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 quartzSmoky quartz

► Olivine Green (Peridote)► Turquoise Blue

NB: Table 1.1

Absorption and colour

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

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