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MOHS' SCALE OF HARDNESS

     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). Since hardness depends upon the crystallographic direction (ultimately on the strength of the bonds between atoms in a crystal), there can be variations in hardness depending upon the direction in which one measures this property. One of the most striking examples of this is kyanite, which has a hardness of 5.5 parallel to the 1 direction ( c-axis), while it has a hardness of 7.0 parallel to the 100 direction ( a-axis). Talc (1), the softest mineral on the Mohs scale has a hardness greater than gypsum (2) in the direction that is perpendicular to the cleavage. Diamonds (10) also show a variation in hardness (the octahedral faces are harder than the cube faces). For further information see articles from the American Mineralogist on microhardness, the Knoop tester, and diamonds.

      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 microfractures on the surface, or displacing atoms (in metals) of the mineral. Generally, minerals with covalent bonds are the hardest while minerals with ionic, metallic, or van der Waals bonding are much softer.

   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. One 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 - this is an area where practice is important). 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.

 

Mineral Hardness    
Diamond 10 Zaire 1 cm. 14 carats
Corundum 9 variety ruby, India 6 cm.
Topaz 8 Mursinsk, Russia, 5cm across Seaman Museum specimen
Quartz 7 variety amethyst, Guerro, Mexico 16 cm.
Orthoclase 6 Orthoclase (white) on quartz, Baveno, Italy Orthoclase crystal is 3 cm tall. Seaman museum specimen.
Apatite 5 Durango, Mexico. Crystal is 7.5 cm. tall. Seaman museum specimen.
Fluorite 4 Elmwood mine, Tennessee 2.5 cm. (note phantom)
Calcite 3 Elmwood Mine, Tennessee 8 cm. (twinned)
Gypsum 2 Wyoming 12 cm. Note "fishtail" twin on left
Talc 1 Rope's Gold Mine, Michigan (green) 4 cm. across talc mass

 

Why is hardness important?

    The effects of high hardness are important in many fields. Abrasives are used to form and polish many 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 - a variety of corundum, is used in many abrasive products that do not require the hardness (or expense) 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 harness is also important in sedimentary rocks. Harder minerals tend to be able to travel longer distances down river systems. Quartz can often undergo several cycles of erosion, transportation and lithification ( change of sediments to rock). Zircons are persistent minerals in the environment and can often tell geologists the types of rock that were the original source rock for metamorphic or sedimentary rocks.

    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.

 Niagara Falls, N. Y. from American side 5/2002 
Niagara Falls, New York


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