Gemstone Color Causes

I. Traditional Gemmological Color Causes

Traditional gemmology is mainly based on the chemical composition and external structure characteristics of gemstones, and divides gemstone color into self-color, other color and false color.

1. Self-color

Color caused by elements that are part of the basic chemical components of gemstone minerals. These color-causing elements are mostly transition metal ions, such as ferroaluminum garnet, turquoise, malachite, and blue chalcopyrite.

2. His color

Color caused by impurity elements contained in gemstone minerals. He color gem in very pure colorless, when it contains trace color-causing elements, can produce color, different trace elements can produce different colors. For example, spinel, whose chemical composition is mainly Mg Al2O4, is colorless when pure, blue when it contains trace elements of Co, brown when it contains trace elements of Fe, and red when it contains trace elements of Cr. In addition, different valence states of the same element can produce different colors, such as Fe3 + is often brown, Fe2 + is light blue. The same element of the same valence in different gemstones can also cause different colors, such as Cr3 + in corundum to produce red, in beryl to produce green.

3. False colors

False colors are not directly related to the chemical composition and internal structure of the gemstone, but to the physical action of light. Gemstones often exist in some small parallel arrangement of inclusions, out of the dissolution of the lamellar crystals, parallel resolution, and so on. Their refraction of light, reflection and other optical effects produce the color is false color. False color is not inherent in the gem itself, but false color can add a lot of charm for the gem, this aspect of the specific content has been in the section of the special optical effects of gemstones for a more detailed description.

Second, modern science, the cause of the color of gemstones

With the development of science, people found that the color of gemstones does not only depend on its chemical composition, but more importantly, depends on its internal structure. The modern scientific theory of color genesis breaks the boundaries of self-color and other color in the traditional color genesis theory, and reveals the nature of gemstone color genesis from the perspectives of crystal field theory, molecular orbital theory and energy band theory.

(1) The coloring of ions by electronic leaps within ions (crystal field theory)

The object of crystal field theory is the transition metal elements and certain lanthanide and actinide elements in the crystal structure of gemstones. It sees the crystal field as a kind of electrostatic interaction between positive and negative ions, and calls the positively charged cations the center ions, and the negatively charged anions and complex anions collectively known as coordination ions, or ligands for short. The difference between crystal field theory and other theories is that it treats the ligand as a point charge, and the essence of the point charge action is the generation of an electrostatic potential field force, which is also called a crystal field. Crystal field leaps include d-d leaps and f-f leaps. The transition metal elements in the fourth and fifth periods of the periodic table contain 3d and 4d orbitals, respectively, and the lanthanides and actinides contain 4f and 5f orbitals, respectively. In the presence of ligands, the five energy-equal d orbitals of the transition elements and the seven energy-equal f orbitals of the lanthanide elements split into several groups of d orbitals and f orbitals of unequal energy. When their ions absorb light energy, the d or f electrons in the low-energy state can jump to the d or f orbitals in the high-energy state, and these two types of leaps are known as d-d leaps and f-f leaps, respectively. These two types of leaps are called d-d leaps and f-f leaps, respectively. Since these two types of leaps must occur under the action of the ligand field of the ligand body, they are also called ligand field leaps.

The best examples of gemstone color changes caused by d-d electron leaps of transition metal elements are ruby, emerald and variscite, the ultraviolet-visible absorption spectra of which are shown in Figure 1-4-11.

Figure 1-4-11 UV absorption spectra of ruby, emerald and varnish

A - ruby; B - varnish.C - emerald< /p>

The chromogenic ion in ruby is Cr3+, and the free ion spectral terms derived from the 3d3 electronic grouping state of Cr3+ are as follows: the fundamental spectral term is 4F, and the excitation spectral terms are 4P, 2G, 2D, and so on. In the octahedral field, the splitting from the fundamental spectral term 4F into three energy levels, i.e., 4A2, 4T2, and 4T1. The absorption spectral characteristics of ruby show two strong and broad absorption bands in the visible region, which are caused by the leaps between the 4A2→4T2, and 4A2→4T1 energy levels, respectively. d electrons in the process of the leaps between the 4A2→4T2, and 4A2→4T1 energy levels, respectively, absorb 2.25 and 3.02 e V of energy, and the rest of the absorbed residual energy combines to form the ruby color (see Figure 1-4-12).

The characteristics of the absorption spectrum of emerald show (see Figure 1-4-13) that in the visible region, there are two strong and broad absorption bands, which are caused by the leaps between the 4A2→4T2 and 4A2→4T1 energy levels, respectively. d electrons in the process of the leaps between the 4A2→4T2 and 4A2→4T1 energy levels, respectively, absorbing 2.04 and 2.92 e V energy, and the remaining residual energy after absorption is combined into the color of emerald (see Figure 1-4-12). residual energy combines to form the emerald color.

Figure 1-4-12 UV absorption spectrum of ruby

Figure 1-4-13 UV absorption spectrum of emerald

Metamorphite's chemical composition (BeAl2O4) is between ruby and emerald, and the metal ions affecting the aluminum-oxygen octahedron are only one kind of Be. Therefore, the strength of electric field of Cr3+ ions and the ligands around them is lower than that of ruby and higher than that of emerald. emerald, the nature of the chemical bond between its metal-oxygen ions is also between ruby and emerald. The energy absorbed by the 4A2→4T2 transition of the Cr3+ ion in metasomatite is 2.16 eV, which is between that of ruby (2.25 eV) and emerald (2.04 eV), while the energy absorbed by the 4A2→4T1 transition (2.98 eV) is not much different from that of ruby and emerald. In the visible region, the chances of transmitting red and blue-green light in a metamorphic stone are nearly equal, and so the conditions of the light source in the external environment (color temperature) determine the color of the metamorphic stone. For example, fluorescent lamps with a higher color temperature have more blue-green components, resulting in the superposition of blue-green components in the variscite, which appears as a blue-green color. Conversely, a low color temperature in an incandescent light source results in an overlay of red components in the change stone, which appears red (see Figure 1-4-14).

Figure 1-4-14 UV Absorption Spectrum of Metamorphite

(II) Charge Migration between Ions for Color (Molecular Orbital Theory)

The state function of individual electrons in a molecule is known as a molecular orbital. According to the molecular orbital model, it is believed that all orbitals in a molecule extend over the entire molecule. The electrons occupying these orbitals are not localized to a particular atom, but are present throughout the molecule. According to the molecular orbital theory, electrons can jump from one atomic orbital to another, and this electron jump is called charge migration.

Some molecules are both electron donors and electron acceptors, and when electrons are excited by radiant energy to jump from the outer orbitals of the donor to the acceptor, strong absorption occurs, a spectrum known as the charge migration spectrum. Along with the charge transfer, a strong absorption band is produced in the absorption spectrum, and if the charge transfer band appears in the visible range, the corresponding color is produced. Charge migration can take many forms; it can occur between homonuclear atomic valence states as well as between heteronuclear atomic valence states.

1. Charge migration between metal-metal atoms

Charge migration between metal-metal atoms can be divided into charge migration between homonuclear atomic valence states and charge migration between heteronuclear atomic valence states.

(1) charge migration between the valence states of homonuclear atoms

Charge migration between the valence states of homonuclear atoms comes from the interaction between the two atoms of the same transition element with different valence states, when two homonuclear atoms with different valence states are distributed in different types of lattice points, and there is an energy difference between the two, the electrons can be transferred, and the spectral absorption bands are generated, which results in the gemstone presenting the color. . The production of the blue-violet color of cordierite is a typical example of this condition. In cordierite, Fe3+ and Fe2+ are in tetrahedral and octahedral positions, respectively, the two ligands to *** prisms, when visible light irradiation to cordierite, one of its Fe2+ d-electron absorbs a certain amount of energy of the light leaps to Fe3+, the absorption band of this process is located in the 17,000cm-1 (equivalent to the yellow), so that cordierite shows blue. Blue, green tourmaline and aquamarine are also colored due to charge migration between Fe2+-Fe3+.

(2) charge migration between the valence states of heteronuclear atoms

Figure 1-4-15 UV absorption spectrum of sapphire

The typical example of charge migration between the valence states of heteronuclear atoms is sapphire (see Figure 1-4-15), in sapphire Fe2+ and Ti4+ are located in the neighboring octahedron connected by the face, Fe, Ti ions at a distance of 0.265 nm, and the d-orbitals of both overlap along the crystallographic axis. When electrons run from Fe2+ into Ti4+, Fe2+ transforms into Fe3+ and Ti4+ transforms into Ti3+, i.e., Fe2++Ti4+→Fe3++Ti3+. This process of charge migration is accompanied by a spectral absorption energy of 2.11 eV, with the center of the absorption band located at 588 nm, which results in a blue color in the c-axis direction of the sapphire only through the blue color. When two octahedra are connected by a prism in the perpendicular c-axis direction, the charge transfer absorption band is then slightly displaced in the long-wave direction, giving the sapphire a bluish-green color in the very light direction. Charge migration between the valence states of heteronuclear atoms is also responsible for the coloration of blue zoisite and brown rhodochrosite.

2. Other types of charge migration

In addition to the two types of charge migration mentioned above, there is also charge migration between non-metal and metal atoms and between non-metal and non-metal atoms.

The common type of charge migration between non-metal and metal atoms in gemstones is O2-→Fe3+. The charge migration between 02- and Fe3+ strongly absorbs violet and blue light in the visible spectrum, resulting in a golden-yellow color of the gemstone. The color of golden beryl and golden sapphire are caused by the charge migration between 02-→Fe3+.

(C) Electron Leap Between Energy Bands for Color (Energy Band Theory)

Energy band theory is a quantum mechanical model for studying gemstone materials and is a further development of molecular orbital theory. It better explains the color mechanism of natural colored diamonds and the reason for its diamond luster. Energy band theory: solid electrons are not bound to an atom, but for the entire crystal **** have, and in the crystal internal three-dimensional space of the periodic potential field movement. The energy of the electrons in motion has certain upper and lower limits, and the energy region allowed for these electron motions is called the energy band. The difference between it and crystal field theory and molecular orbital theory is: crystal field theory and molecular orbital theory mainly applies to the local ions and the electrons on the atomic group, the electrons are fixed domain, is the local state between the jump; energy band theory is the opposite, it is considered that the electrons are non-domain, is not localized between the electrons between the jump. Energy bands can be divided into: ① conduction band (also known as the empty band), formed by the energy level of the unfilled electron a high-energy band. ② band gap (also known as forbidden band), the valence band of the uppermost surface (also known as the Fermi surface) and the conduction band of the distance between the lowermost surface, the width of the forbidden band with the mineral bonding is different; ③ valence band (also known as the band), by the atomic orbitals have been filled with electrons of the energy levels of the low-energy bands, when natural light through the gemstone, the gemstone will absorb the energy to make electrons from the valence band to the conduction band jump, the energy required depends on the width of the band gap, i.e., the top of the valence band and the bottom of the conduction band of the electrons to leap. The energy required depends on the width of the band gap, i.e. the energy difference between the top of the valence band and the bottom of the conduction band, also known as the energy gap, and is generally expressed as ΔEg. Different gemstones have different colors due to different energy intervals. As in crystal field theory, the energy absorbed by electrons returning from the conduction band to the valence band is still emitted as light. For example, the energy interval of the band gap of type IIa diamond (ΔEg=5.4e V) is larger than the energy of visible light, i.e., the energy absorbed when the electron leaps from the valence band to the conduction band is 5.4e V. Therefore, the absorption mainly occurs in the ultraviolet region, and there is no absorption of the visible energy, so theoretically, type IIa diamonds are colorless (see Fig. 1-4-16); since type Ib diamonds contain a trace amount of lone nitrogen atoms, nitrogen atoms Outer electron (1s22s22p3) than carbon atoms (1s22s22p2) more than one, the extra electron in the forbidden band to generate an impurity energy level (nitrogen donor energy level), thereby narrowing the energy interval of the band gap, electrons from the impurity energy level jump to the conduction band of the energy absorbed by the energy of the 2.2e V (564 nm), so the type diamonds show an orange-yellow (see Figure 1-4-17).

(D) lattice defects in color

The phenomenon that the arrangement of plasmas deviates from the lattice structure law (the plasmas make periodic translational repetitions in the three-dimensional space) is called lattice defects in the local range of the crystal structure of gemstones. The cause is related to the thermal vibration of the mass points inside the gem crystal, external stress, high temperature and high pressure, irradiation, diffusion, ion implantation and so on.

For example, diamond crystals crystallized in the high-temperature and high-pressure environment of the upper mantle are rapidly carried to the near-surface by the host magma (kimberlite magma or potassium-magnesium brilliant porphyry magma), and the rapid change in the temperature and pressure conditions and the mutual collision between the crystals and the surrounding rock materials are prone to lead to the intrusion of diamond crystals with the structure of the local changes, and lattice defects induced, so that a part of the originally colorless color of diamonds The color of some of the originally colorless diamonds is changed, resulting in the formation of brownish-yellow, brownish-yellow and pinkish-red diamonds.

Figure 1-4-16 Diagram of electron transport in a type IIa diamond

Figure 1-4-17 Diagram of electron transport in a type Ib diamond

Color centers, as a special case of lattice defects, refer to lattice defects in gemstones that can selectively absorb visible light energy and produce color. It is a typical type of structural coloration. The types of color centers are complex, but the most common are electron centers (F-centers), hole centers (V-centers) and impurity ion centers.

1. Electron centers (F centers)

Electron centers (F centers) are caused by anionic vacancies in the crystal structure of a gemstone. In the case of the entire gemstone crystal, when an anion is absent, the vacancy becomes a positively charged electron trap, which captures electrons. If a vacancy traps an electron and binds it to that vacancy, this electron is in an excited state and selectively absorbs energy at a certain wavelength and becomes colored. Thus, an electron core is composed of an anionic vacancy and an electron bound by the electric field of this vacancy. For example, fluoride ions in purple fluorite crystals leave the normal lattice site and form an anionic vacancy (which lacks a negative charge), and this structural site displays a positive charge, forming a positively charged electron trap. In order to maintain the electrical neutrality of the crystal, the anionic vacancy must trap a negative electron, resulting in color.

2. Void heart (V heart)

Void hearts (V hearts) are caused by the absence of cations in the crystal structure. In terms of electrostatic interaction, the absence of a cation is equivalent to the addition of a negative charge in the vicinity, and an anion in the vicinity must become a "cavity" in order to maintain electrostatic equilibrium. Therefore, the cavity heart is composed of a cation vacancy capture a "cavity". For example, the Al3+ impurity, which replaces Si4+ with a homogeneous form in smoke crystals, forms a positively undercharged position in the lattice site (positive charge trap), and in order to maintain temporary electroneutrality, the Al3+ ion must be surrounded by a corresponding positively monovalent cation. When a crystal is irradiated, the O2- with the nearest neighbor will lose an excess electron and a hole will remain, forming a cavity heart (V heart). Irradiation of gemstones using charged particles (accelerated electrons, protons), neutrons or rays from an irradiation source ultimately results in the formation of an electron-hole heart or an ion-deficient heart in the gemstone through the interaction of the charged particles, neutrons or Y-rays with the ions, atoms or electrons in the gemstone. As in the case of irradiated diamonds, blue topaz, etc., the nature of irradiation is to provide the energy to activate the displacement of electrons, lattice ions or atoms, resulting in the formation of an irradiated damage heart.