Many minerals have a streak that's surprisingly different from it's outward color. Why is this? What causes the optical properties to change if the mineral is powdered?
Thanks!
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It mostly has to do with the fact many minerals are partially translucent.
Trace impurities or even crystal structure can dominate the color of a translucent material but when ground in to a fine powder (streak) they can no longe do so and you can see the true color.
In other minerals surface oxidation can mask the true color.
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$\begingroup$ Do you have any idea of the scale of such impurities to the size of the powdered fragments. If there are still impurities in the powdered fragments, wouldn't it also affect the absorption of light? $\endgroup$ Commented May 19, 2018 at 22:26
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1$\begingroup$ Its about the depth, in a translucent material reflection and distortion from a trace impurity builds up, it does not have the thickness to do this in a powder. As an example red veins when viewed through thick skin appears blue due to subsurface scattering, many mineral have this same translucent quality. $\endgroup$– JohnCommented May 19, 2018 at 22:41