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Calcite crystals are semi-transparent but turn white when crushed. I used to think that it has something to do with the scattering of all wavelengths of light from small particles. But that cannot be the whole story. By that explanation crushed carbon should also appear white (which is not the case).

So my question is why do some substances appear white when crushed while others do not.

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  • $\begingroup$ If you downvote, please leave a comment. $\endgroup$ – mithusengupta123 Jan 14 at 13:25
  • $\begingroup$ You probably mean calcite as crystallic calcium carbonate. Calcium is a soft alkali earth metal, reacting with water and forming gaseous hydrogen and calcium hydroxide. $\endgroup$ – Poutnik Jan 14 at 13:37
  • $\begingroup$ @Poutnik Corrected... thanks $\endgroup$ – mithusengupta123 Jan 14 at 13:43
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    $\begingroup$ @mithusengupta123, ignore the downvotes, some people have no clue why they are doing it! The problem with anonymous downvotes is that we don't know whether it is Einstein or a high school drop out who has cast a vote. I am sure this phenomena has to do with random scattering/reflection process. For solids you look at the diffuse reflectance spectrum. The they reflect all visible wavelengths, they appear white. $\endgroup$ – M. Farooq Jan 14 at 14:09
  • $\begingroup$ Read about Kubelka-Munk theory developed for powder's color. $\endgroup$ – M. Farooq Jan 14 at 14:12
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It depends if the substance in bulk is transparent, highly reflective (metallic) or highly absorptive.

Carbon in its graphite allotrope is highly absorptive, and even finely divided is still black.

Carbon as diamond is transparent. Crushed diamonds are white -- look at a diamond file or abrasive disk. What happens is that incident light is scattered as it encounters particle after particle, refracted or reflected repeatedly, based on particle size, shape and index of refraction (nD). A higher nD increases the amount of light reflected from particles. For that reason, titanium dioxide, $\ce{TiO2}$, which has a comparatively high index of refraction (~2.5), makes an excellent white pigment.

Metals reflect light because the "cloud" of electrons on the surface return a reflected wave out of phase with the incident wave. However, finely-divided metals have fewer electrons to share, and electrons start behaving as discrete, localized charges. Finely divided silver appears black, for example. This is the basis for traditional silver halide photography. Powdered $\ce{AgCl}$ appears white, but where hit by light, it yields metallic silver nanoparticles, which appear black (as a negative image).

If your interested, look up the effect of surface plasmons on metallic reflectivity, which can yield surprising colors, such as gold in ruby glass.

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