Is salt (monocrystal sample) white or transparent?

Question

EDIT:

I have made cleat that this question is about a monocrystal sample and is not answered by the referred other question.

I have read this question:

What color is antimatter?

where OP is saying:

Salt (NaCl) is white.

and David White says in a comment:

Salt is not white. It's transparent. That white appearance is due to reflections off all of the small cubical surfaces.

and Ruslan in a comment says:

salt is white even if transparent, just as blue vitriol is blue despite being transparent. Even a smooth surface still reflects some light, which is the very reason why a bunch of grains scatters light to appear white.

So the question remains:

Question:

1. Is salt (monocrystal sample) white or transparent?

Answer 1

The measured electronic gap between valence and conduction bands in a perfect $\text{NaCl}$ crystalline structure is $8.5\ \text{eV}$ at normal pressure and temperature. It is quite a large gap, and correspondingly, the visible light cannot be absorbed, resulting in a perfectly transparent crystal (see, for instance, this picture).

However, growing a perfect $\text{NaCl}$ crystal is not that easy. Most natural crystals (found in salt mines or by evaporation of seawater) contain many defects, partly due to impurities, water included, and partly to structural defects like F centers, Schottky defects, or dislocations. Most of them give a translucent white appearance to the crystal, although some of them, for example, the F centers. may color the crystal.

Answer 2

EDIT: The answer below responds to the original question which was about salt in general, not just monocrystal sample.

It's both:)

Now let's expand on that a bit. As a material NaCl is transparent to visible light. If you grow a high quality crystal you will be able to see through it quite clearly. In fact is is used as a window material for optical spectroscopy see for example these sodium chloride windows.

Now most of us are familiar with the NaCl as table salt, which looks like a white powder. The main difference is that while a salt window will have two parallel surfaces (front and back), a heap of salt has many and at random orientations. And its refractive index is 1.5 - quite different from that of air (which is 1). There will be a refraction and reflection on each air-salt interface. Each grain will in general change the direction of light passing through it. The reflections on each interface add to it and as a result the light is scattered very quickly (after interacting with a few grains). Because there is no absorption the light eventually makes it out of the salt heap. But any information about where it came from is scrambled. So the powdered salt appears white under illumination.

The refractive index of salt is quite close to that of a vegetable oil. If you mix the two the refractions and deflections will be less pronounced. You can notice that the salt powder becomes more transparent in oil. It is far from perfect (probably because the oil does not go into all the small cracks and other salt grain imperfections) but still visible.

Answer 3

Answer: transparent.

Infrared spectrometers suspend liquid specimens between blocks of monocrystal sodium chloride. The blocks are transparent to both visible and IR light.

My Chemistry professor pleaded, "PLEASE don't clean them with water !!!"

Answer 4

A small addition to the other excellent answers. Here is an example of especially large salt crystals grown by nature, visible in the museum of natural history in Vienna:

If you look carefully, you will see that the crystals are transparent despite their large size (you can see the rock behind), at the same time, some white features are also apparent, which stem from growth defects in the crystal. There may be even clearer salt crystals somewhere else, depending on how the crystal is grown, but I have not seen any so far.

Image source