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Geology is the Way

Color in minerals

Minerals show a wide range of color resulting from the interaction between a light source and their crystal structure. The color we see in a mineral corresponds to the wavelengths of the electromagnetic spectrum emitted or transmitted by a mineral in the visible light (i.e. the part of the electromagnetic spectrum we can ‘see’) between violet (wavelength λ = 350 nm) and red (λ = 750 nm). White light is the sum of all the wavelengths between these two extremes.

Visible light in the electromagnetic spectrum
Visible light is the part of the electromagnetic spectrum comprised between wavelengths of 390 and 760 nm which our eyes can detect. Radiation with smaller wavelength is called ‘ultraviolet’, whereas radiation beyond the red color (i.e. with larger wavelength) is called ‘infrared’. Source: wikimedia.

There are several mechanisms that can produce color in minerals starting from a source of white light. The simplest mechanism is the reflection of light and the selective absorption of some wavelengths in the visible spectrum. Transparent, colorless minerals do not interact with light, which is able to cross their crystal lattice. On the other hand, a white opaque mineral reflects all wavelengths of the visible spectrum emitting white light. A mineral that is able to absorb all wavelengths does not emit light and appears black.

Colored minerals may be transparent or opaque. In both cases, colors arises from the selective absorption of some visible light wavelengths and the reflection of some colors. A specific color may appear either because all wavelengths are absorbed except for a specific one corresponding to a given color or because some wavelengths are absorbed and the reflected visible light wavelengths combine, resulting in a specific color. For example, if all light is absorbed and only orange is reflected, a material will appear orange. Orange forms also if all wavelengths of the visible light are reflected and blue is selectively absorbed, in this case for the selective subtraction of part of the spectrum from white light.

Origin of color in minerals
The wavelengths of the visible light reflected or absorbed by minerals determine the color we see in minerals. Transparent colorless minerals do not absorb or reflect light. White minerals reflect all light. Black minerals absorb all light. Colored minerals selectively absorb or reflect part of the visible light.

How do minerals absorb and emit light?

absorption and emission of electromagnetic radiation by electrons in shells
The absorption of electromagnetic radiation by an electron make it jump to a higher energy level. Electrons then rearrange to occupy the lowest energy levels available in the shells and emit radiation in the process. Source: William D. Nesse, Introduction to Mineralogy.

The color we see in a mineral corresponds to the electromagnetic spectrum emitted by the mineral in the visible light, but minerals can absorb and transmit electromagnetic radiation at different wavelengths that are invisible to the human eye, like infrared, ultraviolet, or X-rays. The radiation whom a mineral interacts with is related to the shells occupied by electrons around nuclei within a crystal structure. Electrons are, indeed, found in different shells with different energy levels and can ‘jump’ to a higher energy level if they receive radiation with an energy equal to that required to reach a vacant energy orbital: this radiation is absorbed from the spectrum of the incident radiation. Electrons in atoms tend to occupy the lowest energy levels available, so if one electron moves to a higher energy level, the electrons in the orbitals of the atom will re-arrange to occupy the vacant lower energy shells, emitting electromagnetic radiation in the process. Colored minerals are those that are able to absorb and emit light in the visible light (photoluminescence). When re-entering in the lower energy configurations, electrons may emit radiation that is not present in the absorbed radiation, called fluorescence.

Crystal field theory and color
The elements that are generally able to interact with and emit visible light are transition metals with partially filled 3d orbitals that contain unpaired electrons. These elements are for example Ti, V, Cr, Mn, Fe, Co, Ni and are known as chromophore elements (from the Greek chroma, ‘color’, and phoros, ‘bearing’). Minerals that contain chromophore elements are primary constituents of their formula are said to be idiochromatic (e.g. cinnabar, azurite) , whereas minerals in which chromophore elements are present as impurities are said allochromatic (e.g. pink in rose quartz). When these elements occur within a crystal structure, the electrons in their orbitals interact with neighboring anions, like oxygen (crystal field), and the structure of the orbitals become degenerate: the repulsion between electrons and anions in a chemical bond modifies the geometry of the orbitals, reducing the energy needed to move electrons to higher energy levels. Moreover, the same orbitals within an atom in a crystal structure have not the same energy level anymore, but some electrons may end up with slightly higher or lower energy. In a solid, the outer orbitals become energy bands.

Atomic orbitals
Types of atomic single electron orbitals for the lowest four quantum numbers. Image by Geek3 via wikimedia.

Ruby is a variety of corundum [Al2O3], in which part of Al3+ is substituted by Cr3+. The electrons in the degenerated 3d orbitals of Cr3+ in the crystal lattice of corundum absorb violet and green/yellow light and transmit red light. In Emerald, Cr3+ substitutes Al3+ in the structure of beryl [Be3Al2Si6O18], producing the distinctive emerald green color which is due to the selective absorption of yellow, red, and violet light and transmission of mostly blue and green. Beryl has many other colored varieties like light blue aquamarine and yellow heliodor, both of which are colored by Fe with different valence and occupying different sites in the crystal structure.

Corundum crystal
Colorless twinned crystal of corundum. 3.9 x 2.5 x 1.4 cm. Eheliyagoda, Ratnapura, Ratnapura, Sabaragamuwa, Sri Lanka. Photo © Robert M. Lavinsky via wikimedia.commons.
Ruby crystal
Crystal of corundum var. ruby. 3.4 x 3.3 x 2.3 cm. Luc Yen, Yenbai Province, Vietnam. Photo © Robert M. Lavinsky via wikimedia.commons.


Aquamarine crystals
Beryl var. aquamarine on muscovite. Nagar Hunza Valley, Pakistan. Photo © Gunnar Ries via wikimedia.commons.
Emerald crystals
Beryl var. emerald on quartz. Kagem Emerald Mine, Distrikt Kafubu Emerald, Ndola, Copperbelt, Zambia. Photo © Robert M. Lavinsky via wikimedia.commons.

Color centers

color centers in minerals
The presence of holes filled by electrons (a – fluorite) or spare electrons produced by radioactive decay around trace transition metals (b – amethyst) creates a color center. The spare electron can bump into the high energy orbitals of the neighboring cations and color the mineral. Source: William D. Nesse, Introduction to Mineralogy.

Color in minerals may also be related to crystal defects with additional or missing electrons in the structure. In fluorite [CaF2], a missing F anion can be replaced by a spare electron that occupies an hole in the

structure and is held in place by the surrounding positive charges. Energy from visible light is enough to allow this electron to jump in the highest energy orbitals of the neighboring cations before returning back to the hole. This produces the violet color in some fluorites.

The violet color of amethyst is produces by a defect caused by radioactive decay in quartz [SiO2] that contains impurities of Fe3+ substituting Si4+ (the electrical neutrality is balanced by the presence of H+ ions in the structure): radioactive decay from neighboring sources of radiation expels an electron from oxygen around [FeO4] color centers and the spare electron can bump into the higher energy levels of Fe interacting with visible light and producing a violet color. A similar process results in the dark color of smoky quartz, in which Al3+ substitutes Si4+. The coloration of amethyst and smoky quartz is hence related to transient defects caused by radioactive decay and heating of the crystal allows electrons to reconfigure themselves and the color to disappear.

violet fluorite cubes
The violet color of fluorite is produced by missing F in the structure replaced by free electrons. The color can be removed by heating. Size: 4.7 x 3.8 x 2.4 cm. Berbes Mining area, Ribadesella, Asturias, Spain. Photo © Robert M. Lavinsky via wikimedia.commons.

Band theory and color
In a solid, the outer electrons are paired with the electrons of neighboring atoms into a valence band. The energy band with higher energy than the valence band is called ‘conduction band’, because the electrons in this band can move within the solid and conduct electricity. Some minerals are conductive because the energy levels of the valence band and the conduction band overlap: these are, for example, metals like iron, in which the outer electrons are not bound to a specific atom but move in an electron cloud within the crystal lattice. Other minerals, characterized by a greater amount of covalent bonds, are characterized by a band gap between the valence band and the conduction band. The width of the gap determines the type of radiation absorbed by a mineral and, hence, its color. If the band gap is larger than the energy level of visible light, no light is absorbed and the mineral is transparent (e.g. diamond). If the band gap is smaller than the energy level of visible light, all light is absorbed and the mineral appears black or grey (e.g. galena). An intermediate energy level, between 2 and 3 eV, causes the absorption of only the short wavelength part of the visible light spectrum, coloring the mineral in warm colors (red to yellow), depending on the absorbed wavelengths.

Band theory
The difference in energy levels between the valence band and the conduction band in semiconductor minerals may match that of visible light and produce a color. Source: William D. Nesse, Introduction to Mineralogy.
Origin of color in diamonds
The color of diamonds is caused by the presence of nitrogen (yellow diamonds) or boron (blue diamonds), whose outer electrons lie in the band gap. Source: Nassau K. (1978). American Mineralogist.

Diamonds are generally colorless because the band gap width is larger than visible light. The presence of nitrogen or boron in the structure of diamond alters this situation. Nitrogen has one extra electron with respect to carbon, which enters a level within the band gap. This electron can bump into the conduction band by absorbing violet, resulting a yellow coloration in diamond. Boron has an electron less than carbon and can host in its 2p orbitals some of the electrons of carbon. These orbitals lie in the band gap and from there electrons can bump to the conduction band absorbing yellow light and generating a blue color as a result.

Blue diamond
A diamond crystal with octahedral habit and light blue color. Photo © Jeff-o-matic via Flickr.



oil on water
Color produced by an oil film on water. Source: curiosity via Quora.

Color can be produced in some minerals in the same way a film of oil on water results in rainbow-like color patterns. When light encounters an oil film, it is splat in two rays: one is reflected by the oil surface, the second one is refracted by the oil film and then reflected by the contact between oil on water. Light is slower by about 1/3 within the oil film, due to the refraction index of oil. Therefore, when the two rays emerge from the oil, their wavelengths are not in phase anymore and the two rays can reinforce or cancel one another, producing color by constructive or destructive interference. A similar phenomenon happens in some minerals like plagioclase, characterized by nanoscale unmixing between Ca-rich and Na-rich compositions (e.g. peristerite and labradorite): light is reflected on the surface of the mineral and refracted by layers (lamellae) with different composition and refraction indices that cause destructive or constructive interference when the rays recombine.

oil water color interference explanation
The constructive (or destructive) interference between reflected and refracted light in an oil film on water produces color depending on the thickness of the oil film.
The presence of submicroscopic unmixing produces iridescent colors in some plagioclase, in this example in labradorite. Photo © Prokofiev via


Color in transmitted light (under the petrographic microscope)
When we observe mineral at the petrographic microscope, we use a transmitted light system emitting polarized light (i.e. light vibrating on a single plane). The color we see in a mineral is not produced by the wavelengths of light that are reflected but by those that can pass through the mineral. In general, the more a mineral absorbs light, the more it appears colored in thin section. Colorless mineral do not absorb any wavelength of light. On the other end of the spectrum, opaque minerals absorb all light and appear black. A pleochroic mineral absorbs polarized light unevenly depending on the orientation of its crystal structure with respect to the incoming light beam. More about pleochroism here.

glaucophane blueschist
Minerals in thin section show a variety of colors depending on their absorption of transmitted light. This sample contains colorless minerals (appearing white), colored minerals (green, blue, and violet) and opaque minerals (black). The light blue to violet colors are produced by the same mineral, which is characterized by different absorption of polarized light depending on its orientation (pleochroism). Blueschist from Ghinivert (Montgenevre area, Italian Alps). PPL. Width: 2 mm. Photo © Alessandro Da Mommio.

Nassau, K. (1978). The origins of color in minerals. American mineralogist63(3-4), 219-229.
Rossman, G. R. (1994). Colored varieties of the silica minerals. Reviews in Mineralogy29, 433-433.

Proprietà dei Minerali


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