Mineral Color

Mineral color is our perception of the wavelengths of light that reach the eye after being reflected from, or transmitted through, a mineral. Color is determined not by what light the mineral reflects, but by what it absorbs - the colour we see is whatever is left over. A red mineral absorbs the blue end of the spectrum heavily and reflects red light. A white mineral absorbs almost nothing and reflects essentially the entire visible spectrum. A black mineral absorbs all wavelengths. ^[1]

The physics of color in minerals is reasonably well understood, even though detailed colour studies exist for only about 10% of all mineral species. The absorbing processes that produce mineral color fall into four categories: crystal field transitions, charge transfer transitions, band theory effects, and colour centres. Each is explained on its own page; this page addresses the broader framework of how we perceive color and the non-electronic causes of color.

How the Eye Perceives Color

Visible light occupies wavelengths between about 400 nm (violet) and 700 nm (red). The energy of light increases with decreasing wavelength - violet light is more energetic than red. The eye distinguishes wavelengths using three types of colour receptors, most sensitive at about 660 nm (red), 500 nm (green), and 420 nm (blue-violet). ^[1]

Monochromatic light - light of a single wavelength - produces one of the spectral colours. Yellow monochromatic light at 570 nm stimulates the red and green receptors roughly equally and the blue-violet receptor very little; the brain interprets this combination as yellow. Polychromatic light (multiple wavelengths) is also perceived as a single colour even if no single wavelength corresponding to that colour is present. Television screens and computer monitors exploit this by mixing only red, green, and blue-violet light to produce the complete range of perceived colours. ^[1]

When the full visible spectrum reaches the eye simultaneously (as in sunlight), the result is perceived as white. Complementary colour pairs - two colours that together stimulate all three receptors equally - are also perceived as white. Some colours, including purple and brown, have no counterpart in the visible spectrum and are produced entirely by mixtures: purple from red plus violet, brown from red plus blue plus yellow. ^[1]

About 4% of the population - mostly male - have colour blindness, in which one or more receptor types are absent or non-functional. For most tasks this poses no difficulty, but the identification of mineral colours in optical mineralogy can be affected. Awareness of the limitation allows practical workarounds: paying closer attention to non-colour optical properties, or using the subtle colour distinctions that lie outside the normal colour vocabulary. ^[1]

Color Consistency: Idiochromatic vs. Allochromatic

Idiochromatic Minerals

Minerals that contain a chromophore element as a fixed part of their chemical composition show consistent, predictable colours. These are called idiochromatic (self-coloured) minerals. A compositional range may produce a range of colours - for example, the Fe content of an olivine controls how deeply coloured it is - but the colour always follows from the composition in a predictable way. Opaque minerals are also considered effectively idiochromatic because colour is controlled by bond character, not variable impurities. ^[1]

Allochromatic Minerals

Minerals that lack chromophore elements in their essential composition are called allochromatic (foreign-coloured) - their colour comes from impurities, colour centres, trace chromophores, or inclusions, not from the fundamental chemistry of the mineral. They are typically pale or colourless in their pure form. Quartz is the canonical example: pure SiO₂ is colourless, but it may be purple (amethyst, from trace Fe³⁺), pink (rose quartz, from trace Ti⁴⁺), smoky brown (from colour centres involving Al), or virtually any other colour depending on included material. ^[1]

Color from Mechanical Causes

Not all mineral colour arises from electronic absorption processes. Finely dispersed inclusions of other minerals can colour a host that would otherwise be pale. Hematite (Fe₂O₃) is a widely distributed inclusion mineral; if included in quartz or calcite, it can stain them various shades of red or brown. Jasper - a red-brown variety of fine-grained quartz widely used in lapidary work - owes its colour to disseminated hematite. Graphite (C) can turn normally white or colourless calcite (CaCO₃) black. The milky-white colour of milky quartz comes from vast numbers of microscopic to submicroscopic fluid inclusions - mostly water - trapped during crystal growth. ^[1]

Chatoyancy

Chatoyancy is a silky sheen - a single, narrow band of reflected light - seen in minerals made of parallel fine fibres (such as satin spar gypsum, CaSO₄·2H₂O) or minerals that contain abundant parallel fibrous inclusions. The fibres act collectively as a reflective grating aligned in one direction. Moving the sample relative to the light source makes the band of light appear to slide across the surface, mimicking the play of light on a cat’s eye.

Asterism

Asterism is a related phenomenon in which inclusions produce a star pattern rather than a single band. Corundum and quartz - both hexagonal - may contain fine fibrous rutile (TiO₂) inclusions aligned parallel to the three 2-fold symmetry axes at right angles to c. Light scattered from these three sets of inclusions combines to produce a six-pointed star. Star sapphire and star ruby are the most commercially prized examples. ^[1]

Play of Color

Some minerals show rapidly shifting flashes of spectral colour as the sample is tilted - the play of color. In precious opal (fire opal), the structure consists of roughly uniform spheres of silica gel packed together with a spacing close to the wavelength of visible light. These spheres act as a diffraction grating, splitting incoming polychromatic white light into its spectral components. Some plagioclase, called moonstone, shows a similar effect from a closely spaced lamellar exsolution structure. ^[1]

Iridescence

Iridescence is a play of colour on a mineral’s surface rather than from its interior. It is common in pyrite, chalcopyrite, and bornite, whose oxidised surfaces develop a thin coating that produces thin-film interference colours - the same effect seen on a film of oil on water. Cracks inside transparent minerals such as quartz can also produce iridescent colour from thin-film interference along the crack walls. ^[1]

Streak

Streak is the colour of a mineral when it is reduced to a fine powder. It is a more reliable identification property than hand-specimen colour because the powdered form eliminates the effects of grain size, surface oxidation, and many types of included material that make hand-specimen colour variable. ^[1]

Minerals with dominantly ionic and covalent bonding tend to have pale or white streaks even when their hand-specimen colour is dark. Because these minerals are intrinsically transparent, individual powder particles transmit most of the incident light - only a small fraction is absorbed - and nearly the complete spectrum is reflected back to the eye as pale or white. Hand-specimen colour may vary widely in these minerals because of impurities, but the streak remains consistent because the fine powder eliminates the optical effects of inclusions and surface coatings. ^[1]

Minerals with metallic bonding have richly coloured or black streaks. Being opaque, they absorb incident light strongly even in powdered form. Streak colour may actually be darker than hand-specimen colour because the irregular, fine-grained surfaces of the powder absorb more light than a polished flat surface would. Streak colour is tested by rubbing the mineral on an unglazed white porcelain streak plate (hardness ~7). For minerals harder than 7, the streak is obtained by pulverising a small fragment with a knife blade or rock hammer. ^[1]

References

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

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