Electrical Conductivity in Minerals

The electrical conductivity of a mineral - its ability to carry an electric current - is controlled by the energy structure of its electrons, which in turn depends on the type of chemical bonding that holds the mineral together. ^[1] Understanding conductivity matters because it connects directly to other observable properties: a mineral’s color, transparency, and metallic luster all follow from the same band-gap physics that determines whether it will carry a current.

Band Structure

When atoms bond together to form a mineral, their discrete electron energy levels spread and merge into continuous energy bands. ^[1] Two bands matter most for electrical behavior. The valence band is the highest fully occupied band - it holds the electrons responsible for chemical bonding. The conduction band sits above it and is normally empty. The energy difference between these two bands - the band gap - determines whether a mineral conducts electricity, insulates against it, or does something in between. ^[1] When the gap is zero or the bands overlap, electrons move freely and conduction is easy. When the gap is large, electrons are locked in place and the mineral insulates.

Conductors

Metallic minerals conduct electricity readily because their valence and conduction bands overlap with no gap between them, allowing electrons to move freely throughout the structure at any energy input. ^[1] Copper is the standard mineralogical example: its band structure shows complete overlap between the bands, giving electrons an unobstructed pathway through the lattice. This is why native copper, gold, and silver - all of which have metallic bonding - are consistently strong electrical conductors regardless of conditions. Metallic minerals have high electrical conductivity precisely because electrons are free to migrate throughout the conduction bands without needing to overcome any energy barrier. ^[1]

Insulators

Minerals bonded by ionic or covalent mechanisms do not conduct electricity under normal conditions because their valence electrons are confined to specific bonding orbitals with a large energy gap separating them from the conduction band. ^[1] In practice, band gaps above about 3 eV are large enough that ordinary voltages, heat, and visible light cannot bridge them. Quartz illustrates the extreme case: its experimentally determined band gap is approximately 8.9 eV, far larger than the energy of any visible photon (which ranges from about 1.8 to 3.1 eV). ^[1] This is why quartz is both electrically inert and transparent: visible light simply does not carry enough energy to excite electrons across the gap.

Semiconductors

Some covalently bonded minerals have band gaps that are real but small enough that modest amounts of energy - from heat, light, or an applied voltage - can push electrons from the valence band into the conduction band, allowing them to conduct. ^[1] Galena (PbS) is the classic mineral semiconductor, with a band gap of only 0.42 eV. ^[1] Because this value is smaller than the energy carried by any photon of visible light, every photon that strikes galena has more than enough energy to excite an electron across the gap. Galena therefore absorbs all visible light, which explains its characteristic opaque, metallic grey-to-black appearance in hand specimen.

Temperature Effects

Temperature affects conductivity in opposite directions for metallic minerals versus ionic or covalent ones, and this contrast is diagnostically useful. In ionic and covalent minerals, heating adds thermal energy to electrons, making it more likely that some will be kicked across the band gap into the conduction band - so conductivity increases with temperature. ^[1] In metallic minerals, the situation is reversed. Heating causes the atoms in the lattice to vibrate more energetically, and these vibrations scatter the electrons that are trying to flow through the conduction bands, impeding their movement and reducing conductivity. ^[1] The practical consequence is clear: heating a true metallic mineral makes it a worse conductor, while heating a silicate or oxide makes it marginally better - a distinction that persists all the way through their hand-specimen physical properties and optical behavior.

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