The softest mineral on the Mohs scale is talc. Talc is comprised of the elements magnesium, silicon, and oxygen. It is used to make talcum powder. Talcum powder is commonly used to help with skin rashes. Talc in its base form is used to help eliminate excess fluid in the lungs and close up gaps in the lungs where fluid tends to accumulate.
Why is talc considered to be the softest material on the Mohs scale? But first, what is Mohs scale? It was created by German mineralogist Friedrich Mohs who arranged different minerals in terms of their hardness. He compiled a representative scale of these minerals in 1812, ranking different materials on it.
What determines the hardness of a material? The hardness of a material is determined by its ability to scratch other materials. The minerals were arranged with the softest materials at the top, with lower ranked numbers. The higher the rank, the lower on the scale the mineral was placed. If a material was ranked four on Mohs scale, it would be able to scratch the material ranked number three. A material with a score of five would be able to scratch anything with a rank lower than it.
Here is an illustration of Mohs scale:
The image shows ten minerals (along with a couple miscellaneous materials) ranked as previously explained. The lower the rank, the softer the material. The hardest material, diamond, is nearly impervious to other materials harming it. In fact, when cutting and shaping diamonds, another diamond or a laser is needed to make any sort of headway.
The ten minerals used are baselines for the level of hardness they adhere to. Any material can and is ranked on this scale. The ten used are just used to compare against, giving an easy way for new materials to be given a score.
A limitation of Mohs scale is that it does not show that actual hardness of the material, just what it can cut. Diamond, while it is ranked the highest, is actually 42 times harder than the next highest rank, corundum.
Another limitation of Mohs scale is that minerals nearly universally have variation in their hardness depending on the direction hardness is measured. Hardness depends on the crystallographic direction of the material. Basically, how the atoms are bonded and what type of charge the material has determines its hardness in a certain direction as compared to another. Talc is actually rated harder when measured in a direction perpendicular to its material than gypsum, the next rank up on the scale. But in every other direction it is softer, hence its rating as the softest mineral.
While Mohs scale is inherently limited in its usefulness, it does give a good way to generally know how hard a material is. It provides this function with near unerring accuracy. Mohs scale is one of the first methods used to test the physical properties of a new material.