Sea salt comes from the evaporation of seawater and contains trace amounts of minerals stripped from table salt. Table salt has added iodine during the refining process, an essential element for human nutrition and man-made anti-clumping agents. At first, this salt looks gray or darker, but some refine and factory processes purify it to look white.
Sodium chloride crystal is made up of sodium ions, which are cations and chloride ions. The chloride ions being comparatively larger, occupy the corner and face-centered positions. The sodium ions being smaller in size goes to fill up the voids. Thus sodium ions occupy the octahedron voids in a cubic structure. Each chloride ion is surrounded by six sodium ions extending the structure while six chloride ions surround each sodium ion. So the coordination number is six. Sunlight or electric light passes through these coordinates and absorbs colors.
Sunlight travels through space in different wavelengths, some of which are reflected into space once they hit the earth’s atmosphere. Some others are scattered once they enter the atmosphere, and there is a large portion of wavelengths eventually make it to the Earth’s surface.
Sea salt’s crystals aren’t white. They’re completely transparent. The diameter is larger than the wavelength of the visible spectrum, reflecting and scattering light equally, making them white. Some of them are denser than others because they contain more crystals. Visible light finds it difficult to pass through those crystals. It means that reflectivity is greater than invisibility.
White light from the sun comes in and bounces back to our eyes. The light comes in, and it’s bent as it passes through the first crystal and then bends again as it passes through the second crystal. So it zigzags its way through the pile of salt, and eventually, it finds its way out to our eyes. That’s why we see salt as white.
Read More: Why Makes The Sea Water Salty?