The relative scale of metals and mineral hardness
Mohs scale of mineral hardness is referred to both a set of standard minerals: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, diamond, used to determine the relative hardness by scratching a test object with a reference element, and actually a ten-scale of relative mineral hardness. Standards, the following 10 minerals are used as the reference elements for the mineral hardness scale in the order of increasing hardness: talc, gypsum, calcite, fluorite, apatite, orthoclase (feldspar), quartz, topaz, corundum, diamond.
Mohs scale was proposed as a relative hardness scale in 1811 by a German scientist Friedrich Mohs. Despite the fact that the formulations belongs to the beginning of XIX century, this relative hardness scale is widely used to the present time. It is now possible to buy a set of standard minerals for the educational process and for the jewelry industry.
Mohs scale is used for a quick comparative diagnostics of minerals. Thus, if a reference element, having hardness of 5, scratches the sample, and the latter leaves a mark on the surface of a reference sample of hardness 4 , the intermediate mineral would have its hardness at 4.5 (4 ‘) Mohs scale. At the same time we must keep in mind that Mohs scale is not a linear scale. Numbers on the hardness scale only indicate the order of hardness distribution, but do not have any quantitative significance. The hardness scale in any case must not be interpreted like a diamond (10) is twice harder than apatite (5). The absolute hardness values ”(sometimes referred to as the true relative) show an entirely different picture. In addition, please be aware that some minerals hardness in different directions can vary greatly due to crystal structure. Metal hardness has a more stable absolute values”, but as a matter of practice we rarely deal with a pure metals, and the alloy hardness depends on all of its components.
Mohs hardness scale in the technical literature is usually shown as a comparative table. For clarity, it is sometimes accompanied by photographs of minerals – hardness scale references.
Mohs scale is even more convenient because one can usein actual practice such “handy tools” as their fingernail (Mohs hardness of 2 ‘), a cent or any other U.S. coin (hardness slightly less than 3), knife (Mohs hardness of 5 ‘), glass (5 ‘), high-tensile steel file (hardness 6 ‘), sandpaper of synthetic corundum (has Mohs hardness of 9), fine sandpaper for wood “garnet paper” (hardness 7 ‘).