Does Granite Conduct Electricity?

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Granite, a common type of igneous rock formed from the solidification of magma, has an electrical conductivity that is influenced by various factors, including the amount of free water, temperature, and the presence of hydrous accessory minerals and structural water in major minerals. Understanding the electrical conductivity of granite is crucial in various fields, such as geophysics, geothermics, and electrical engineering.

Factors Affecting Electrical Conductivity of Granite

Free Water Content

The amount of free water present in granite is a significant factor that affects its electrical conductivity. Studies have shown that the conductivity of granite is considerably controlled by the presence of free water. In the deep crust, where granite is mostly dry, the conductivity is relatively low.

Temperature

Temperature is another important factor that influences the electrical conductivity of granite. As the temperature increases, the conductivity of granite can increase by more than half an order of magnitude, indicating the onset of melting. This suggests that the structural water in feldspars, a major mineral component of granite, can influence the conductivity to a moderate degree.

Hydrous Accessory Minerals

The presence of hydrous accessory minerals, such as muscovite, biotite, and amphibole, in granite can also affect its electrical conductivity. These minerals start to dehydrate at around 700 K at ambient pressure, but their influence on the bulk conductivity of granite is relatively minor.

Quantifiable Data on Electrical Conductivity of Granite

does granite conduct electricity

Conductivity Measurements

In a study by Guo et al. (2023), the electrical conductivity of two natural granites and two synthetic aggregates compositionally similar to the natural samples was measured. The study found that the average calculated data of the electric properties of granite agreed with the measured data, with conductivity values ranging from 36.2 pS/m (measurement) to 53.5 pS/m (calculation).

Another study measured the electrical conductivity of two granite samples from southern Tibet, which were found to be 0.0024 ± 0.0001 S/m and 0.0030 ± 0.0001 S/m, respectively. These values are relatively low compared to other rock types, such as basalt, which has a conductivity of approximately 0.1 S/m.

Influence of Hydrous Minerals and Structural Water

The presence of hydrous accessory minerals and structural water in major minerals can also influence the electrical conductivity of granite. As mentioned earlier, the hydrous accessory minerals in granite start to dehydrate at around 700 K, but their effect on the bulk conductivity is minimal.

However, the increase in conductivity of granite by more than half an order of magnitude at 1273 K and 1 GPa suggests that the structural water in feldspars could influence the conductivity to a moderate degree. This indicates that the electrical conductivity of granite is a complex property that is influenced by multiple factors.

Theoretical Considerations

Electrical Conductivity Equation

The electrical conductivity of a material, such as granite, can be described by the following equation:

σ = ne²τ/m

Where:
– σ is the electrical conductivity (S/m)
– n is the number of charge carriers per unit volume (m^-3)
– e is the charge of an electron (1.602 × 10^-19 C)
– τ is the average time between collisions of the charge carriers (s)
– m is the effective mass of the charge carriers (kg)

This equation shows that the electrical conductivity of a material is directly proportional to the number of charge carriers, their charge, and the average time between collisions, and inversely proportional to their effective mass.

Charge Carrier Mechanisms

In the case of granite, the charge carriers responsible for electrical conductivity can be either electrons or ions. The specific charge carrier mechanism depends on the mineral composition and the presence of defects or impurities in the crystal structure.

For example, the presence of hydrous accessory minerals and structural water in feldspars can introduce ionic charge carriers, such as protons (H+) or hydroxide ions (OH-), which can contribute to the overall electrical conductivity of granite.

Practical Applications

The electrical conductivity of granite has various practical applications, including:

  1. Geophysical Exploration: Measurements of the electrical conductivity of granite can be used in geophysical exploration techniques, such as magnetotelluric surveys, to map the subsurface structure and identify potential mineral deposits or geothermal resources.

  2. Electrical Engineering: The low electrical conductivity of granite makes it a suitable material for electrical insulation in various applications, such as high-voltage power transmission lines and electrical equipment.

  3. Geological Modeling: Understanding the electrical conductivity of granite can help in the development of more accurate geological models, which are essential for understanding the Earth’s interior structure and dynamics.

Conclusion

In summary, the electrical conductivity of granite is a complex property that is influenced by several factors, including the amount of free water, temperature, and the presence of hydrous accessory minerals and structural water in major minerals. Quantifiable data on the electrical conductivity of granite, such as conductivity values ranging from 36.2 pS/m to 53.5 pS/m, and the influence of hydrous minerals and structural water, provide valuable insights for various applications in geophysics, geothermics, and electrical engineering.

References:

  1. Karato, S. (2011). Electrical conductivity of minerals and rocks. In Electrical Conductivity in Geophysics and Geothermics (pp. 15-42). Springer, Berlin, Heidelberg.
  2. Guo, H.K., Wang, X.Z., Zhang, X.B., Özaydin, S., Li, S., & Clark, S. (2023). The electrical conductivity of granite: The role of hydrous accessory minerals and the structure water in major minerals. Earth-Surface Processes and Landforms, 48(5), 811-823.
  3. Xu, Y.G., & Shankland, T.J. (2008). The electrical conductivity of sandstone, limestone, and granite. Journal of Geophysical Research: Solid Earth, 113(B5), B05203.
  4. Yoshino, T., Hirose, K., & Hirschmann, M.M. (2010). Electrical conductivity of olivine + basaltic and olivine + carbonatite melts with equilibrium melt geometry. Journal of Geophysical Research: Solid Earth, 115(B11), B11205.
  5. Ni, L., Hirschmann, M.M., & Shankland, T.J. (2011). Electrical conductivity of hydrous basaltic melt. Earth and Planetary Science Letters, 307(3-4), 367-375.