HELP on MOHs SCALE of HARDNESS (original scale 1-10)
Mohs hardness is a highly non-linear partially subjective scale of hardness. The original scale, on which the data presented is based, goes from 1 (being very soft) to 10 (being very hard). The hardness of substance is determined by whether the measured material will scratch a reference material. By this method, Moh was able to assign hardness values to 10 common substances, which pinpoint the scale of hardness. Thus talc has a hardness of 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9 and diamond 10. Note that carbon has representations at both ends of the scale, with graphite at 0.5 and diamond at 10. By ascertaining whether the test substance will scratch one of the above reference materials or not determines its hardness. The scale is highly non-linear, with a greater gap between 8 and 10 than between 1 and 8. A newer and extended scale that goes some way to filling in the large steps between 8 and 10 goes from 1 to 15. The new scale (not used in the graphs) gives talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, vitreous pure silica 7, quartz 8, garnet 9, topaz 10, fused zirconia 11, fused alumina 12, silicon carbide 13, boron carbide 14 and diamond 15. Other more scientific scales of hardness such as Vickers Hardness or the Knoop Test measure the indentation produced in the test substance by a diamond point under a known force. Other similarly measured hardness scales are Brinell Hardness, Meyer Hardness and Rockwell Number. All hardness scales are arbitrary.
Please note that for some elements, the value of hardness as determined by Mohs method is dependant upon the purity of the element. Slight impurities can dramatically increase the hardness of some metals, for example tungsten. Also, the value of hardness as determined by Mohs method can depend upon the orientation of the substance (which crystal axis is aligned in the scratch direction), and the rake angle (the angle at which the element makes contact with the material to be scratched). For instance, graphite varies from 0.5 to 1. These variables can make the determination of Mohs hardness subject to considerable error such that a range of values are obtained. For example, silver has a Mohs hardness in the range 2.5 to 4. The values quoted here err on the softer side of the range. For other elements with a narrower range, I have simply taken the arithmetic mean of the range.
Hardness is not the same as toughness; diamond, although extremely hard, may cleave along planes of weakness when knocked. Despite being harder than quartz, emerald and zircon are brittle and easily chipped. The toughest gems, jadeite, nephrite and agate all have harnesses of 7 or less, but resist fracture by means of their structure of microcrystalline interlocking crystals or fibres.
Caesium is the softest element at just 0.2 whilst carbon in the form of diamond the hardest at 10.
