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Rock Magnetism


 

The research field of rock magnetism includes the study of all those basic magnetic properties of rocks that are not directly represented by their “fossil” (or remanent) magnetism, that constitutes the object of paleomagnetism. Among such properties the most widely used are the magnetic susceptibility (k), the hysteresis properties (in particular, the determination of the coercivity spectra, the saturating field and the saturation magnetization, fig. 1), a variety of artificially imparted remanences produced in the laboratory (isothermal remanent magnetization, IRM, anhysteretic remanent magnetizatio, ARM) e the variation of the magnetic state with temperature (determination of the Curie/Néel temperature and of the magnetic phase transitions). 
 
Figure 1 ihysteresis properties

Magnetic parameters and units (SI)

 

Parameter Unit Comments
B T Magnetic induction
H A/m Magnetic field
m Am2 Magnetic moment
N adimensionale Demagnetizing factor
λ adimensionale Magnetostriction constant
K, Kl, Ku J/m3 Anisotropy costant

 

Parameter Volume-specific Mass-specific Comments
RM A/m Am2/kg Magnetizzation (NRM, IRM, ARM...)
k -   Magnetic susceptibility
χ   m3/kg Magnetic susceptibility
kARM - m3/kg ARM acquired per unit of steady field

 

Raptios Units Comments
S-ratio - IRM-0.3T/SIRM
ARM/k; IRM/k Am2/kg   A/m (mass-specific RM)   (volume-specific RM)
ARM/χ; IRM/χ A/m (mass-specific RM)  
ARM/SIRM adimensionale  
kARM/χ adimensionale (mass-specific kARM)

 

The whole of the above mentioned magnetic properties allows to determine the magnetic mineralogy of rocks and is fundamental for a correct interpretation of paleomagnetism. The identification of the magnetic minerals present in a rock is actually necessary for a proper evaluation of the geological processes which caused the rock to acquire and preserve a stable remanent magnetization. The study of the concentration, composition and grain size of the magnetic minerals in a sediment and of their variation in a stratigraphic sequence or within a sedimentary basin constitute the basis for the research field of environmental magnetism.
Moreover, many magnetic properties of rocks are anisotropic, that is they depend on the direction in which they are measured. The study of the magnetic anisotropy of rocks is particularly focused on magnetic susceptibility (which is given by the ratio between the magnetization induced in an applied low-field and the intensity of the field itself) and on some artificial remanent magnetizations (which represent the magnetization measured on a rock sample when no external magnetic field is acting upon it).


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