American Mineralogist, Volume 63. pages 219-229, 1978
The origins of color in minerals
KURT NASSAU
Bell Laboratories Murray Hill, New Jersey 07974
Abstract
Four formalisms are outlined. Crystal field theory explains the color as well as the fluores- cence in transition-metal-containing minerals such as azurite and ruby. The trap concept, as part of crystal field theory, explains the varying stability of electron and hole color centers with respect to light or heat bleaching, as well as phenomena such as thermoluminescence. The molecular orbital formalism explains the color of charge transfer minerals such as blue sapphire and crocoite involving metals, as well as the nonmetal-involving colors in lazurite, graphite and organically colored minerals. Band theory explains the colors of metallic minerals; the color range black-red-orange- yellow-colorless in minerals such as galena, proustite, greenockite, diamond, as well as the impurity-caused yellow and blue colors in diamond. Lastly, there are the well-known pseudo- chromatic colors explained by physical optics involving dispersion, scattering, interference, and diffraction.
Introduction The approach here used is tutorial in nature and references are given for further reading or, in some Four distinct physical theories (formalisms) are instances, for specific examples. Color illustrations of required for complete coverage in the processes by some of the principles involved have been published which intrinsic constituents, impurities, defects, and in an earlier less technical version (Nassau, 1975a). specific structures produce the visual effects we desig- Specific examples are given where the cause of the nate as color. All four are necessary in that each color is reasonably well established, although reinter- provides insights which the others do not when ap- pretations continue to appear even in materials, such plied to specific situations. An outline with examples as smoky quartz (Nassau, 1975b), which have been is given in Table I. extensively investigated over many years. This classification goes well beyond the traditional Specific causes for color in the majority of minerals idiochromatic ("self-colored," as from a major in- are in fact not known, and detailed investigations, gredient), allochromatic ("other-colored," as from often requiring precision spectroscopy, impurity an impurity), and pseudochromatic ("false-colored" analysis down to ppm levels, magnetic resonance, as originating from physical optical effects), although and sometimes radiation and even laboratory syn- these three are, in fact, indirectly included in several thetic procedures, may be necessary for unambiguous places in Table I. The current approaches transcend conclusions. As an example, the deep blue of dia- the conventional wisdom which attempts to attribute monds such as the "H ope" almost certainly origi- blue-green colors to copper, deep blue to cobalt, red nates in merely a few boron atoms per million of or green to chromium, and so on. To give only one carbon atoms (this also provides the electrical con- example: deep blue colors may be produced by ductivity). Previously the blue color had been attrib- idiochromatic copper (shattuckite), allochromatic uted to aluminum present at much higher concentra- cobalt (blue spinel), by a color center (M axixe-type tion, but synthesis demonstrates that aluminum in irradiated beryl), by charge transfer between iron and this instance does not produce a blue color, whereas titanium (blue sapphire), by molecular orbital effects boron does (Chrenko, 1973). in S3 units (lapis lazuli), by boron acceptors in a Many different systems of designation are used for band gap (blue diamond), and by pseudochromatic the energy of quantized systems (e.g. energy as wave light interference (blue regions in opal). numbers in cm-I, electron volts eV, kcaIlmole, etc.; 0003-004X/78/0304-0219$02.00 219 220 NASSAU: COLOR IN MINERALS
Table I. Twelve types of color in minerals
Color Cause Typical minerals Formalism
Transition metal compounds Almandite, malachite, turquoise Crystal field theory Transition metal impurities Citrine, emerald, ruby Crystal field theory Color centers Amethyst, fluorite, smoky quartz Crystal field theory
Charge transfer Blue sapphire, crocoite, lazurite Molecular orbital theory Organic materials Amber, coral, graphite Molecular orbital theory
Conductors Copper, iron, silver Band theory Semiconductors Galena, proustite, pyrite, sulfur Band theory Doped semiconductors Blue diamond, yellow diamond Band theory
Dispersion "Fire" in faceted gems Physical optics Scattering Moonstone, "stars", "eyes" Physical optics Interference Iridescent chalcopyrite Physical optics Diffraction Opal Physical optics or the equivalent light wavelength in nm, J.Lm,or A, There are no unpaired electrons when all electron or its frequency in see-lor Hertz, Hz). The eV scale shells are completely full or empty as in Cr6+, CeH, used in this article and its relationship to the spec- or Cu+, when this type of color cannot result. Of trally pure colors is illustrated in Figure I. these transition elements, Fe is the most plentiful (about 5% of the earth's crust) and is therefore the The crystal field formalism dominant color contributor in minerals. Color best explained by the crystal field formalism Consider now a mineral containing, say, some tri- involves predominantly ionic crystals containing ions valent chromium. The electronic configuration of with unpaired electrons. These usually originate in Cr3+ is Is22s22p63s23p63tf; here only the three un- elements with partially filled d shells such as V, Cr, paired electrons in the 3d shell can interact with vis- Mn, Fe, Co, Ni, and Cu, or in elements with partially ible light so as to produce absorption and color. Due filled f shells such as the actinides and lanthanides. to the quantization of energy, a system such as Cr3+ can exist only in a limited number of states, the "energy levels." These energy levels can be shown on ELECTRON WAVELENGTH VOLTS a diagram such as Figure 2, by their energy content COLOR CX,) lev) with respect to the "ground state," i.e. the state of the U ion when it is not interacting with any kind of radia- L SW 2537 T tion and is in its lowest energy state. R A V I 4 0 L D E LW 3660 3 T VIOLET------4000 3 t BLUE INCREASING C INCREASING 5000 Q) TEMPERATURE GREEN ENERGY > YELLOW ~B 2 >- <00: 7000 W RED RED------Z FLUORESCENCE w uw I z z N 0 w 10,000---- ;::
The actual calculation of such energy levels is quite field is well illustrated by comparing ruby to emerald, complex and, for a given atom, has to include the in which Cr3+ also substitutes for AI3+. The distorted following parameters: the valence state, the symmetry six-coordinated site and AI-O distances in emerald of the ion's environment (i.e. the coordination poly- are similar to those in ruby, but the effect of the hedron of charges which produces the "crystal field," added Be and Si and the change in structure from the also sometimes called the "ligand field") including hexagonal close-packed oxygen framework of Alz03 any distortions from ideal tetrahedral, octahedral, to the more open structure of Be3AlzSis018 leads to etc.; and the strength of the crystal field, i.e. the metal-oxygen bonds of higher covalency than in nature and strength of the bonding. The result of ruby, a lower electron density on the oxygens, and such a calculation is an energy level scheme such as accordingly produces a crystal field of reduced that of ruby, (about 1% Cr3+ substituted for AI3+ in strength. The resulting relatively small shifts of the C the distorted octahedral sites of Alz03 with very and D absorption bands in Figure 2f essentially elimi- strong bonding resulting in an Al-O distance of 1.91 nate the red transmission but intensify the blue-green A) shown in Figure 2a. transmission, leading to the characteristic emerald The crystal field calculation not only gives the ex- color. The fluorescence level B is almost unchanged, cited energy levels (only the first three are shown in and here again the strong red fluorescence adds to the Figure 2a) with respect to the ground state A, but quality of the gemstone color when not quenched by also a set of "selection rules," covering the probabil- iron impurities. A similar change in crystal field re- ity of parallel electron paths. For example, excitation sults in a change from red to green as the concentra- (light absorption) can result in this instance in a tion of CrZ03 in Alz03 is increased, resulting in a less change from the ground state A to states C and D ionic bonding and a reduced crystal field. with high probability, but the change from A to B is Alexandrite (Cr3+ in AlzBe04) has an absorption relatively forbidden (Fig. 2b). De-excitation cannot scheme intermediate between those of ruby (Fig. 2e) occur from D or C back to A but must first pass and emerald (Fig. 2f). The almost equally balanced through state B with the emission of heat to the red and blue-green transmission bands result in the lattice as in Figure 2c; this can now be followed by eye perceiving either the one or the other as domi- light emission, the red fluorescence of ruby, from nant, depending on the energy distribution of the level B back to the ground state A as in Figure 2d. incident light (appearing green in daylight and red in Additional complications not so far mentioned in- incandescent light), and are responsible for the so- clude temperature and other broadening of energy called "alexandrite effect." Note that a different ion, levels and the fact that, since the crystal field can V3+, in a different lattice, Alz03, produces almost the differ with orientation in an anisotropic crystal, so identical effect in natural or synthetic "alexandrite- can the energy level scheme, resulting in pleochroism colored corundum." The alexandrite effect has, in (dichroism in the case of ruby). The actual unpola- fact, been observed in a number of different minerals rized absorption spectrum of ruby shown in Figure 2e (White et al., 1967). illustrates how the broadened absorbing D and C A last complication of crystal-field-caused color is levels produce violet and green-yellow absorption that the same ion may enter several different sites in a bands. The remaining light transmission produces the mineral, thus providing several sets of energy level strong red color of ruby with its blue (purple) cast. schemes and resulting in superimposed absorption Some of the energy absorbed into the C and D ab- bands. If there is transfer of electrons among differ- sorption bands (as well as into other absorption ent sites, then the molecular orbital formalism de- bands at higher energies in the ultraviolet not shown) scribed below can produce additional absorptions. reappears as red fluorescence from energy level B Detailed treatments of crystal field concepts have (which remains quite narrow), thus adding to the red been given by Burns (1970) and Wood (1969), and glow of this gemstone. It can be seen to be a coinci- may also be found in recent inorganic texts such as dence that both the color as well as the fluorescence that of Cotton and Wilkinson (1972). Intermediate of ruby is red. It should be noted that the presence of treatments by Loeffler and Burns (1976), Frye (1974), Fe impurity can completely "quench" the fluores- and Fyfe (1964) can be recommended. cence by siphoning off energy from the fluorescence The crystal field formalism is appropriate for three level B without significantly affecting the absorption- types of color: those caused by transition metals ei. caused color. ther as a major ingredient or as an impurity, and that The effect of a change in the strength of the crystal produced by color centers.
------222 NASSAU: COLOR IN MINERALS
Table 2. Idiochromatic coloration in transition metal compounds* be present. Laboratory synthesis of minerals with the controlled addition of impurities may even be needed Cerium: Parisite (yellow) ChrQffiium: Crocoite (orange); phoenicrocoite (red); for unambiguous explanations of color. uvarovite (green) Cobalt: Erythrite, roselite, spherocobaltite As discussed above, the same impurity can cause (pink) widely different colors, such as Cr-caused ruby red Copper: Azurite, chrysocDlla, turquoise (blue); dioptase, malachite (green); cuprite (red) and emerald green. However, there are some emer- Iron: Lazulite (blue); olivine (green); almandite, alds which derive their color either partially or totally ludlockite (red); cacoxenite, goethite (yellow) from vanadium (Wood and Nassau, 1968). Manganese: Rhodochrosite, rhodonite (pink); spessartite (orange); ganophyllite (yellow) Color centers Nickel: Bunsenite (green) Uranium: Autunite, carnotite (yellow); curite, The unpaired electron which produces color by masuyite (orange) light absorption into excited states does not have to *Charge transfer may also be present in some of be located on a transition-element ion; under certain these. circumstances it can be located on a non-transition- element impurity ion or on a crystal defect such as a Idiochromatic colors caused by major transition-metal missing ion. Both of these can be the cause of color constituents centers. If an electron is present at a vacancy, we have Several examples of this type of color are given in an "electron" color center; if an electron is missing Table 2 and many more can be deduced with some from a location where there usually is an electron certainty from merely knowing the chemical compo- pair, we have a "hole" color center. Color centers can sition of minerals containing essential transition-ele- also be treated in the band formalism, and a good ment ingredients. Nevertheless, there are various pit- review is that of Schulman and Compton (1962). falls: some valence states need not cause color (e.g. 1\1any color centers are known, but the exact color- monovalent Cu); the color may be disguised or modi- causing mechanism has been established in only a fied by impurities; molecular orbital effects may also very few instances. One of these is the purple "F- be operative; and so on. center" or Frenkel defect of fluorite, one of many types of color center which can form in fluorite. Fig- AI/achromatic colors caused by transition-metal impu- ure 3A is a two-dimensional representation of the rities CaF2 structure. There are several ways by which an Some well-established examples of impurity- F- ion can be missing from its usual position: this can caused color are given in Table 3. The touchstones of occur during growth or when energetic radiation dis- this type of color are the nature of the streak, which places an F- ion from its usual position to another usually represents the color of the pure compound, point in the crystal; we can also create such centers by and the occurrence of such a mineral in various col- growing fluorite in the presence of excess Ca, or by ors; it is usually safe to assume that the lightest color removing some F from a crystal by the application of represents the pure mineral. The cause of color can- an electric field. not necessarily be ascribed to an impurity just be- Since the crystal must remain electrically neutral, cause the impurity is known to be present in a signifi- an electron usually occupies the empty position to cant amount. Other impurities at a lower produce the F-center or "electron color center" as concentration may be involved, or a color center may shown in Figure 38. This unpaired electron can now exist in excited states, the energy of which is con- Table 3. Allochromatic coloration by transition metal impurities trolled by the same crystal field factors described previously; color and fluorescence can now occur by Chromium: Emerald, grossularite, hiddenite, Cr- jade, Cr-tourmaline (green); alexandrite the same types of transitions as described above. (green-red); ruby, spinel, topaz (red) In the case of smoky quartz, there is a "hole color Cobalt: Lusakite (blue) Iron: Aquamarine, tourmaline (green); chrysoberyl, center." A necessary precursor to this color center is citrine, idocrase, orthoclase (yellow); "greened amethyst" (green); jade (green- the presence in the quartz (Fig. 4A) of impurity AI3+ yellow-brown) substituting for SiH ions with some alkali (e.g. N a+) Manganese: Morganite, spodumene, tourmaline (pink); andalusite (green, yellow) or a hydrogen ion (H+) nearby to maintain electro- Nickel: Chrysoprase, Ni-opal, (green) Vanadium: Apophyllite, V-emerald, "tsavorite" neutrality (Fig. 48). Interstitial AI does not serve as a V-grossularite (green); "alexandrite- precursor to form smoky quartz. Most natural quartz colored sapphire" (green-red) contains substitutional Al at the several hundred ppm NASSAU: COLOR IN MINERALS 223
These and other color centers are listed in Table 4. Note that irradiation with energy as low as ultraviolet light can produce color centers, and that some of these colors are not stable to light exposure. These phenomena can be described on energy level dia- A grams as shown in Figure 5, using the trap concept. The diagram of Figure 5a could be that of smoky quartz. Starting with colorless quartz (containing the substitutional AI precursor) in the ground state A, radiation is absorbed with the system moving into the excited state D. Due to a selection rule the system must move to state B, as shown in Figure 5b, before it can return to the ground state. B State B, the color center, is a "trapped" state with Fig. 3. Fluorite structure (schematic): (A) normal, (B) containing an energy barrier from B to C which must be sur- an "F-center" where a fluorine ion has been replaced by an electron. mounted before the color center can disappear. While in State B light can be absorbed into other excited level, but this in itself does not produce color, since levels such as E and F (Fig. 5c), and cause the smoky there are no unpaired electrons. If such quartz is color. This color will remain until enough energy is irradiated with X-rays, gamma rays, neutrons, etc., supplied (e.g. by heating to 400°C) to permit the or if it was exposed to radioactive materials emitting system to overcome the barrier and return to the low levels of such radiation over geologic time peri- ground state, as in Figure 5d. The ground state is ods, then one of a pair of electrons on an oxygen colorless because the transition to the lowest absorb- adjacent to each Al can be ejected from its position as ing excited level 0 requires a larger energy than that shown in Figure 4B, leaving unpaired electrons. present in visible light. In case of a hole color center, These "hole" defects (the hole indicates that there is the trap is not physically located at the center (see now only one electron where there had formerly been below); the energy diagram, applying to the hole-trap a pair) can now have excited energy levels and light- complex" is the same as that for an electron color absorbing transitions which produce the smoky color center. In the energy level scheme of Figures 5a to 5d, of quartz (Nassau, 1975b). the energy of formation of the color center (A to D) is The electron displaced in the process depicted in large and the energy for its destruction or "bleach- Figure 4B becomes "trapped" somewhere else in the ing" (B to C) is also relatively large. The light-stable quartz crystal without causing any additional light color centers of Table 4 are of this type. absorption. If the smoky quartz is now heated (say to 400°C-see Nassau and Prescott, 1977) these dis- placed electrons can be released from their traps and return to the hole center; all electrons are again paired and the quartz returns to colorless. The bleaching temperature is thus not a characteristic of the color center itself and often varies. On re-irradia- tion the color can once again be restored. A RADIATION Amethyst is a similar color center, involving Fe instead of AI. Depending on the location and envi- ronment of the Fe the color obtained on heating amethyst is either yellow (citrine as in most amethyst) or green (the "greened amethyst" from Montezuman, Rio Prado, Brazil; Rose and Lietz, 1954). Even though the yellow or green color after heating is an B allochromatic transition-metal impurity color, the Fig. 4. Quartz structure (schematic): (A) normal, (B) containing amethyst color itself derives from a color center. Irra- AI" substituted for an SiH with an H+ for charge neutrality. diation after heating can recreate the color center and Radiation ejects one of a pair of electrons from an 0'- and leaves the amethyst color. the "hole" color center of smoky quartz. 224 NASSA U: COLOR IN MINERALS
Table 4. Color centers Phosphorescence involves an even smaller bleach- ing energy 8 to e than that necessary for light bleach- Essentially stable to light: Amethyst, fluorite (purple); smoky quartz (brown to black)' ing. This occurs when the return to the ground state irradiated diamond (green, yellow, brow~, black, blue, pink); some natural and irradiated topaz involves the emission of light of energy correspond- (blue) ing to the e to A return to the ground state and when Ra idl fadin on li ht ex osure: Maxixe-type beryl intense blue ; some irradiated topaz the"thermal excitations present at room temperature (brown); irradiated sapphire (yellow)' ultra- violet light irradiated hackmanite (p~rple) are sufficient to carry the system slowly over the Other colors probably due to color centers: barrier at C. Phosphorescence is shown by some Sylvite (blue); halite (blue and yellow)' zircon (brown); calcite (yellow); barite' fluorite, calcite, diamond, etc. Fluorescence and c:lestite (blue); pleochroic haloes in fluorite, rnlca, etc. phosphorescence "lifetimes" grade into each other; rapid phosphorescence with a very small barrier may The scheme of Figure 5e illustrates a system with a seem to be merely fluorescence. If the trap 8e is a I~rge formation energy but a small bleaching energy. little deeper, thermoluminescence can result; the fluo- SInce 8 to e is here within the range of energies rescence occurs at a fairly well-defined temperature present in visible light, the system will bleach by light- on heating, when the thermal excitation permits rapid energy-induced leakage over the barrier e at the same leakage out of the trap. Phosphorescence and time that color is observed from absorption into E thermoluminescence are not confined to color centers and F. This state of affairs applies to most of the (any more than fluorescence is), but the energy level light-bleaching color centers listed in Table 4. For schemes can nevertheless be represented as in Figure example, Maxixe-type deep blue irradiated beryl 5 wherever such phenomena occur. (Nassau et al., 1976) and brown irradiated topaz will The bleaching process in the case of a hole center involves the release of electrons trapped elsewhere in both fade after some days exposure to light or on heating for a short period to 100 or 200oe. When the crystal. Since these electrons may be trapped at kept in the dark, the color will persist over many various sites, there may be a broad bleaching range in years (more than 60 years in the case of Maxixe beryl; a crystal and also different bleaching temperatures in different specimens as was, in fact, observed in a Nassau et al., 1976). If ~oth formation and bleaching energies are low, series of smoky quartz specimens (Nassau and Pres- cott, 1977). This implies a variation in the barrier the dIagram of Figure 5f applies, for example, to height e, although the color center itself (B, E, F in photochromic hackmanite. In this colorless material Fig. 5) remains the same. even ultraviolet light with energy less than 5eV (note The touchstone for a color center is bleaching by that gamma rays range up to over I million eV) is light or heat and reformation of the color on irradia- sufficient to be absorbed via level 0 and produces a tion. This is not foolproof, however (compare the red color center (8, E, F) which bleaches rapidly on blue to green change in beryl discussed in the final exposure to light. section), and years of investigations employing opti- cal spectroscopy, paramagnetic resonance, and other techniques have been required to characterize fully D D the few color centers which are well understood.
The molecular orbital formalism 'n~ This applies to several different types of situations :} ENERGY where electrons are involved which are not simply B B B located on single atoms or ions as in the crystal field D formalism, but must be considered to be present in F C E multi-centered orbits. The results vary depending on whether metal-metal, metal-nonmetal, or nonmetal- A A A A A nonmetal centers are involved. In the latter two cases (01 (bl {CI (dl {el {fl the type of bonding is usually predominantly co- LARGE AD, LARGEBC LARGE AD. SMALL AD, SMALLBC SMALL BC valent. The result of a molecular orbital treatment is similar to that of a crystal field treatment; both for- Fig. 5. Energy-level schemes showing "traps" involved in phosphorescence, thermolumincescence, and bleaching color malisms result in a set of energy levels and associated centers. transition probabilities. Some examples are summa- NASSA U: COLOR IN MINERALS 225 rized in Table 5. An introduction to molecular orbital Table 5. Colors caused by molecular orbital transitions theory has been given by Ballhausen and Gray ( 1964); charge transfer has been reviewed by Robin A. Metal-metal charge transfer: Fe2+_Fe3+/Fe3+_Fe2+: Cordierite, vivianite and Day (1970) and organic color by Mason (1970). (blue), magnetite, etc. (black?) Fe2+_Ti4+/Fe3+_Ti3+: Kyanite, blue sapphire Metal-metal charge transfer (blue) Mn2+_Mn4+/Mn3+_Mn3+: Manganite, bixbyite, etc. Consider blue sapphire, a variety of corundum, (black?) AI20a, with both Fe and Ti impurities. Each of the B. Metal-nonmetal charge transfer: impurity ions can exist in two valence states, and Cr6+_02-: Crocoite (orange) therefore in two combinations: (a) Fe2+ and Ti'+; and V5+_02-: Vanadinite (orange) (b) Fe3+ and Ti3+. A single electron can be caused to Metal-Sulfur: Pyrite, marcasite, etc. (see transfer from the Fe to the Ti by light absorption and band-gap semiconductors) back again. Since state (b) has a higher energy than C. Electrons not on metal ions: state (a), the transition from (a) to (b) involves the S3: Lazurite in lapis lazuli (blue) TI electrons: Graphite (black) absorption of energy, producing a broad intense ab- Organic pigments: amber, ivory (brown); coral sorption band at the red end of the spectrum and (red, black); pearl (pink, green, blue, black); resulting in the well-known deep blue color. The Fe- bitumen, lignite, etc. (brown to black) Ti distances are 2.65A in the c direction, along the optic axis, but 2.79A in the perpendicular direction; to the central metal, i.e. 0 -> Cr in this case. The therefore, the observed dichroism is expected. result is a broad absorption at the blue end of the Another charge transfer process involving only Fe, spectrum and the transmittance of the other wave- namely (c) Fe2+ + Fe3+ and (d) Fe3+ + Fe2+ can also lengths, leading to an orange color. occur. In the case of sapphire this process produces Similar remarks apply to vanadinite, wulfenite, absorption only in the infrared; it is, however, of and scheelite. The last is not colored when pure, since color significance in the hydrated iron phosphate vi- the charge transfer absorption is not in the visible, vianite. When fresh, this compound contains only but the bright fluorescence of scheelite derives from Fe2+ and is almost colorless. As air-oxidation con- an excited molecular orbital state of the charge trans- verts some of the Fe2+ to Fe3+, charge transfer be- fer 0 -> W in the WO~- units. comes possible and results in the deep blue color. The As in crystal field calculations, the point symmetry iron in vivianite is present in two different sites, and and geometric distortions of the molecular group and one of these oxidizes preferentially; there is tri- the strength of bonding are important parameters. chroism with the deep blue color seen only with light Charge transfer transitions usually have high transi- polarized along the direction connecting the two dif- tion probabilities, thus giving intense colors, and ferent Fe sites, i.e. along the Fe2+-Fe3+ direction. tend to dominate crystal field transition colors when The color of cordierite has also been attributed to both are present, although they may also occur in the this type of Fe2+-Fe3+ charge transfer. It is likely that ultraviolet part of the spectrum. most black minerals containing Fe, Mn, Ti, etc., such Charge transfer is also present in the cha]cogenides as magnetite, ilmenite, manganite, schor!, etc., owe such as pyrite, with S -> metal electron displacement. their color to some kind of charge transfer. It should The optical properties of these types of minerals are, be mentioned that the disproportionation 2 Fe3+ -> however, best understood using the band formalism Fe2+ + FeH could also be involved. discussed below.
Metal-nonmetal charge transfer Electrons not on metal ions A substance such as crocoite, PbCrO" does not There are several minerals the colors of which have any unpaired electrons, since Pb2+, Cr6+, and are best described by molecular orbitals not in- 02- all have closed electron shells. There are, how- volving any metal ions. A particularly striking ever, CrO~- molecular units which are covalent]y color is the deep blue of lazurite (lapis lazuli), bonded, and the bonding electrons within such units (Na,Ca)s(A]Sih202.(S2'SO,), which contains sulfur must be considered to belong to the unit as a whole, molecular units and no unpaired electrons. Many i.e. to be in "mo]ecu]ar orbita]s." A molecular orbital different possibilities have been discussed, but the treatment shows that there are excited states, corre- excited levels of S3 molecular units appear to explain sponding to the transfer of electrons from the oxygen the color (Cotton et al., 1976). 226 NASSAU: COLOR IN MINERALS
In graphite there are sheets of connected six-mem- ENERGY FROM bered rings of carbon atoms, and the "7r-electrons" FERMI can move more or less freely along these sheets. The SURFACE result is strong light absorption, specular reflection, ------C ABSORPTION and anisotropic electrical conductivity, somewhat similar to that in metals, as discussed below. -' - b--- Lastly, there are a number of mineral substances FERMI - -- -- a I i SURFACE which owe their color to organic pigments, many of ENERGY 1 biological origin. Some of these are listed in Table 5 and here color, once again, originates in molecular orbital type of transitions within covalently-bonded molecular units. A NUMBER OF _ B The band theory formalism ELECTRONS The crystal field and molecular orbital theories Fig. 6. Band diagram of a typical metal (A) showing light apply to electrons located on ions, at defects, and on absorption transitions in (B). groups of atoms. Band theory treats electrons as be- longing to the crystal as a whole and applies to a wide vidual energy levels are broadened into "bands," as range of materials, as indicated in Table 6. General shown in Figure 6A. At each energy there is room for accounts of band theory can be found in solid-state only so many electrons (the density of states) and the physics texts such as that by Kittel (1976). available number of electrons fills the band structure up to the "Fermi surface." In most metals there is The band theory of metals one continuous band extending to high energies, but In a crystal of a metal such as copper or an alloy the density of states is different at different energies. such as brass, each metal atom contributes its outer The surface of such a metal can absorb light of any electrons, those usually involved in chemical bond- energy, with electrons from the filled part of the band ing, to a joint pool. These electrons are free to move being excited up to empty parts of the band, as shown throughout the whole crystal while being constrained in Figure 6B. Most of this light is immediately re- only by the periodic potential associated with the emitted at the surface (the metallic luster), but the atoms in the crystal structure. The relatively free efficiency of this process depends on the variation of movement of such a pool of electrons in a metal the density of state with energy, i.e. the shape of the results in the high electrical and thermal con- band above the Fermi surface. This produces the ductivities as well as in the metallic luster and specu- color differences among metals and alloys such as lar reflection. In a typical metal, there are more than those listed in Table 6. Since there are still selection 1023electrons per cm3, all essentially equivalent to rules (based on which energy level it was that broad- each other, i.e. "degenerate." The result of quantum ened into the various parts of the band), the actual considerations applied to such a system is that indi- identification of the shape of the band with the color of the metal is very complex. Table 6. Band theory colors Band theory and semiconductors
Conductors (metallic color and luster) There is a large group of minerals where the bond- Elements: Copper, gold, iron, silver, mercury, ing is predominantly covalent (electron sharing) and etc. Alloys: Amalgam, iridosmine, meteoritic nickel- where the average number of bonding electrons is iron exactly four per atom. This is the case for elements of Semiconductors Narrow-band-gap: Altaite, galena (opaque, gray the fourth column of the periodic table such as dia- to black); cobaltite,marcasite, pyrite, smaltite (opaque, metallic) mond, IV-IV compounds such as moissanite (SiC), Medium-band-gap: Cinnabar, proustite, pyrargyrite II-VI compounds such as greenockite (CdS), and (red); realgar (orange); greenockite, orpiment, sulphur (yellow) many others. Band theory applied to such materials Wide-band-gap: Diamond, sphalerite, zincite (colorless) shows a gap between two separate bands, the lower of Wide-band- a semiconductors with im urities which, the "valence band," is exactly filled with elec- Donor impurity: Nitrogen in diamond yellow) Acceptor impurity: Boron in diamond (blue) trons, while the upper, the "conduction band," is completely empty. As shown in Figure 7A, the Fermi NASSA U: COLOR IN MINERALS 227 surface is thus at the top of the valence band and the CONDUCTION ENERGY BAND COLOR BAND ENERGY GAP REMAING ~~~~f COLOR ENERGY BAND space between the top of the valence band and the SURFACE ev eV l 4 4 ~ bottom of the conduction band is the "band-gap," ENERGY -COLORLESS - VIOLET 3 BAND -YELLOW the energy of which is usually designated Eg. t GAP BLUE GREEN -ORANGE I 1 YELLOW Light absorption occurs as in Figure 7B, with ab- --FERMI SURFACE RED -RED sorption possible at all energies above the band-gap -BLACK energy Eg, but not below. Because of limitations asso- o ciated with the band-gap, the luster is not usually A NUMBER OF_ C metallic except at very small values of Eg. If the band- ELECTRONS gap is smaller than the visible-light range (Fig. 7C), Fig. 7. Band diagrams of a typical semiconductor (A), showing light absorption transmissions (B); the dependence of color on the then all light energies can be absorbed and a dark energy of light in (C) leads to the color remaining for various band gray or black color results, as in galena with Eg less gap energies in (D). than OAeV. Such "narrow band-gap semiconductor" materials provide rectifying and amplifying charac- Consider nitrogen impurity atoms substituting for teristics in suitable geometries (silicon and germa- carbons in a diamond crystal (Eg about 5 1/2 eV). nium diodes, transistors, solar cells; note that galena Since N has one more electron than C, each nitrogen crystals served as point contact rectifiers in the early donates one extra electron above the Fermi surface "crystal" radio sets). Minerals such as pyrite can also and these "donor" electrons form an impurity level be treated as narrow-band-gap materials (Pridmore within the band-gap, as shown in Figure 8A. The and Shuey, 1976). If a narrow-band-gap semiconduc- nitrogen impurity level is still 4eV below the con- tor is heated to a high enough temperature so that duction band (a "deep" donor) and is, in fact, some- thermal excitation can bridge the band-gap, then the what broad. It can only absorb a little violet at 3eV, electrons excited into the conduction band will cause thus giving a yellow color. A typical deep-yellow the material to behave as a metal. diamond may contain one nitrogen atom for every At the other extreme, "large-band-gap semicon- 100,000 carbon atoms. The reduction in the band- ductors" with Eg larger than the range of visible light gap is insufficient to permit electrical conductivity at will not be able to absorb in the visible and will thus room temperature. be colorless, as in pure diamond and sphalerite with Boron, having one electron less than carbon, acts Eg about 5 1/2 eV and 3 1/2 eV respectively. in a similar way to produce "acceptor levels" in the With an intermediate band-gap of about 2eV, as in band-ga'p. These are "hole" levels which can accept proustite, only red light is transmitted; all other col- electrons from the valence band and are located only ors have energies larger than Eg and therefore are OAeV above the top of the valence band, as shown in absorbed, as in Figures 7C and 70. With a band-gap Figure 8B. Here again the impurity band is not just a of about 2.5eV, as in greenockite, only blue and violet single level, but has a complex structure, resulting in in Figure 7C are absorbed and the resulting color in Figure 70 is yellow. The overall sequence of band- /-:- CONDUCTION gap colors is thus black/red/ orange/yellow / color- BAND less. When mixed crystals of CdS (yellow, Eg =2.5eV) and CdSe (black, Eg = 1.6eV) are prepared, the inter- mediate orange and red colors are indeed obtained T (Fig. 12 in color in Nassau, 1975a). Some band-gap 4ev BAND semiconductor minerals are listed in Table 6. Semi- GAP 1 NITROGEN 5 1/2eV conducting ore minerals are the subject of a book by DONOR Shuey (1975). _-1_o~4 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. The presence of certain impu- A B rities can affect both the conductivity as well as the Fig. 8. Band diagrams for (A) yellow N-containing diamond and light absorption. (B) blue B-containing diamond. 228 NASSAU: COLOR IN MINERALS a blue color. AI, which also has one less electron than spinel (presumably to precipitate a second phase). It C and is present in significant amounts in some blue is therefore possible that the bluish moonstone effect diamonds, had been previously thought to provide is caused by light scattering from very fine particles. the acceptor; however, laboratory synthesis showed The effect is sometimes termed opalescence and is that B additions gave blue and that Al did not also seen in the white to bluish body color of some (Chrenko, 1973). Since the boron acceptor level is opal; it is unrelated to the fire in opal. "shallow," ambient thermal excitation can raise elec- One other distinction may need emphasis. The col- trons from the valence band into the acceptor level, ors deriving from interference in a thin film, as in the and the resulting holes in the valence band can then tarnish film on bornite, is the well-known sequence conduct electricity. Type lIB blue conducting dia- black/gray /blue-gray /white/yellow / orange/red (end monds such as the "Hope" contain typically one of the first order)/violet/blue/green/etc. These colors boron atom per million carbon atoms. become washed out along the sequence and are not Blue irradiated diamonds do not conduct electric- pure spectral colors. The exact color depends only ity, thus providing one distinction between the boron on the film thickness, the refractive index of the film, acceptor and the irradiation-produced blue color cen- and the nature of the incident light. ter in diamond. Gemological testing (Liddicoat, In the case of diffraction there is also interference 1977) is required to distinguish between nitrogen- of light waves, but from a regular diffraction grating donor yellow diamond and irradiated yellow dia- as in opal. In this case the colors are spectrally pure mond. and depend only on the grating spacing and the angle of observation. A given region of an opal can diffract Color caused by physical optics effects only a limited range of colors. For example, for a flat- Electrons are not involved in the last four types of slab of opal with face-centered cubic sphere packing colors summarized in Table 7. These effects are well- with a spacing of 3A, the color range is green to red; covered in optical mineralogy and optics texts and an with 2.5A blue to yellow, and with 2A violet to blue- extended discussion is therefore not appropriate. green. A detailed description of opal has been given With respect to scattering, it may be noted that the by Sanders (1968) and labradorite is discussed by luster in pearl originates in overlapping platelets of Bolton et al. (1966). aragonite. The terminology used for adularescence, aventurescence, schiller, etc. is very confused. The Concluding remarks effect observed in moonstone orthoclase and also in The effect of irradiation and heat on the color of some albite (a "floating" white or bluish shimmer) is minerals well illustrates the danger of rash con- usually attributed to a layer structure. Light scattered clusions concerning the origins of these colors. Usu- from colloidal particles can produce a very similar ally irradiation produces electron or hole color cen- effect, as in unhomogenized milk or in some moon- ters, while heating permits the trapped electrons and stone imitations made by heat-treating synthetic holes to recombine again. The process is reversible and heat-bleached amethyst can usually be recolored by irradiation. An apparently similar change, the Table 7. Pseudochromatic colors caused by physical optics effects converting of golden or green beryl to blue aquama- rine on heating can also be reversed by irradiation. Color Based on Dispersion: Nevertheless, in this case a color center is not in- "Fire" in high dispersion gem-stones such as dia- mond, zircon, rutile, and strontium titanate. volved, but merely a valence change which eliminates Color Based on Scattering: Chatoyancy as in eat's eyes, tiger's-eye. a yellow-producing Fe3+ absorption at the violet end Asterism as in star corundum, garnet, quartz. of the spectrum on heating; the resulting Fe2+ does Luster as in pearl, foliated talc, brucite, apophyllite, fibrous asbestos, gypum not absorb in the visible and the original color can be (satin spar), etc. Aventurescence as in sunstone, aventurine albite, restored by irradiation (Wood and Nassau, 1968). In aventurine quartz, schiller spar, silver-sheen the case of pink tourmaline both an electron color obsidian. Adularescence as in moonstone (bluish), milky opal. center as well as a hole color center have been pro- Color Based on Interference: posed, but an examination of a large number of speci- Interference effects in thin films such as the tarnish film on chalcopyrite, columbite, and mens by irradiation and heating indicated the occur- bornite and within the cracks of iris quartz. Color Based on Diffraction: rence of at least seven differently-behaving pinks Diffraction grating produced by periodic spacings (Nassau, 1975c). Clearly, much caution must be used as in opal, labradorite, and iris agate. in explaining specific colors. NASSAU: COLOR IN MINERALS 229
Further complications arise from the fact that Kittel, C. (1976) Introduction to Solid State Physics, 5th ed. John more than one type of color-causing mechanism can Wiley and Sons, New York. be present in a mineral. Color may be caused by very Liddicoat, R. T., Jr. (1977) Handbook of Gem Identification, 11th ed. Gemological Institute of America, Los Ange]es. low level impurities in the presence of much larger Loeffler, B. M. and R. G. Burns (1976) Shedding light on the color inactive impurities (e.g. the B vs. Al in diamond of gems and minerals. Am. Scientist, 64, 636-647. discussed above). Adding to such difficulties are the Mason, S. F. (1970) Color and electronic states of organic mole- many early guesses as to color causes which, due to cules. In K. Yenkataraman, Ed., The Chemistry of Synthetic repeated copying from one text to another, frequently Dyes, p. 169-221. Academic Press, New York. Nassau, K. (1975a) The origins of color in minerals and gems. end up as apparent "facts." Lapidary J.. 29, 920-8, 1060-70, 1250-8, ]521. Quantum theory provides the framework for the - (l975b) A reinterpretation of smoky quartz. Phys. Status various causes of color in crystals, and it is encourag- Solidi. A29, 659-663. ing that this field is presently under intensive investi- - (l975c) Gamma ray irradiation induced changes in the gation, since color is, after all, one of the most strik- color of tourmalines. Am. Mineral., 60, 710-7]3. - and B. E. Prescott (1977) Smoky, blue, greenish-yellow and ing characteristic of the majority of minerals and other irradiation-related colors in quartz. Mineral. Mag., 41, gems. 30]-312. -, - and D. L. Wood (1976) The deep blue Maxixe-type color center in beryl. A m. Mineral., 61, 100-107. Acknowledgments Pridmore, D. F. and R. T. Shuey (1976) The electrical resistivity of Helpfu] discussions with D. L. Wood and M. B. Robbins of the galena, pyrite, and chalcopyrite. Am. Mineral, 6/, 248-259. Bell Labs and B. M. Loeffler of Massachusetts Institute of Tech- Robin, M. B. and P. Day (1970) Mixed valence chemistry-a nology are gratefully acknowledged. survey and classification. In H. J. Eme]eus and A. G. Sharpe, Eds., Advances in Inorganic Chemistry and Radiochemistry, Yol. 10, p. 247-422. Academic Press, New York. References Rose, H. and J. Lietz (1954) Ein griln verfiirbbarer Amethyst. Ballhausen, C. J. and H. B. Gray (1964) Molecular Orbital Theory. Naturwissenschaften. 4/, 448. Benjamin, New York. Sanders, J. Y. (] 968) Diffraction of light by opals. Acta Crystallogr.. Bolton, H. c., L. A. Bursill, A. C. McLaren and R. G. Turner A24, 427-434. (1966) On the origin of the color in labradorite. Phys. Status Schulman, J. H. and W. D. Compton (1962) Color Centers in Solidi. 18,221-230. Solids. Pergamon Press, New York. Burns, R. G. (1970) Mineralogical Applications of Crystal Field Shuey, R. T. (1975) Semi-Conducting Ore Minerals. Elsevier, Am- Theory. Cambridge University Press, Cambridge, England. sterdam. Chrenko, R. M. (1973) Boron, the dominant acceptor in semi- White, W. B., R. Roy and J. M. Crichton (]967) The alexandrite conducting diamond. Phys. Rev., B7, 4560-4567. effect: an optical study. Am. Mineral.. 52, 867-71. Cotton, F. A., J. B. Harman and R. M. Hedges (1976) Calcu- Wood, D. L. (1969) Spectra of ions in crystals. In S. Nudelman lations of the ground state electronic structures and electronic and S. S. Mitra, Eds., Optical Properties of Solids, p. 571-593. spectra of di- and trisulfide radical anions by the scattered Plenum Press, New York. wave-SCF-Xa method. J. Am. Chem. Soc., 98,1417-1427. - and K. Nassau (1968) The characterization of beryl and - and G. Wilkinson (1972) Advanced Inorganic Chemistry, emerald by visible and infrared absorption spectroscopy. Am. 3rd ed. Interscience, New York. Mineral. 53, 777-800. Frye, K. (1974) Modern Mineralogy. Prentice Hall, New York. Fyfe, W. S. (1964) Geochemistry of Solids. McGraw-Hili, New Manuscript received, June 20, 1977; accepted York. for publication. December 2, 1977.