Electrical conductivity through rocks and minerals varies significantly with many factors. Some minerals such as galena, pyrites,magnetite are commonly poor conductors in massive form yet their individual crystals have high conductivity since the electrons move rapidly while carrying charges for conduction. Other factors such as porosity, pore fluid contents triggers Electrolytic conduction and impacts sedimentary rocks to have high conductivity. Here below I have explained how electrical conduction through rocks occurs.
1: Electrolytic Conduction
Electric conduction is due to movement of ions within an electrolyte
It may depends on the following,
- Type of ion: Electrons are solely responsible for charge carrying for Electrolytic Conduction. However holes may also be charge carriers but at lower conduction compared to electrons.
- Ionic concentration: The more the concentration of charged ions the more the electric conduction, however if they present with opposite charges it may trigger strong force of attraction which may result in decreased conduction. Also the more the number of ions above a certain threshold level may result in decrease in conductivity due to increased collisions of mobile ions.
- Size of ions: The size of ions varies inversely proportional to charge density. The smaller the size of ion the larger the charge density, hence high conductivity and vice versa is true.
- Mobility: when more free electrons are available the more the Electric conduction. Electrons have high mobility compared to protons.
2: Electronic (Ohmic) conduction
Is the process by which materials such as metals allows electrons to move rapidly so as to carry up the charge. Electrons move by drifting velocity when an electric field is applied.
3: Dielectric conduction.
As the term described, A dielectric material can be defined as an electrical insulator that can be polarized by an applied electric field. Examples of dielectric are Mica, paper, glass, some metal Oxides just to mention a few.
Hence Dielectric conduction is the conduction due to results of polarization.
Atoms and ions acquire an electric polarization and act like an electric dipole.
The degree of polarization is measured by a constant term known as Dielectric Constant or Relative Permittivity,
Dielectric constant or Relative permittivity:
Dielectric constant of the medium is the ratio of electrostatic force acting between two point charges in air/ vacuum to the electrostatic force acting between those charges in the presence of medium. It is denoted by K or £r.
Dielectric constant, K = F (air)/F (medium)
K = [Q×q/4Ï€£o×r²] ÷ [Q×q/4Ï€£×r²]
K = 1/£o÷1/£
K = £/£o = £r
£ = £ok
Different materials have different dielectric constant such as,
Water, k = 80, air, k = 1
How can dielectric polarization be explained?
It is the process by which an atom of dielectric material develops two partial charges with the nucleus being the positive charge center and the electron cloud being the negative charge center when an electric field is applied on it.
It is an applied field across it that causes an electron cloud to move in opposite to the field direction hence results in an induced dipole.
The process continues until the point where the force due to the field and the attractive Coulomb force between the charged electron cloud and the nucleus become at equilibrium.
Dielectric conduction is disregarded in electrical resistivity surveying but it may be used in
- Complex Resistivity measurements
- Spectral Induced Polarization.
Note: In most cases in rocks electric conduction by pore fluids acts as an electrolyte.
Range of resistivity for geological materials is 1.6 × 10-8 Ω⋅m for native silver to 1016 Ω⋅m for pure sulfur
Sedimentary rocks are the most conductive due to their high pore - fluid contents
Metamorphic rocks lie at intermediate but in overlapping resistivities while igneous rocks have high resistivities.
Note: In sedimentary rocks since pore fluid is controlled by pore spaces, then It is important to consider the Age of the rock, since it impacts this property.
Younger rocks: High porosity and permeability
Older rocks: Low porosity and permeability due to exposed to secondary infilling of interstices by mineralization, compaction e.t.c
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