Colour in Minerals :

Colour : The term colour is used to describe at least three subtly different aspects of reality.

1. describes the property of an object as green grass2. characteristics of light rays green grass reflects green light absorbing light of other colors3. class sensations the brain interpretation of the specific manner in which the eye

perceives the light selectively reflected from grass results in the perception of green

According to Dean B. Judd Colour is that aspect of the appearance of objects and lights which depends upon the

spectral composition of the radiant energy reaching the retina of the eye and upon its temporal and Spatial distribution thereon

Conventional Wisdom of colour attributes blue green colour- to copper, deep blue - cobaltRed chromium.

However, chromium compounds can be red, orange, yellow, brown, green and even lilac colour

Only a detailed study using variety of tools spectroscopic techniques, chemical analysis, irradiation and heating required to identify the cause of colour.

Mineralogy : colour is often classified according to three types :

1. Idiochromatic or self coloured2. Allochromatic or other coloured3. Pseudochromatic or false coloured.

Colour theory :

Four distinct theories which cover the whole range of colours occurring in minerals

1. Crystal field theory : covers both idiochromatic and allochromatic colours caused bytransition elements (copper in turquoise and Cr in ruby) as well as colour centers amethyst

unpaired electrons are main cause for colour in minerals

2. Molecular Orbital Theory : applies where electrons orbit around more than one atom or ion and explainscharge transfer process colours

3. Band Gap Theory : interprets a wide series of colours ranging from metallic as in gold, slivercopper unique semiconductor colour black (galena), red (cinnabar), orange (realgar) or yellow(sulphur), colouless (diamond) - colour involve no impurities.

4. Physical Optical effects : group of colour effects dispersion produced fired in diamond, scatteringproduced star eye effects (star ruby) or interference produced by colour in thin films diffraction produced colours in opal, labradorite..

Color Cause Typical minerals TheoryTransition metal compounds Almandite, malachite, turquoise Crystal field theoryTransition metal impurities Citrine, emerald, ruby Crystal field theory

Color centers Amethyst, fluorite, smoky quartz Crystal field theoryCharge transfer Blue sapphire, crocoite, lazurite Molecular orbital theory

Organic materials Amber, coral, graphite Molecular orbital theoryConductors Copper, iron, silver Band theory

Semiconductors Galena, proustite, pyrite, sulfur Band theoryDoped semiconductors Blue diamond, yellow diamond Band theory

Dispersion "Fire" in faceted gems Physical opticsScattering Moonstone, "stars", "eyes" Physical optics

Interference Iridescent chalcopyrite Physical opticsDiffraction Opal Physical optics

Twelve types of color in minerals :

Idiochromatic coloration in transition metal compounds

Cerium: Parisite (yellow)

Chromium : Crocoite (orange); phoenicrocoite (red); uvarovite (green)

Cobalt : Erythrite, roselite, spherocobaltite (pink)

Copper : Azurite, chrysocolla, turquoise (blue); dioptase, malachite (green); cuprite (red)

Iron: Lazulite (blue); olivine (green); almandite, ludlockite (red); cacoxenite, goethite (yellow)

Manganese : Rhodochrosite, rhodonite (pink); spessartite (orange); ganophyllite (yellow)

Nickel: Bunsenite (green)

Uranium : Autunite, carnotite (yellow); curite, masuyite (orange)

* Charge transfer may also be present in some of these.

Chromium : Emerald, grossularite, hiddenite, Cr-jade, Cr-tourmaline (green); alexandrite (green-red); ruby, spinel, topaz (red)

Cobalt: Lusakite (blue)

Iron: Aquamarine, tourmaline (green); chrysoberyl, citrine, idocrase, orthoclase (yellow); "greened amethyst" (green); jade (green-yellow-brown)

Manganese: Morganite, spodumene, tourmaline (pink); andalusite (green, yellow)

Nickel: Chrysoprase, Ni-opal, (green)

Vanadium: Apophyllite, V-emerald, "tsavorite" V-grossularite (green); "alexandrite-colored sapphire" (green-red)

Allochromatic coloration by transition metal impurities

1. The crystal field Theory :

Color best explained by the crystal field theory involves predominantly ionic crystals containing ions with unpaired electrons.

Elements with partially filled d shells such as V, Cr, Mn, Fe, Co, Ni, and Cu, or in elements with partially filled f shells such as the actinides and lanthanides.

CFT - It is successful in describing the magnetic properties, colors, hydration enthalpies of transition metal complexes, but it cannot provide an adequate description of bonding.

Crystal field theory was developed by the physicists Hans Bethe and John Hasbrouck van Vleck. It was combined with molecular orbital theory to form ligand field theory, which delivers insight into the process of chemical bonding in transition metal complexes

Pure crystal field theory assumes that the interactions between the metal ion and the ligands are purely electrostatic (ionic). The ligands are regarded as point charges.

Shape of d orbitals important in order to get an idea of which of these orbitals will interact with the ligands (point charges).

The bonding between a transition metal and the ligands is due to the attraction between the positively charged metal ion and the electrons of the ligand.

Crystal field theory describes how the ligands affect the d electrons and split them in to higher and lower (in terms of energy) groups - the energy difference between the two sets is given the symbol .

This crystal field splitting (i.e. the size of o) depends on several factors:

- the nature of the metal ion. - the metal's oxidation state. A higher oxidation state leads to a larger splitting. - the arrangement of the ligands around the metal ion. -the nature of the ligands surrounding the metal ion.

The stronger the effect of the ligands then the greater the difference between the high and low energy 3d groups.

The splitting of d orbital energies and its consequences are at the heart of crystal field theory

The most common type of complex is octahedral; here six ligands form an octahedron around the metal ion. The ligands point directly at the metal d-orbitals and cause a large splitting.

Tetrahedral complexes are the second most common type; here four ligands form a tetrahedron around the metal ion, and since in this case the ligands' electrons aren't oriented directly towards the d-orbitalsthe energy splitting will be lower than in the octahedral case.

Transition metals form ions with partly filled d-orbitals. There are 5 d-orbitals which each can contain two electrons.

These five d-orbitals are degenerate - they have the same energy when there are no ligandsaround the metal. .

When a ligand approaches the metal ion, the electrons from the ligand will be closer to some of the d-orbitals and farther away from others.

The electrons in the d-orbitals and the electrons in the ligand repel each other (because they're both negatively charged), and so d-electrons closer to the ligands will have a higher energy than ones further away because they feel more repulsion.

Thus, the d-orbitals will split in energy. What determines the way that the orbitals split is the orientation of the ligands with respect to the metal d orbitals.

Colour and Fluorescence of Ruby and Emerald :

Energy levels, transitions, and color absorptions in ruby (a) to (e) and in emerald (f)

1 % Cr3+ substituted for Al3+ in distorted octahedral sites of A12O3 with very strong bonding resulting in an Al-O distance of 1.91 A

The crystal field theory is appropriate for three types of color: - caused by transition metals either as a major ingredient or as an impurity, - and that produced by color centers.

Idiochromatic colors caused by major transition-metal constituents

many more can be deduced with some certainty from merely knowing the chemical composition of minerals containing essential transition-element ingredients.

Nevertheless, there are various pitfalls: some valence states need not cause color (e.g. monovalent Cu)

Allochromatic colors caused by transition-metal impurities

For this type of color are the nature of the streak - usually represents the color of the pure compound, and the occurrence of such a mineral in various colors;

The cause of color cannot necessarily be ascribed to an impurity just because the impurity is known to be present in a significant amount.

Other impurities at a lower concentration may be involved, or a color center may be present. Laboratory synthesis of minerals with the controlled addition of impurities may even be needed for unambiguous explanations of color.

same impurity can cause widely different colors, such as Cr-caused ruby red and emerald green

some emeralds which derive their color either partially or totally from vanadium

Color centers :

The unpaired electron which produces color by light absorption into excited states do not have to be located on a transition element ion - but - under certain circumstances it can be located on a nontransition-element impurity ion or on a crystal defect such as a missing ion.

Both of these can be the cause of color centers. If an electron is present at a vacancy, we have an "electron" color center; if an electron is missing from a location where there usually is an electron pair, we have a "hole" color center.

Purple "F center" or Frenkel defect of fluorite, one of many types - color center in fluorite.

There are several ways by which an F- ion can be missing from its usual position: this can occur during growth or when energetic radiation displaces an F- ion from its usual position to another point in the crystal;

Or on growing fluorite in the presence of excess Ca, or by removing some F from a crystal by the application of an electric field.

Since the crystal must remain electrically neutral, an electron usually occupies the empty position to produce the F-center or "electron color center" This unpaired electron can now exist in excited states, the energy of which is controlled by the same crystal field factors

R. V. Karanth

Quartz structure (schematic): (A) normal, (B) containing Al3+ substituted for Si4+ with an H+ for charge neutrality.

Radiation ejects one of a pair of electrons from an O2-and leaves a "hole" color center of smoky quartz

Smoky quartz, - "hole color center." Necessary precursor to this color center is the presence in the quartz is impurity Al3+ substituting for Si4+ ions with some alkali (e.g. Na+) or a hydrogen ion (H+) nearby to maintain electroneutrality

Interstitial Al does not serve as a precursor to form smoky quartz. Most natural quartz contains substitutional Al at the several hundred ppm

(a-d) : large formation energy and bleaching energy(e) : Large formation energy but less bleaching temp required(f) : Small formation and bleaching energy required

(Smoky Quartz)

(Maxixe type blue beryland Irradiated topaz)

(Phosphorescence-Small bleaching energy BC

-Large phosphorescence(BC slightly deeper)

-Small phosphorescence(BC shallower)

-If deeper BC level would result in TL)

(Photo chromatic Hackmanite, Na8Al6Si6O24(Cl2,S)Colourless + UV = Pink/red, bleaches fast)

Amethyst - similar color center - involving Fe instead of Al. Depending on the location and environment of the Fe the color obtained on heating amethyst is either yellow (citrine as in most amethyst) or green (the "greened amethyst" ).

Though iron a transition element impurity involved in colouration should not be considered asallochromatic as it is present only as a precursor for amethyst color - itself derives from a color center.

Irradiation after heating can recreate the color center and the amethyst color.

2. The molecular orbital Theory

Applied to several different types of situations where electrons are involved - are not simply located on single atoms or ions, but must be considered to be present in multicentered orbits.

The results vary depending on whether metal-metal, metal-nonmetal, or nonmetal-nonmetal centers The result of a molecular orbital treatment is similar to that of a crystal field treatment; both theories result in a set of energy levels and associated transition probabilities.

Metal-metal charge transfer (Intervalence charge transfer)

Blue sapphire -corundum, Al2O3, contain both Fe and Ti impurities. Each of the impurity ions can exist in two valence states two combinations possible.

(a) Fe2+ and Ti4+; and (b) Fe3+ and Ti3+.

The MO theory treats molecular bonds as a sharing of electrons between nuclei. Unlike the V-B theory, which treats the electrons as localized balloons of electron density,

The MO theory says that the electrons are delocalized. - means that they are spread out over the entire molecule

when two atoms come together, their two atomic orbitals react to form two possible molecular orbitals. One of the molecular orbitals is lower in energy. It is called the bonding orbital and stabilizes the molecule. The other orbital is called an anti-bonding orbital. It is higher in energy than the original atomic orbitals and destabilizes the molecule.

Below is a picture of the molecular orbitals of two hydrogen atoms come together to form a hydrogen molecule:

The MO Theory has five basic rules:

1. The number of molecular orbitals = the number of atomic orbitals combined2. Of the two MO's, one is a bonding orbital (lower energy) and one is an anti-bonding orbital (higher energy)3. Electrons enter the lowest orbital available4. The maximum no. of electrons in an orbital is 2 (Pauli Exclusion Principle)5. Electrons spread out before pairing up (Hund's Rule)

Below is a molecular orbital energy diagram for the hydrogen molecule. The two AO's or atomic orbital combine to form 2 MO's - the bonding and the anti-bonding molecular orbital.

- the five rules have been followed, the electrons having been placed in the lowest energy orbital (rule 3) and have paired up (rule 4) and there are only two electrons in the orbitals (rule 5).

Colors caused by molecular orbital transitions

A. Metal-metal charge transfer:

Fe2+ - Fe3+ / Fe3+ - Fe2+ : Cordierite, vivianite (blue), magnetite, etc. (black?)Fe2+ - Ti4+ / Fe3+ - Ti3+ : Kyanite, blue sapphire (blue)Mn2+ - Mn4+ / Mn3+ - Mn3+: Manganite, bixbyite, etc. (black?)

B. Metal-nonmetal charge transfer:Cr6+ - 02- . Crocoite (orange)V5+ - 02- . Vanadinite (orange)Metal-Sulfur: Pyrite, marcasite, etc. (see band-gap semiconductors)

C. Electrons not on metal ions:S-3: Lazurite in lapis lazuli (blue)pi electrons: Graphite (black)Organic pigments: amber, ivory (brown); coral (red, black); pearl (pink, green, blue black); bitumen, lignite, etc. (brown to black)

Electrons not on metal ions

Several minerals - colors are best described by molecular orbitals not involving any metal ions.e.g. - deep blue of lazurite (lapis lazuli), (Na,Ca)8(AlSi)12O24(S2,SO4), contains sulfur molecular units and no unpaired electrons.

The excited levels of S-3 molecular units appear to explain the color

A single electron can be caused to transfer from the Fe to the Ti by light absorption and back again.

Since state (b) has a higher energy than state (a), the transition from (a) to (b) involves the absorption of energy, producing a broad intense absorption band at the red end of the spectrum deep blue color.

The Fe-Ti distances are 2.65A in the c direction, along the optic axis, but 2.79A in the perpendiculardirection results in dichroism

Another charge transfer process involving only Fe, Fe2+ + Fe3+ and Fe3+ + Fe2+ can also occur. - homonuclear metal-metal charge transfer

R. V. Karanth

The color of cordierite has also been attributed to this type of Fe2+ - Fe3+ charge transfer

black minerals containing Fe, Mn, Ti, etc., such as magnetite, ilmenite, manganite, schorl, etc., owe their color to some kind of charge transfer.

Charge transfer transitions usually have high transition probabilities, thus giving intense colors, and tend to dominate crystal field transition colors when both are present, although they may also occur in the ultraviolet part of the spectrum.

Color - hydrated iron phosphate vivianite. When fresh, this compound contains only Fe2+ - colorless.

Air-oxidation converts some of the Fe2+ to Fe3+, charge transfer becomes possible and results in the deep blue color. iron in vivianite is present in two different sites, and one of these oxidizes preferentially. Tri- chroism with the deep blue color seen only with light polarized along the direction connecting the two different Fe sites, i.e. along the Fe2+ - Fe3+ direction

e.g. PbCrO4, Wulfenite and Scheelite (WO-4 units), Lapis lazuli (electrons not on metal ions, S-3 unit)

How impure must the crystal be for it to appear colored? For sapphire, only 0.01% of titanium and iron are needed as the electron transfers very easily

The process absorbing the photon and driving the system from the initial to the excited state is effectively instantaneous.

It actually takes about a femtosecond or 10-15 second.The process is instantaneous and leaves the system in an excited state

Emerald Green Beryl Aquamarine

heliodor Bixbite

Morganite Goshenite

Be3Al2Si6O18

R. V. Karanth

Colourless beryl

Yellow beryl

Greenish - yellow

Blue beryl

Irradiation (2 Mev)

Heating ~ 300oC

Heating ~ 400oC

3. The Band Gap Theory

The crystal field and molecular orbital theories apply to electrons located on ions, at defects, and on groups of atoms. Band theory treats electrons as belonging to the crystal as a whole and applies to a wide range of materials - metallic conductors and semiconductors

In a crystal of a metal such as copper or an alloy such as brass, each metal atom contributes its outer electrons, those usually involved in chemical bonding, to a joint pool. These electrons are free to move throughout the whole crystal. Free movement of such a pool of electrons in a metal results in the high electrical and thermal conductivities as well as in the metallic luster and metallic reflection

typical metal, there are more than 1023 electrons per cm3 all equivalent to each other

Applying quantum consideration individual energy levels broaden into bands

At each energy levels there is room for so many electrons (density of states) The available number of electrons fill the band structure up to the Fermi Surface

Metals have one continuous bands extending to higher energies, but density of state different fordifferent energies..

Surface of metal can absorb light of energy, transition occurs between filled part the outer empty parand most of the light is remitted at the surface (metallic luster)

Efficiency of this process depends on the shape of the band above the Fermi SurfaceResulting in colour difference in metals and alloys.

Colour of semiconductors

There is a large group of minerals where the bonding is predominantly covalent (electron sharing) andwhere the average number of bonding electrons is exactly four per atom.

-elements of the fourth column of the periodic table such as diamond, IV-IV compounds such asmoissanite (SiC), II-VI compounds such as greenockite (CdS).

Band theory applied to such materials shows a gap between two separate bands, lower - "valence band," filled with electrons, while the upper - "conduction band," is completely empty

wavelength

Photon/energy

wavenumber

If the band gap is smaller than the visible-light range - all light energies can be absorbed and a dark gray or black color results, as in galena

If a narrow-band-gap semiconductor is heated to a high enough temperature so that thermal excitation can bridge the band-gap, then the electrons excited into the conduction band will cause the material to behave as a metal

large-band-gap semiconductors" larger than the range of visible light will not be able to absorb in the visible and will thus be colorless, - pure diamond and sphalerite with Eg about 5.4 eV and 3.5 eV

With an intermediate band-gap of about 2eV, as in proustite, only red light is transmitted; all other colors have energies larger than Eg and therefore are absorbed

With a band-gap of about 2.5eV, as in greenockite, only blue and violet are absorbed and the resulting is yellow.

The overall sequence of band gap colors is thus black/red/orange/yellow/colorless.

Impurities in semiconductors

Medium- and large-band-gap semiconductors do not conduct electricity at room temperature when pure, since the ambient excitation is insufficient to bridge the band-gap.

Presence of certain impurities can affect both the conductivity as well as the light absorption.

Consider nitrogen impurity atoms substituting for carbons in a diamond crystal (Eg about 5.4 eV). N has one more electron than C, each nitrogen donates one extra electron above the Fermi surface and these "donor" electrons form an impurity level within the band-gap,

The nitrogen impurity level is still 4eV below the conduction band (a "deep" donor). It can only absorb a little violet at 3eV, thus giving a yellow color.

A typical deep-yellow diamond may contain one nitrogen atom for every 100,000 carbon atoms. The reduction in the bandgap is insufficient to permit electrical conductivity at room temperature.

Boron - one electron less than carbon, acts in a similar way to produce "acceptor levels" in the band-gap.

These are "hole" levels which can accept electrons from the valence band and are located only 0.4eV above the top of the valence band.

The impurity band is not just a single level, but has a complex structure. resulting in a blue color.

Since the boron acceptor level is "shallow," ambient thermal excitation can raise electrons from the valence band into the acceptor level, and the resulting holes in the valence band can then conduct electricity.

Type IIB blue conducting diamonds such as the "Hope" contain typically one boron atom per million carbon atoms.

Blue irradiated diamonds do not conduct electricity, thus providing distinction between the boron acceptor and the irradiation-produced blue color center in diamond.

Diamonds can contain many elements present as substitutional (i.e., replacing carbon atoms in the lattice) impurities: nitrogen, boron, hydrogen, oxygen, sulfur, nickel, cobalt, and iron have all been detected.

Diamond TypesDiamonds can be scientifically classified into 4 types, known as type 1a, 1b, 2a, and 2b.

Diamonds are made of carbon, and are extremely pure, but in almost all diamonds there are tiny proportions of other elements, interspersed within the carbon as part of their crystal structure. These "impurities" are not what are known as inclusions, and are so small as to be invisible even under a very powerful microscope.

Type 1 Diamonds Type 1 diamonds contain nitrogen. About 98% of all diamonds are type 1a

Type 1aIf the nitrogen atoms are clustered together within the carbon lattice, then the diamond is said to be a Type 1a diamond. Because these diamonds absorb blue light, they can have a pale yellow or brown color. 98% of diamonds are Type 1a.

Type 1bIf the nitrogen atoms are evenly spread out throughout the carbon lattice, then the diamond is said to be a Type 1b diamond. These diamonds absorb green light as well as blue light, and have a darker color than type 1a diamonds. Depending on the precise concentration and spread of the nitrogen atoms, these diamonds can appear deep yellow ("canary"), orange, brown or greenish. Less then 0.1% of diamonds belong to Type 1b.

Type 2Type 2 are diamonds that absorbed no, or very few, nitrogen atoms.

Type 2aThese diamonds can be considered as the "purest of the pure" - they contain no, or minuscule amounts of impurities and are usually colorless. Unless, that is, the carbon tetrahedrons that make up the diamond were twisted and bent out of shape while the diamond rose to the surface of the earth. An imperfect carbon lattice will make the diamond absorb some light, which will give it a yellow, brown or even pink or red color. 1-2% of diamonds belong to Type 2a.

Type 2bThese diamonds contain no nitrogen - but they do contain boron, which absorbs red, orange and yellow light. These diamonds therefore usually appear to be blue, although they can also be grey or nearly colorless. All naturally blue diamonds belong to Type 2b, which makes up 0.1% of all diamonds.

Pseudochromatic colors caused by physical optics effects

1. Color Based on Dispersion:

"Fire" in high dispersion gem-stones such as diamond, zircon, ruble, and strontium titanate.

2. Color Based on Scattering:

Chatoyancy as in cat's eyes, tiger's-eye.

Asterism as in star corundum, garnet, quartz.

Luster as in pearl, foliated talc, brucite, apophyllite, fibrous asbestos, gypum (satin spar), etc.

Aventurescence as in sunstone, aventurine albite, aventurine quartz, schiller spar, silver-sheen obsidian.

Adularescence as in moonstone (bluish), milky opal.

3. Color Based on Interference:

Interference effects in thin films such as the tarnish film on chalcopyrite, columbite, and bornite and within the cracks of iris quartz.

4. Color Based on Diffraction:

Diffraction grating produced by periodic spacings as in opal, labradorite, and iris agate

Asterism

Chatoyancy(Cats eye)

Adularescence

Infrared spectrum of Type IaB diamond. (1) region of nitrogen impurities absorption, (2) B2 peak, (3) self absorption of diamond lattice, (4) hydrogen peaks