Mohs Hardness Scale

Physical Science Tutorial

Mohs Hardness Scale

Intro

One of the most diagnostic properties of minerals is their hardness. This is determined by a minerals ability to scratch another rock, or on the other hand, a minerals ability to resist scratching. The softest minerals can be scratched with your fingernail, minerals a little harder can be scratched with a penny, minerals harder than these can be scratched with a pocket knife, and minerals harder than these can be scratched with a nail. The minerals that are Not scratched by a nail can scratch glass, and the hardest minerals scratch each other.

Some minerals have been selected to serve as relative reference points, and have been put on a scale of 1-10. With 1 being the softest and 10 being the hardest. They are:
1: Talc (Fingernail)
2: Gypsum (Fingernail)
3: Calcite (Penny)
4: Fluorite (Penny-Pocket Knife)
5: Apatite (Pocket Knife)
6: Feldspar (Pocket Knife-Nail)
7: Quartz (Scratches Glass)
8: Topaz (Scratches Quartz)
9: Corundum (Scratches Topaz)
10: Diamond (Scratches Corundum)

Minerals other than these can be classified directly to their reference mineral, or, fall between. Such as a 2.5 if a penny AND a fingernail can scratch it.

Sample Problem

Jack found a mineral and he isn’t sure if it is a Ruby (a red gemstone that consists of Corundum), or just a piece of Fluorite (which can also be red due to impurities). So, Jill tells Jack that he should use the Mohs hardness scale to determine what mineral it is. If Jack CANNOT scratch the unknown mineral with his pocket knife did he find an expensive Ruby, or just a piece of red Fluorite.

Solution

Since Fluorite is a 4 on the Mohs hardness scale it can therefore be scratched with a pocket knife. And, given that a Ruby is made of Corundum, it cannot be scratched with a pocket knife because Corundum has a hardness of 7. Therefore, since he could NOT scratch it with his pocket knife Jack found a Ruby!
(Assuming Jack was absolutely sure that the mineral was either a Ruby or piece of red Fluorite)



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