| Moh's
Hardness A rough measure of the resistance of a smooth surface to scratching or abrasion, expressed in terms of a scale devised (1812) by the German mineralogist Friedrich Mohs. The Mohs hardness of a mineral is determined by observing whether its surface is scratched by a substance of known or defined hardness. To give numerical values to this physical property, minerals are ranked along the Mohs scale, which is composed of 10 minerals that have been given arbitrary hardness values. The minerals contained in the scale are shown in the Table below; also shown are other materials that approximate the hardness of some of the minerals. If the species being tested is fine-grained, friable, or pulverulent, the test may only loosen grains without testing individual mineral surfaces; thus certain textures or aggregate forms may hinder or prevent a true hardness determination. For this reason the Mohs test, while greatly facilitating the identification of minerals in the field, is not suitable for accurately gauging the hardness of industrial materials such as steel or ceramics.
Another disadvantage of the Mohs scale is that it is not linear;
that is, each increment of one in the scale does not indicate a
proportional increase in hardness. For instance, the progression
from calcite to fluorite (from 3 to 4 on the Mohs scale) reflects
an increase in hardness of approximately 25 percent; the progression
from corundum to diamond, on the other hand (9 to 10 on the Mohs
scale), reflects a hardness increase of more than 300 percent.
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