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Unformatted text preview: Physical Properties of Minerals I-Optical Properties 1- Color: Generally speaking, color results from the interaction between light and the minerals. Keep in mind that this interaction includes: transmission, refraction, reflection, scattering, or absorption. Visible light wavelength = 350 750 nm. Not always a diagnostic property (cf. p. 107 of your text). Most minerals with a metallic luster (see below) have diagnostic colors that vary little, whereas many minerals whose luster is non-metallic have variable colors. Origin of color: (i) Crystal Field Transitions : Occur when the mineral contains a transition element (chromophore) in which the orbital energies have been affected by crystallization (known as the crystal field effect or crystal field splitting; Fig. 1). With some empty orbitals, electrons can absorb certain wavelengths of the visible spectrum to jump to the next empty orbital, hence rendering a color to the mineral (if the mineral absorbs light of the red wavelength, the resulting color will be blue). When some or all of these electrons return to their original orbital, emission of energy (fluorescence) of a particular wavelength takes place, which can also contribute to the color of the mineral (as in the case of Ruby). Crystal field transitions depend on: (i) the type of transition element or chromophore, (ii) its coordination # in the structure, (iii) its oxidation state, and (iv) the strength of the crystal field imposed by such coordination. Crystal Field transitions are responsible for the color of minerals predominated by ionic bonds; e.g. the green color of olivine, the red colors of ruby and almandine garnet, and the yellow color of chrysoberyl. (ii) Valence and conduction band transitions : Applies to minerals with significant metallic bonding. In this case, the mineral absorbs light energy which may be used to promote electrons from one valence band to the next conduction band. When these electrons then return to their original valence band, the energy is released in the form of light, the wavelength of which dictates the color. This light appears as being reflected from the mineral. Note that if the band gap (difference in energy between the valence and conduction bands is smaller than the energy range of visible light), the latter is entirely absorbed and the mineral appears black. Valence and conduction band transitions are responsible for the color of such minerals as pyrite, cinnabar, and chalcopyrite. (iii) Molecular Orbital transitions (Intervalence charge transfer) : These occur when valence electrons transfer back and forth between adjacent ions, each time releasing a fixed amount of energy. If the wavelength of such energy falls within the visible light spectrum, the mineral acquires a color. 2 Examples include glaucophane, kyanite, beryl (aquamarine) and corundum (some ruby & sapphire), all of which owe their colors to electrons hopping between Fe 2+ and Fe 3+ in different sites....
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This note was uploaded on 02/27/2012 for the course GLY 314 taught by Professor Staff during the Spring '09 term at Marshall.
- Spring '09