Mohs Scale of Harness

What is the Mohs Scale of Hardness?

The Mohs scale of mineral hardness is a scale characterising the scratch resistance of various minerals through the ability of a harder material to scratch a softer material.

How did the Mohs Scale come about?

The Mohs’ hardness scale was developed in 1822 by Frederich Mohs. This scale is a chart of relative hardness of the various minerals (1 – softest to 10 – hardest).

Why is Hardness Important?

The effects of high hardness are important in many fields. Abrasives are used to form and polish a lot of different substances. Diamonds are an important mineral component in cutting tools for the manufacturing of metals and other substances, forming dies for the drawing of wires, and for cutting cores in oil wells and mineral exploration. Emery is used in many abrasive products that do not require the hardness of diamond tools. Garnets were used as an abrasive in sandpaper. Talc is an extremely soft mineral that has been used in bath powders (talcum powder).

Mineral hardness can also be seen in the topography of many landscapes. Quartz bearing rocks are often more resistant to weathering and will produce the capstones that protect the tops of buttes and mesas from erosion.

Tell Me More!

Mohs’ hardness is a measure of the relative hardness and resistance to scratching between minerals. Other hardness scales rely on the ability to create an indentation into the tested mineral (such as the Rockwell, Vickers, and Brinell hardness – these are used mainly to determine  hardness in metals and metal alloys). The scratch hardness is related to the breaking of the chemical bonds in the material, creation of micro-fractures on the surface, or displacing atoms (in metals) of the mineral.

When doing the tests of the minerals it is necessary to determine which mineral was scratched. The powder can be rubbed or blown off and surface scratches can usually be felt by running the fingernail over the surface. One can also get a relative feel for the hardness difference between two minerals. For instance quartz will be able to scratch calcite with much greater ease than you can scratch calcite with fluorite. You must also use enough force to create the scratch (if you don’t use enough force even diamond will not be able to scratch quartz). You also have to be careful to test the material that you think you are testing and not some small inclusion in the sample. This is where using a small hand lens can be very useful to determine if the test area is homogenous.

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