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Minerals possess many properties. Color is usually the first and most easily observed. For many minerals, color is a reliable diagnostic property used for identification. For others, it is the most variable and unreliable property. But what causes color in minerals? This is something that was debated among geologists for years. It is only in the last 15-20 years that we have really begun to understand the causes.
Factors Influencing Color
The major factors responsible for the production of color in minerals fall into five categories:
- The presence of an element essential to the mineral composition
- The presence of a minor chemical impurity
- Physical defects in the crystal structure
- The mechanical mixture of very fine impurities
- The presence of finely-spaced structures in the mineral
What we see as color is the result of our brain's interpretation of the dominant wavelength of light. Minerals are colored because certain wavelengths of incident light are absorbed, and the color we perceive is produced by the remaining wavelengths that were not absorbed. Some minerals are colorless. This means that none of the incident light has been absorbed.
Color and Atomic Structure
Electrons in atoms are located in orbitals around the nucleus. The number of orbitals present is related to the atom's position on the periodic table. The energy of each orbital is precisely defined and is different than that of the adjacent orbitals. Orbitals closer to the nucleus are lower energy than the outer orbitals.
The lowest energy state of an electron is called the ground state, and the absorption of electromagnetic radiation (such as light) can cause the electron to jump to a higher-energy excited state. The energy difference between the two states is called the energy gap. When light strikes an atom or ion, it may absorb some of the energy and transfer electrons to a higher energy state, but only if the energy of the incoming radiation is the same as the energy gap for the electron.
In a mineral there may be many electrons belonging to a specific element that can be excited. If so, a particular wavelength of light may be strongly absorbed. If the energy of the incoming radiation is greater or less than the energy gap, light will pass through the structure without being absorbed.
Crystal Field Transitions
The transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) have partially-filled inner (3d) orbitals. The electrons in these orbitals can be excited by energy in the visible spectrum and these transitions are the basis for the production of color. Crystal field theory accounts for these electronic transitions.
A good example of this phenomenon is seen in the gems ruby and emerald where the same element can produce completely different colors. In these, trace amounts of Cr3+ replaces Al3+ and is responsible for the red color of ruby and the green color of emerald. The color produced by chromium in these two gems is different because of the influence of the surrounding atoms in the crystal structure (the crystal field) and differences in the nature and strength of chemical bonds.
In rubies, there is strong absorption in the regions of violet, green, and yellow light that yields the characteristic red color. In emeralds, the presence of Si and Be atoms results in bonds between the metals and oxygen which are more covalent than in ruby. This affects the electric field around chromium and causes the absorption of yellow and red light and the transmission of green and blue light in emeralds.
Color can also be caused by structural defects in minerals. For example, an excess electron that is unattached to any single atom may be trapped in a structural defect such as a void due to a missing ion. A hole, or the absence of an electron, can have the same effect. These are what are called color centers.
In the mineral fluorite, the purple color is due to what is called a Frenkel defect. A fluorine ion has been moved from its normal site in the mineral to another site. An electron remains in the hole to maintain electrical neutrality. This electron can exist in the ground state and in various excited states. The movement of the electron among these various states causes color and fluorescence. If the fluorite is heated, the structure returns to normal and the color fades.
Molecular Orbital Transitions
In minerals that contain metals that can exist in different valence states, movement of the electrons results in selective absorption of light. Sapphires contain minor amounts of Ti and Fe. Both elements can exist in two valence states: Ti3+- Ti4+, and Fe2+- Fe3+, and therefore in two combinations - Ti3+ Fe3+, or Ti4+ Fe2+. An electron is transferred between Ti and Fe that results in the absorption of light in the yellow through red part of the spectrum, causing the deep blue color of sapphires. In the mineral magnetite, Fe is present as both Fe2+ and Fe3+ which results in colors from deep blue to black.
Impurities may produce color in minerals. Normally colorless calcite can be colored black by MnO2 or carbon. Tiny specks of red or green minerals can impart their color to minerals. For example, chlorite (green) in quartz, and hematite (red) in feldspar, calcite and jasper.
In some minerals the presence of closely spaced structures produces an iridescence or play of colors. For example, the colors seen in precious opal results from the interference of light reflected from submicroscopic layers of nearly spherical particles arranged in a regular pattern. The layering produces a pearly or milky opalescence.
In some varieties of plagioclase feldspar, closely spaced intergrowth structures produce an iridescence in blues and greens with changing angles of incident light (see the display of feldspar minerals). This is seen in moonstone which is a light-colored variety of plagioclase (albite to oligoclase). The iridescence seen in the rock anorthosite, commonly used as a facing stone on buildings, is due to the presence of dark gray plagioclase. Plagioclase exhibits essentially complete solid solution between two endmembers: albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The intergrowth structures in plagioclase and the compositional range are as follows:
- Peristerite intergrowths - An2-15 (albite to oligoclase)
- Boggild intergrowths - An47-58 (andesine to labradorite)
- Huttenlocher integrowths - An60-85 (labradorite to bytownite)