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Chapter 11 Color
Introduction • Color: our perception of wavelengths of light reflected by or transmitted through material (mineral) • Light can be: • Transmitted • Absorbed • Scattered • Refracted • Reflected by a crystal • Color not determined by general structure • rather depends on trace elements or mineral defects • Al2O3: white – corundum; red – ruby; blue – sapphire • Quartz (SiO2): amethyst – purple; yellow – citrine; pink – rose quartz, etc.
Absorption • No absorption: • White light transmitted through crystal, crystal appears colorless • Preferential absorption: • Some wavelength absorbed; combination of remaining spectrum gives COLOR • Absorption in visible range controlled by electron transitions between different energy levels of electrons • In a similar way as internal electrons being displaced from orbitals by electromagnetic radiation and producing X-rays (in XRD), light can be observed by electrons in the outer orbitals, displacing it to a higher energy level. But as the electron returns to the lower energy, radiation is emitted again of which certain energy is absorbed – corresponding to the energy difference between the two energy levels. Thus, a specific colour is absorbed so that the resultand colour is emitted to be observed
Cause of differential colour absorption • Crystal field transitions • Caused by chromophore elements that are very active in colour development since they are: • Elements with partly filled 3d orbitals: • Transition elements: Ti, V, Cr, Mn, Fe, Co, Ni, Cu • Elements with partly filled 4d orbitals: • Actinides (Ac, Th, U, etc.) or lanthanides (La, Ce, Nd, etc.) • Molecular orbital transitions • Caused by an electron that is shared by the orbitals of adjacent cations of variable charges • As it ‘hops’ between the two cations energy is absorbed and released • Eg.: Fe2+ and Fe3+; Ti4+ and Fe2+ - both resulting in a blue colour • Colour centre transitions • Structural defects: vacancies or interstitial impurities called colour centers • Fluorite, smoky quartz, amethyst and citrine
AbsorptionColors in important minerals • Fluorite Purple or Green • Halite Blue or yellow • Topaz Blue or yellow • Corundum Red (Ruby) Blue (Sapphire) • Garnet Yellow-orange (Spessartine) Green (Demantoid) Dark red (Almandine) • Beryl Deep green (Emerald) Blue-green (Aquamarine) Pink (Morganite) Yellow (Heliodore) • Cordierite Blue • Kyanite Blue • Tourmaline Pink (Rubellite) • Quartz Violet (Amethyst) Citrine (Yellow) Rose quartz Smoky quartz • Olivine Green (Peridote) • Turquoise Blue NB: Table 1.1
Fluorescence and phosphorescence • Fluorescence • Halite, Fluorite; Scheelite; Barite; Diamond (Table 11.2) • Irradiation with ultraviolet light; re-emit light in visible range • Depends on trace elements and defects • Activator elements: Cr, Mn, U, W • Suppressor elements: Fe, Co, Ni • Phosphorescence • Similar to fluorescence, but time needed for electrons to return to ground states after radiation • Fluorite, Calcite, Aragonite, Willemite • Will emit light some time AFTER radiation has been stopped
Fluorescence and phosphorescence Fluorescence Phosphorescence
Dispersion • Differential refractive index with change in wavelength • Diamond • Good example: unusually high dispersion • White light into rainbow spectrum • Refraction of each wavelength is repeated many times • Sparkling, brilliant color pattern
Luster • Perceiving scattering and reflection of light by crystals • Two main types: • Metallic • Reflect light like a metal • Generally opaque under transmitted light microscopy • Nonmetallic • Most light enters crystal, therefore crystal transmits most light • Generally these minerals are light colored • Divided into: • Vitreous (glassy) • Pearly • Greasy • Adamantine (brilliant)
Microstructure • Submicroscopic structures – add color effects • Inclusions such as: • Hematite in jasper: tiger’s eye • Rutile in corundum: star sapphire • Separation into lamellae of different compositions • Unmixing • Twinning • Zoning
Fluorite Sodalite Halite Garnet Apatite Beryl Tourmaline Calcite Dolomite Quartz Zircon Rutile Microcline Orthoclase Plagioclase Serpentine Lepidolite Biotite Muscovite Hornblende Andalusite Olivine Orthopyroxene Clinopyroxene Epidote Natrolite Topaz Barite Spodumene Common minerals