Rock salt is a form of coarsely ground sodium chloride. Sodium chloride, or salt, is employed in many industries, as well as used in cooking. It helps set dyes in fabrics and is used in the process of producing detergent and soap as well as being used as grit on roads. Rock salt has a crystalline structure, is used as a desiccant and can also be used as an extinguishing agent.
Mineral Classification and Chemical Symbol
Rock salt is also known by the mineral name of halite. Rock salt is that which is sedimentary -- that is, it is found in hard layers underground. According to ThinkQuestLibrary.org, not all salt is extracted from oceans or evaporated salt lakes. Rather, sedimentary salt is found in locations of former seas. Halite's chemical symbol is NaCI, meaning it contains one molecule each of chlorine and sodium.
Pure rock salt is colourless. However, when found underground it is generally not completely pure, so may have yellow, red, grey or brown hues. It is either transparent or translucent and when you shine a light on it, its lustre is vitreous, meaning it appears shiny and glassy.
Rock salt forms in crystals with a simple cubic symmetry. When it is broken, it will break evenly into cubes and when it shatters, the pieces will be of different sizes and shapes.
Hardness and Weight
Mineralogists rate rock salt at 2 to 2.5 for hardness. This means it is quite soft, its surface able to be scratched with a fingernail. It is rated 2.1 to 2.3 for "specific gravity" meaning it is light in weight.
As a Desiccant and Extinguisher
Rock salt has hygroscopic properties, meaning it is able to induce or sustain dryness. This is why salt was historically used as desiccant in food preservation. It is also commonly used as an extinguisher to put out kitchen or grease fires.