Of the many properties of minerals color is the most easily observed. However, in many Minerals color is one of the most changeable and unreliable diagnostic properties. This is caused by the fact that the color of many minerals is due to structural irregularities. An additional coloring agent in minerals is the mechanical admixture of impurities. These impurities give a variety of colors to otherwise colorless minerals. Quartz for example may be green because of the presence of finely dispersed chlorite; calcite may be black because it is colored by manganese oxide or graphite. Because these impurities are not found in all specimens of a specific mineral, the property of color cannot be used as a diagnostic tool. However color can be used as a diagnostic property when the element causing the color is essential to the mineral. That is, the element must always be present in a specific quantity that results in all specimens of the same mineral having a constant color.
Although color usually is not used as a diagnostic tool, it doesn't mean that it isn't an interesting property to investigate. Color is defined as the response of the eye to the visible light range of the electromagnetic spectrum. Visible light represents a range of wavelengths from about 350 to 750 nanometers (1 nm = 10 angstroms).
When white light strikes the surface of a mineral, it may be transmitted, scattered, reflected, refracted, or absorbed. The processes of scattering and reflection are part of the property perceived as the luster of a mineral. If light suffers no absorption, the mineral is colorless in reflected and transmitted light. In contrast, minerals are colored when certain wavelengths that are transmitted through the crystal and reach the eye.
To determine which wavelengths are absorbed by minerals and which are transmitted, an instrument known as a spectrometer is used to quantitatively measure absorptions. The peaks in the output of a spectrometer represent absorption of specific wavelengths of light, and are the result of the interaction of light at these wavelengths with ions, molecules, and bonds in the irradiated structure. The absorptions between 0.4 and 0.7 µm result from chromophores, or transition metal ions such as Fe and Cr, that cause the color. In the infrared region between 1 and 4.5 µm, there are absorptions due to molecules such as H2O and CO2. Beyond 4.5 µm, the absorptions are the result of vibrations of the crystal lattice or the so-called lattice modes. Because these absorptions lie outside of the visible light range, they do not affect the perceived color.
The energies of electrons occur in discrete units, or quanta, and there are well-defined energy differences between these energy levels. When electromagnetic radiation interacts with a material, those photons with wavelengths whose energies correspond exactly to the energy differences between the electronic levels will be absorbed by exciting electrons from one level to a higher energy level. In colored minerals, the energy differences between these electron energy levels are in the range of energy of visible light. Thus, when white light shines on a mineral, certain wavelengths are absorbed, causing excitation of electrons between these levels. These wavelengths are, therefore, removed from the spectrum, and this is the underlying cause for color in minerals.
The electronic process responsible for color in minerals can be classified as crystal field transitions, molecular orbital transitions, and/or color centers. Crystal field theory describes electronic transitions between partially filled d orbitals. Molecular orbital transitions, also known as charge-transfer transitions, occur in minerals when valence electrons transfer back and forth between ions in adjacent sites. Color also can be caused by structural defects. This can be an excess electron that is unattached to any single atom and that is trapped at some structural defect, such as a missing ion or an interstitial impurity. A "hole," the absence of an electron, can have the same effect. These types of color producing defects are known as color centers.
Corundum a crystalline form of aluminum oxide and one of the rock-forming minerals. It is naturally clear, but can have different colors when impurities are present. Transparent specimens are used as gems. Pure corundum is defined to have 9.0 on the Mohs hardness scale; it can scratch almost every other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average Mohs hardness near 8.0.
In addition to its hardness, corundum is unusual for its high density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminum and oxygen. Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands. Corundum is mined in Zimbabwe, Russia, and India. Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York.
In the blood-red Chromium-containing variety of Corundum, also known as ruby, small amounts of Cr (+3 charge) replace Al (+3 charge) in 6-fold sites. Resulting in the absorption in the violet, green, and yellow, with transmission in blue and red. The overall red color of ruby is further intensified by a characteristic red fluorescence. That is, not only does ruby absorb most wavelengths of white light such that red is transmitted, it also emits red light by fluorescence.
In the blue variety of sapphire, an electron from an Fe (+3) is transferred to a Ti (+3), thus producing Fe(+2) and Ti(+4). This charge transfer transition also is a large factor in the blue color.
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Nassau, Kurt Gems Made by Man 1980 Chilton Book Company Radnor, Pennsylvannia