1. Chapter 11 Color

2. 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  Al 2 O 3 : white – corundum; red – ruby; blue – sapphire  Quartz (SiO 2 ): amethyst – purple; yellow – citrine; pink – rose quartz, etc.

3. 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

4. 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.: Fe 2+ and Fe 3+ ; Ti 4+ and Fe 2+ - both resulting in a blue colour ► Colour centre transitions  Structural defects: vacancies or interstitial impurities called colour centers ► Fluorite, smoky quartz, amethyst and citrine

5. Absorption Colors in important minerals ► FluoritePurple or Green ► HaliteBlue or yellow ► TopazBlue or yellow ► CorundumRed (Ruby) Blue (Sapphire) ► GarnetYellow-orange (Spessartine) Green (Demantoid) Dark red (Almandine) ► BerylDeep green (Emerald) Blue-green (Aquamarine) Pink (Morganite) Yellow (Heliodore) ► CordieriteBlue ► KyaniteBlue ► TourmalinePink (Rubellite) ► QuartzViolet (Amethyst) Citrine (Yellow) Rose quartz Smoky quartz ► OlivineGreen (Peridote) ► TurquoiseBlue NB: Table 1.1

6. Absorption and colour

7. 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

8. Fluorescence and phosphorescence FluorescencePhosphorescence

9. 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

10. 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)

11. 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

12. Common minerals ► 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