transition at -10 , -20o C (Morin transition).
Néel temperature: 680o C
 
Js = 0.5 Am2/kg
Ms = 2.5 kA/m
Bc = 40 - 175 mT
Bcr = 270 - 535 mT
Bsat = 1.5 - 5 T
Figura 5
 

 

Maghemite - γ Fe2O3

 
Cubic crystal structure, like magnetite.
Ferrimagnetic.
Curie temperature = 590 - 675o C
It converts to ematite upon heating, at a variable Ti : 250oC < Ti < 900oC (often Ti = 350oC for grains with dimension < 1 x 10-6 m)
 
Js = 70 - 80 Am2/kg
Ms = 355 - 405 kA/m
Bc = 6 - 9 mT     
Bcr = 17 - 23 mT       
Bsat = 0.1 - 0.2 T

 

Goethite - α FeO.OH

 

 

Figure 6 

 

Figure 7 
Orthorombic crystal structure, fig. 6, 7, 8.
Antiferromagnetic.
Néel temperature: 120o C (55-130o C)
It converts to ematite during diagenesis.
 
Js < 1 Am2/kg
Ms < 2 kA/m
Bc = 25 - 252 mT           
Bcr = 504 - 4025 mT
Bsat = ----
Figure 8 

 

Pyrrhotite - FeS1+x ; 0 <= x <= 0.14


 
Fe7S8 : Monoclinic crystal structure.
Ferrimagnetic. 
Phase transition at 34K.
TCurie: 320-325oC, fig. 9
 
Fe9S10 : Hexagonal crystal structure. 
Antiferromagnetic at 20oC.
Phase transition λ at 200-210oC.
TCurie: 275-295oC, fig. 10
 
Js = 5 - 20 Am2/kg
Ms = 25 - 95 kA/m
Bc = 9 - 100 mT           
Bcr = 10 -110 mT           
Bsat = 0.5 -1 T

 

Figure 9 

 

Figure 10 

 


 

Greigite - Fe3S4

 
Cubic “inverse spinel” crystal structure, like magnetite, fig. 11.
Ferrimagnetic.
It decomposes with heating. Maximum temperature of decomposition: 380o C
 
Sketch of the ½ of a unit cell of greigite (Fe3S4) in fig. 11. The crystalline structure of greigite is basically the same inverse spinel structure typical for magnetite (Fe3O4) in which S2- ions substitute O2- ions. Cations are both in tetrahedral (A-site) and octahedral (B-site) coordination with S2- ions. In greigite the easy axis of magnetization is the <100> crystallographic axis.
 
Js = 20-25 Am2/kg
Ms = 80-125 kA/m
Bc = 10 - 70 mT           
Bcr = 45 - 95 mT          
Bsat = 0.2 - 0.3 T
 
Figure 11 
  
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