Band Theory and Mineral Color

Band theory explains mineral colour in metals and semiconductors through the electronic structure of energy bands. When atoms are brought into close proximity in a crystal, their individual electron energy levels spread into continuous bands - the filled lower-energy valence band and the higher-energy conduction band. The relationship between these bands determines whether a mineral is transparent, coloured, or opaque, and controls which wavelengths are absorbed and re-emitted. ^[1]

Metals: No Band Gap

In minerals with significant metallic bonding, the valence band and conduction band overlap - there is no energy gap between them. This means a continuum of energy levels is always available, and electrons can be promoted by any photon regardless of its wavelength. Consequently, metals absorb the entire visible spectrum and are opaque. ^[1]

Opacity does not mean the light disappears - the absorbed energy is mostly converted to heat, and some is re-emitted as light. The re-emitted light is what we perceive as the metallic colour. The efficiency of re-emission is not equal for all wavelengths; it is controlled by the density of energy levels in the conduction band and by quantum mechanics. The colour of a metal therefore reflects which wavelengths are re-emitted most efficiently, not which are most strongly absorbed.

Gold, Silver, and Copper

Gold absorbs light at the red and yellow end of the spectrum most strongly. Counterintuitively, the strongly absorbed wavelengths are also the ones most efficiently re-emitted - the red and yellow photons penetrate only a few hundred atoms into the surface and are immediately re-emitted at greater than 80% efficiency. The blue and violet end penetrates further before being absorbed, so less of it returns to the surface and more converts to heat. The colour we see is therefore dominated by re-emitted red and yellow. Gold sheets thinner than 100 nm transmit blue-green light because at that thickness even the red end cannot all be reflected. ^[1]

Silver re-emits the entire visible spectrum at over 95% efficiency - it is an almost perfect reflector of all wavelengths, which is why it appears neutral white and is widely used as a mirror coating. Copper has an absorption and re-emission pattern similar to gold, giving it a reddish colour. ^[1]

Semiconductors: Band Gap Controls Color

In semiconductors - many of which are sulfide minerals - an energy gap (the band gap, E_g) exists between the valence and conduction bands. Light whose photon energy exceeds the band gap can promote an electron from the top of the valence band to the conduction band and is absorbed. Light below the band gap energy cannot be absorbed and passes through or is reflected. The size of the band gap therefore dictates exactly which wavelengths are removed from the visible spectrum. ^[1]

Small Band Gap: Galena and Pyrite

When the band gap is smaller than the entire visible energy range, all visible wavelengths are absorbed and the mineral is black or grey. Galena (PbS) has a very small band gap and appears grey-black. Pyrite has a band gap of about 0.95 eV - small enough that the full visible spectrum can be absorbed. However, like gold, pyrite re-emits red-end wavelengths more efficiently than violet-end wavelengths, giving it a brassy yellow colour that closely mimics gold in hand specimen. ^[1]

Intermediate Band Gap: Cinnabar

Cinnabar (HgS) has an intermediate band gap - large enough that high-energy short-wavelength light from the violet end of the spectrum is absorbed (promoted across the gap), but too large to absorb the lower-energy red end. Red photons pass through without absorption. The result is a vivid red colour - the wavelengths that survive to reach the eye are precisely those that cannot bridge the gap. ^[1]

Large Band Gap: Diamond

Diamond has a band gap of about 5.5 eV - far larger than the energy of any visible photon (1.8-3.2 eV). No visible wavelength can bridge the gap, so no absorption occurs across the visible spectrum. The result is that pure diamond is colourless and transparent. ^[1]

Impurity-Induced Color: Blue Diamond

Some diamonds are blue rather than colourless. Trace amounts of boron are responsible. Boron has one fewer valence electron than carbon, and when it substitutes for carbon in the diamond structure it creates an energy level just above the top of the valence band - a shallow acceptor state. Electrons can be promoted from the valence band to this level by absorbing red-end photons, leaving blue light to reach the eye. The band gap itself is unchanged; it is the additional impurity level that creates the absorption. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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