Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. 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. As the hardest known naturally occurring substance, diamond is at the top of the scale. 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 the Mohs scale is 4.5.
The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) almost four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer.[1]
| 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 2.5; copper penny, about 3.5; a knife blade, 5.5; window glass, 6.5; 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." Another Mnemonic is "Two Gypsies Called Flo And Fred Queued To Cut Diamonds."
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 | pure 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 to 7.5 | Garnet |
| 7 to 8 | Hardened steel |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
| >10 | Aggregated diamond nanorods |
See also
Notes
- ↑ A sclerometer is an instrument used by mineralogists to measure the scratch hardness of materials.
ReferencesISBN links support NWE through referral fees
- Dieter, George E. (1989). Mechanical Metallurgy, SI Metric Adaptation, Maidenhead, UK: McGraw-Hill Education. ISBN 0-07-100406-8.
- Malzbender, J (2003). "Comment on hardness definitions". Journal of the European Ceramics Society 23: 1355.
- Mineral Gallery. 2000. What is Important About Hardness? Amethyst Galleries. Retrieved May 10, 2007.
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