Characterization and Origins of Diverse Mineral Colours
Dr.Benu Chatterjee
The beautiful colours of minerals have been valued in all societies. A comprehensive review of the colour phenomena in minerals is presented.
Colour is the most eye-catching feature of minerals. The recognition of colours in minerals goes back to pre-historic ages when charcoal and iron oxides (red hematite mineral) were used to colour cave paintings which still retain their original brightness. The present article is a comprehensive review of the various features of the diverse mineral colours, namely mechanics, characterization, complexities and origins of colours.
1. Mechanics of Creating Mineral Colours
The visible white light forms a part of the electromagnetic spectrum we can see. It is made up of waves with wavelengths of 350 to 750 nanometres that correspond to the seven colours of the rainbow with violet and red having the shortest and longest wavelengths respectively. Also, wavelength (λ) and frequency (γ) of light wave are related to energy (E) (photon : a basic unit of electromagnetic radiation) as E = hc/λ = hγ where h is Planck’s constant and c is the speed of light. When a solid sample absorbs light, what we see is the sum of the remaining reflected colours that strike our eyes. If all the light energies are reflected back, the sample will appear colourless or white. However, it will appear black if all the energies are absorbed, leaving none of the visible spectrum reaching our eyes.
Minerals develop colours by interacting with white light when only part of the visible spectrum is absorbed leaving the rest to emit. When this happens, the energies absorbed are removed from the emitted light which will no longer be white, but have complementary colour associated with the emitted wavelengths. The observed colour of a mineral thus corresponds to those wave lengths of light that are least absorbed.
2. Characterization of Mineral Colours - Idiochromatic, Allochromatic and Pseudochromatic
Certain elements termed as chromophores, are pigmenting agents which provide colours to minerals when they are present as a part of the crystal lattice, or as impurity. Chief chromophores are the first row of transition metals in the Periodic Table with atomic numbers 22 to 29, namely titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. Mineral colours are classified into three main groups, namely idiochromatic. allochromatic and pseudochromatic.
Idiochramatic ("self coloured") minerals exhibit their own inherent permanent colours that are derived directly from the presence of one or more chromophores as a constituent of the crystal lattice. The property of chromophores determines which wavelengths of light to absorb and which ones to emit. Examples include Iron-based red cinnabar, copper-based blue azurite and green malachite minerals.
Allochromatic ("other coloured") minerals, originally colourless, develop colours owing to the presence of trace amounts (parts per million) of chromophores as impurity in the host crystal lattice. The colourless quartz mineral for example changes to purple colour (amethyst mineral) when iron is present as impurity in the lattice. Similarly, colourless corundum changes to blue sapphire or red ruby by the presence of trace amounts of either iron and titanium, or chromium respectively.
Pseudochromatic ("false coloured") minerals display colour only because of the tricks played by light. These minerals contain layers that create colours by various features of physical optics discussed later on. The colour may vary, but is often a unique property of the mineral. For example, opal, moonstone and labradorite all reflect light in a characteristic way, but the colours are not true to the types of minerals.
3. Complexities of Mineral Colour and Theories
In mineralogy, colour is one the primary diagnostic features to identify minerals. However, mineral colour is changeable, and unpredictable, and cannot be used for identification. For example, different impurities can cause fluorite mineral to come out in various colours. Also, there are minerals of similar colours. Furthermore, some minerals such as topaz and beryl undergo changes from dull to deeper colour on heating as practised in the gemstone industry. These complex features of mineral colours make it less reliable to identify a mineral from its colour. It is thus important to understand what causes colours in minerals.
In order to show colour, a mineral has to somehow disturb the balance of the light energies with preferential absorption or emission of certain wavelengths/energies of light. The theories of the origins of colours in minerals such as idiochromatic, allochromatic and pseudochromatic which cover most of the familiar minerals, are simple. Delving deeper, Kurt Nassau (1983) has separated causes of colour into fifteen mechanisms based on several groups, such as crystal field effects, physical optics, band gap theory and molecular orbital mechanism. However, in order to keep the present article short and simple, only the crystal field theory and physical optics are discussed. Any discussion of either the band gap theory or molecular orbital formalism is excluded – both of which nonetheless relate to less familiar materials such as organic materials, semiconductors and metal conductors.
3.a. Crystal Field Theory - Transition Metal Ions and Colour Centres
Crystal Field Theory is based on two mechanisms involving transition metal ions and colour centres:
Transition metal ions (simple cases) is based on electron interactions, and the crystal field effects can explain both idiochromatic and allochromatic colours. The innermost electrons of chromophore transition metals are too tightly bound to the nucleus to be excited by light energy (photon) because electrons, after all, generally like to move about in pairs. However, valence electrons in these metal ions are unpaired which along with inner 3-d orbitals partially filled with electrons, are prone to excitation to higher energy orbitals by photons. This phenomenon would satisfy the need for pairing up of electrons, and thus becomes responsible for colours in minerals. Excitatation is achieved by absorbing photon from visible incident light. The absorbed energies are subtracted from that of the incident light, resulting in the observed colour.
Transition metal ions (complex cases) lead to characteristics found in, for example, corundum and beryl. Both corundum (aluminium oxide) and beryl (beryllium aluminosilicate) minerals are colourless because all electrons are paired in the crystal lattice allowing no absorption of light. However, addition of same chromium ion (Cr 3+) as chromophore impurity to either mineral, changes them to become colourful of different shades, namely red (ruby) and green (emerald) respectively. This phenomenon indicates complexity of crystal field interactions. It is believed that the geometry of the atoms in beryl is different from corundum, allowing the host molecules to interact with chromium more weakly. This results in reduced energy differences between the energy levels which eventually results in developing complementary green colour rather than ruby red in corundum.
The colour centres (F-centres) are defects created by removing atoms usually by radiation from the crystal structure. The resulting “hole” may be filled by an electron from a neighbouring atom. This will leave behind an unpaired electron which, as discussed before, is prone to excitation by photons with ultimate creation of colours. The most familiar examples are amethyst and smoky quartz minerals.
3.b. Physical Optics Effects - Scattering, Diffraction, Interference and Dispersion
Colouration of pseudochromatic minerals is best explained by physical optics processes where, unlike crystal field theory, electrons are not involved. Light passing through a mineral interacts with the crystal structure, and colours are produced due to various phenomena such as scattering (which explains blue sky), diffraction (peacock feather), interference (oil slicks) and dispersion (rainbow spectrum through prism) of incident white light.
Scattering occurs due to reflection of light off sub microscopic (the finer the better) particles of solid or liquid. It would cause colours to appear because blue light with shorter wavelength is scattered more than red. As particles become larger, they scatter other colours which join with the blue until the ultimate white colour is reached. Examples include star sapphire, bluish moonstone, and white to bluish colour of some opal. Regularly spaced layers (for diffraction) or thin layers (for interference) of minerals with different indices of refraction will produce colours if the separation of the layers is of the same order of magnitude as the wavelength of light. Opal and feldspars produce flashes of colour via diffraction. The play of colour in labradorite and the tarnish layers on chalcopyrite are good examples of interference colouring. Dispersion is related to mineral’s index of refraction. Lights of different wavelengths bend differently when passing obliquely through a mineral, causing light to spread out into colours of the visible spectrum. This is best seen in faceted gemstones such as diamond, pure rutile and zircon.
A remarkable scientific progress has been made in the past two decades in understanding minerals’ colouring phenomena. Based on our current knowledge of the optical properties of solids, we nowadays make use, for example, of rubies and sapphires in high power solid-state lasers.
Sources
W.Hamilton, A.Woolley and A.Bishop, Minerals, Rocks and Fossils, Hamlyn:: Reed International Books Ltd,1996
Kurt Nassau, The Physics and Chemistry of Color : The Fifteen Causes of Color, John Wiley & Sons, New York, 1983
Kurt Nassau, Scientific American, Vol.243, No.4, October, 124, 1980
Color in Minerals:
Physical Characteristics of Minerals
Color in Minerals
The Mechanism of Color Phenomenon:
Causes of Color in Minerals
Causes of Color in Minerals and Gemstones
Why Minerals Are Colored
Dr.Benu Chatterjee
The beautiful colours of minerals have been valued in all societies. A comprehensive review of the colour phenomena in minerals is presented.
Colour is the most eye-catching feature of minerals. The recognition of colours in minerals goes back to pre-historic ages when charcoal and iron oxides (red hematite mineral) were used to colour cave paintings which still retain their original brightness. The present article is a comprehensive review of the various features of the diverse mineral colours, namely mechanics, characterization, complexities and origins of colours.
1. Mechanics of Creating Mineral Colours
The visible white light forms a part of the electromagnetic spectrum we can see. It is made up of waves with wavelengths of 350 to 750 nanometres that correspond to the seven colours of the rainbow with violet and red having the shortest and longest wavelengths respectively. Also, wavelength (λ) and frequency (γ) of light wave are related to energy (E) (photon : a basic unit of electromagnetic radiation) as E = hc/λ = hγ where h is Planck’s constant and c is the speed of light. When a solid sample absorbs light, what we see is the sum of the remaining reflected colours that strike our eyes. If all the light energies are reflected back, the sample will appear colourless or white. However, it will appear black if all the energies are absorbed, leaving none of the visible spectrum reaching our eyes.
Minerals develop colours by interacting with white light when only part of the visible spectrum is absorbed leaving the rest to emit. When this happens, the energies absorbed are removed from the emitted light which will no longer be white, but have complementary colour associated with the emitted wavelengths. The observed colour of a mineral thus corresponds to those wave lengths of light that are least absorbed.
2. Characterization of Mineral Colours - Idiochromatic, Allochromatic and Pseudochromatic
Certain elements termed as chromophores, are pigmenting agents which provide colours to minerals when they are present as a part of the crystal lattice, or as impurity. Chief chromophores are the first row of transition metals in the Periodic Table with atomic numbers 22 to 29, namely titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. Mineral colours are classified into three main groups, namely idiochromatic. allochromatic and pseudochromatic.
Idiochramatic ("self coloured") minerals exhibit their own inherent permanent colours that are derived directly from the presence of one or more chromophores as a constituent of the crystal lattice. The property of chromophores determines which wavelengths of light to absorb and which ones to emit. Examples include Iron-based red cinnabar, copper-based blue azurite and green malachite minerals.
Allochromatic ("other coloured") minerals, originally colourless, develop colours owing to the presence of trace amounts (parts per million) of chromophores as impurity in the host crystal lattice. The colourless quartz mineral for example changes to purple colour (amethyst mineral) when iron is present as impurity in the lattice. Similarly, colourless corundum changes to blue sapphire or red ruby by the presence of trace amounts of either iron and titanium, or chromium respectively.
Pseudochromatic ("false coloured") minerals display colour only because of the tricks played by light. These minerals contain layers that create colours by various features of physical optics discussed later on. The colour may vary, but is often a unique property of the mineral. For example, opal, moonstone and labradorite all reflect light in a characteristic way, but the colours are not true to the types of minerals.
3. Complexities of Mineral Colour and Theories
In mineralogy, colour is one the primary diagnostic features to identify minerals. However, mineral colour is changeable, and unpredictable, and cannot be used for identification. For example, different impurities can cause fluorite mineral to come out in various colours. Also, there are minerals of similar colours. Furthermore, some minerals such as topaz and beryl undergo changes from dull to deeper colour on heating as practised in the gemstone industry. These complex features of mineral colours make it less reliable to identify a mineral from its colour. It is thus important to understand what causes colours in minerals.
In order to show colour, a mineral has to somehow disturb the balance of the light energies with preferential absorption or emission of certain wavelengths/energies of light. The theories of the origins of colours in minerals such as idiochromatic, allochromatic and pseudochromatic which cover most of the familiar minerals, are simple. Delving deeper, Kurt Nassau (1983) has separated causes of colour into fifteen mechanisms based on several groups, such as crystal field effects, physical optics, band gap theory and molecular orbital mechanism. However, in order to keep the present article short and simple, only the crystal field theory and physical optics are discussed. Any discussion of either the band gap theory or molecular orbital formalism is excluded – both of which nonetheless relate to less familiar materials such as organic materials, semiconductors and metal conductors.
3.a. Crystal Field Theory - Transition Metal Ions and Colour Centres
Crystal Field Theory is based on two mechanisms involving transition metal ions and colour centres:
Transition metal ions (simple cases) is based on electron interactions, and the crystal field effects can explain both idiochromatic and allochromatic colours. The innermost electrons of chromophore transition metals are too tightly bound to the nucleus to be excited by light energy (photon) because electrons, after all, generally like to move about in pairs. However, valence electrons in these metal ions are unpaired which along with inner 3-d orbitals partially filled with electrons, are prone to excitation to higher energy orbitals by photons. This phenomenon would satisfy the need for pairing up of electrons, and thus becomes responsible for colours in minerals. Excitatation is achieved by absorbing photon from visible incident light. The absorbed energies are subtracted from that of the incident light, resulting in the observed colour.
Transition metal ions (complex cases) lead to characteristics found in, for example, corundum and beryl. Both corundum (aluminium oxide) and beryl (beryllium aluminosilicate) minerals are colourless because all electrons are paired in the crystal lattice allowing no absorption of light. However, addition of same chromium ion (Cr 3+) as chromophore impurity to either mineral, changes them to become colourful of different shades, namely red (ruby) and green (emerald) respectively. This phenomenon indicates complexity of crystal field interactions. It is believed that the geometry of the atoms in beryl is different from corundum, allowing the host molecules to interact with chromium more weakly. This results in reduced energy differences between the energy levels which eventually results in developing complementary green colour rather than ruby red in corundum.
The colour centres (F-centres) are defects created by removing atoms usually by radiation from the crystal structure. The resulting “hole” may be filled by an electron from a neighbouring atom. This will leave behind an unpaired electron which, as discussed before, is prone to excitation by photons with ultimate creation of colours. The most familiar examples are amethyst and smoky quartz minerals.
3.b. Physical Optics Effects - Scattering, Diffraction, Interference and Dispersion
Colouration of pseudochromatic minerals is best explained by physical optics processes where, unlike crystal field theory, electrons are not involved. Light passing through a mineral interacts with the crystal structure, and colours are produced due to various phenomena such as scattering (which explains blue sky), diffraction (peacock feather), interference (oil slicks) and dispersion (rainbow spectrum through prism) of incident white light.
Scattering occurs due to reflection of light off sub microscopic (the finer the better) particles of solid or liquid. It would cause colours to appear because blue light with shorter wavelength is scattered more than red. As particles become larger, they scatter other colours which join with the blue until the ultimate white colour is reached. Examples include star sapphire, bluish moonstone, and white to bluish colour of some opal. Regularly spaced layers (for diffraction) or thin layers (for interference) of minerals with different indices of refraction will produce colours if the separation of the layers is of the same order of magnitude as the wavelength of light. Opal and feldspars produce flashes of colour via diffraction. The play of colour in labradorite and the tarnish layers on chalcopyrite are good examples of interference colouring. Dispersion is related to mineral’s index of refraction. Lights of different wavelengths bend differently when passing obliquely through a mineral, causing light to spread out into colours of the visible spectrum. This is best seen in faceted gemstones such as diamond, pure rutile and zircon.
A remarkable scientific progress has been made in the past two decades in understanding minerals’ colouring phenomena. Based on our current knowledge of the optical properties of solids, we nowadays make use, for example, of rubies and sapphires in high power solid-state lasers.
Sources
W.Hamilton, A.Woolley and A.Bishop, Minerals, Rocks and Fossils, Hamlyn:: Reed International Books Ltd,1996
Kurt Nassau, The Physics and Chemistry of Color : The Fifteen Causes of Color, John Wiley & Sons, New York, 1983
Kurt Nassau, Scientific American, Vol.243, No.4, October, 124, 1980
Color in Minerals:
Physical Characteristics of Minerals
Color in Minerals
The Mechanism of Color Phenomenon:
Causes of Color in Minerals
Causes of Color in Minerals and Gemstones
Why Minerals Are Colored