Mohs scale of mineral hardness
Mohs' scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.
Mohs based the scale on ten minerals that are all readily available except the last one, diamond. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on Mohs scale is 4.5.
The table below shows comparison with absolute hardness measures by a sclerometer. Mohs' is a purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond, almost four times as hard as corundum.
| Hardness | Mineral | Absolute Hardness |
|---|---|---|
| 1 | Talc (Mg3Si4O10(OH)2) |
1 |
| 2 | Gypsum (CaSO4·2H2O) | 2 |
| 3 | Calcite (CaCO3) | 9 |
| 4 | Fluorite (CaF2) | 21 |
| 5 |
Apatite
(Ca5(PO4)3(OH-,Cl-,F-)) |
48 |
| 6 | Orthoclase Feldspar (KAlSi3O8) | 72 |
| 7 | Quartz (SiO2) | 100 |
| 8 | Topaz (Al2SiO4(OH-,F-)2) | 200 |
| 9 | Corundum (Al2O3) | 400 |
| 10 | Diamond (C) | 1500 |
On the Mohs scale, fingernail has hardness 2; copper penny, about 3; a knife blade, 5; window glass, 5.5; steel file, 6.5. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.
Some mnemonics traditionally taught to geology students to remember this table are "The Girls Can Flirt And Other Queer Things Can Do" or "To Get Candy From Aunt Fanny, Quit Teasing Cousin Danny".
An alternative table is shown below which has been modified to incorporate additional substances that may fall in between two levels.
Source: American Federation of Mineralogical Societies: Mohs Scale of Mineral Hardness
| Hardness | Substance or Mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 2.5 to 3 | Gold, Silver |
| 3 | Calcite, Copper penny |
| 4 | Fluorite |
| 4 to 4.5 | Platinum |
| 4 to 5 | Iron |
| 5 | Apatite |
| 6 | Orthoclase |
| 6.5 | Iron pyrite |
| 6 to 7 | Glass, Vitreous pure silica |
| 7 | Quartz |
| 7 and up | Hardened steel |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Garnet |
| 11 | Fused zirconia |
| 12 | Fused alumina |
| 13 | Silicon carbide |
| 14 | Boron carbide |
| 15 | Diamond |