Magnetic properties of rocks and minerals are key to understanding Earth's magnetic field. These properties stem from the behavior of electrons in atoms, which can create weak or strong magnetic responses in different materials.
Rocks get their magnetic properties from iron-bearing minerals like magnetite. These minerals can retain magnetization, providing clues about Earth's past magnetic field and tectonic movements. This info is crucial for studying geomagnetic reversals and core dynamics.
Magnetic Material Types
Diamagnetic, Paramagnetic, and Ferromagnetic Materials
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Diamagnetic materials exhibit a weak, negative magnetic susceptibility and are slightly repelled by a magnetic field
These materials do not retain magnetic properties when the external field is removed (water, quartz, calcite)
Paramagnetic materials have a small, positive magnetic susceptibility and are weakly attracted to a magnetic field
Paramagnetic materials also do not retain magnetic properties when the external field is removed (biotite, pyrite, siderite)
Ferromagnetic materials possess a large, positive magnetic susceptibility and are strongly attracted to a magnetic field
These materials can retain magnetic properties, known as remanent magnetization, even when the external field is removed (magnetite, hematite, pyrrhotite)
Factors Determining Magnetic Behavior
The magnetic behavior of materials is determined by the presence and alignment of magnetic moments associated with unpaired electrons in their atomic or molecular structure
In diamagnetic materials, the magnetic moments of electrons cancel out, resulting in a weak, negative magnetic response
Paramagnetic materials have unpaired electrons with magnetic moments that align parallel to an applied field, creating a weak, positive magnetic response
Ferromagnetic materials have a strong interaction between neighboring magnetic moments, leading to a spontaneous alignment and a strong, positive magnetic response
Magnetic Properties of Materials
Magnetic Susceptibility and Remanent Magnetization
Magnetic susceptibility measures how easily a material can be magnetized in the presence of an external magnetic field
It is a dimensionless quantity representing the ratio of induced magnetization to the applied magnetic field strength (SI units, CGS units)
Remanent magnetization is the permanent magnetization that remains in a material after the removal of an external magnetic field
Ferromagnetic minerals align their magnetic moments with the applied field and retain this alignment, resulting in remanent magnetization
Magnetic Anisotropy
Magnetic anisotropy refers to the directional dependence of magnetic properties in a material
It can be caused by the preferred orientation of magnetic minerals, the shape of magnetic grains, or the presence of stress or strain in the rock
The magnetic anisotropy of susceptibility (AMS) is commonly used to study the fabric and deformation history of rocks
AMS reflects the orientation and alignment of magnetic minerals, providing insights into rock formation and deformation processes (sedimentary bedding, metamorphic foliation)
Magnetic Properties of Rocks
Contribution of Rock-Forming Minerals
Iron-bearing minerals, such as magnetite, hematite, pyrrhotite, and goethite, are the primary contributors to the magnetic properties of rocks due to their ferromagnetic behavior
Magnetite (Fe3O4) has a strong, positive magnetic susceptibility and can carry stable remanent magnetization
Hematite (Fe2O3) has a weaker magnetic susceptibility compared to magnetite but can also carry stable remanent magnetization
Pyrrhotite (Fe1-xS) exhibits a strong magnetic susceptibility and can carry remanent magnetization, but its magnetic properties are influenced by composition and crystal structure
Factors Influencing Rock Magnetic Properties
The concentration, grain size, and distribution of magnetic minerals within a rock determine its overall magnetic properties
Rocks with a higher content of ferromagnetic minerals generally exhibit stronger magnetic properties (basalts, gabbros)
Rocks dominated by diamagnetic or paramagnetic minerals have weaker magnetic signatures (limestones, quartzites)
The magnetic properties of rocks can be influenced by factors such as alteration, weathering, and metamorphism, which can modify the composition and distribution of magnetic minerals
Magnetic Remanence in Rocks
Acquisition Processes
Thermoremanent magnetization (TRM) is acquired when ferromagnetic minerals cool below their Curie temperature in the presence of an external magnetic field, typically the Earth's magnetic field
The magnetic moments of the minerals align with the field direction and are "frozen" in place upon cooling (igneous rocks, metamorphic rocks)
Chemical remanent magnetization (CRM) is acquired when new ferromagnetic minerals grow or undergo chemical changes in the presence of an external magnetic field
The magnetic moments of the newly formed minerals align with the field direction (diagenesis, hydrothermal alteration)
Detrital remanent magnetization (DRM) is acquired when magnetic mineral grains align with the Earth's magnetic field during deposition and sediment compaction
The alignment is preserved as the sediment lithifies into sedimentary rock (sandstones, mudstones)
Post-depositional remanent magnetization (pDRM) is acquired when magnetic minerals in soft sediments rotate and align with the Earth's magnetic field after deposition but before lithification
Preservation and Significance
The preservation of remanent magnetization in rocks depends on the stability of the magnetic minerals, the absence of later thermal or chemical disturbances, and minimal exposure to strong, external magnetic fields
Rocks that have preserved their original remanent magnetization provide valuable information about past magnetic field directions and intensities
Paleomagnetic studies utilize remanent magnetization to reconstruct the Earth's magnetic field behavior and to understand tectonic movements and paleogeography (apparent polar wander paths, plate reconstructions)
The study of magnetic remanence in rocks is crucial for understanding geomagnetic field reversals, secular variation, and the evolution of the Earth's core dynamics