Colors from Physical Processes

A variety of physical processes involving scattering and interference phenomena cause color in a variety of minerals.


Iridescence


Thin films of a material with an index of refraction which differs from the host mineral cause the color of some minerals.  The sequence of colors has long been recognized as Newtons colors, the same colors seen in an oil slick on water.  The following examples are representative.

Diffraction Gratings

Regularly spaced layers of minerals with different indices of refraction will produce colors if the separation of the layers in on the order of the wavelength of light.  Familiar examples include opal and feldspars.
 

Inclusions

Inclusions of foreign phases are often responsible for the color of minerals.  The dark, near-black color of some plagioclases is due inclusions of magnetite.  A picture of these inclusions is available in the paper by  Xu et al. (1997) J Geophysical Research 102: 12,139-12,157.
 

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