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