Mohs scale of mineral hardness facts for kids
The Mohs scale ( mohz) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by the German geologist and mineralogist Friedrich Mohs, in his book Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien (English: Attempt at an elementary method for the natural-historical determination and recognition of fossils); it is one of several definitions of hardness in materials science, some of which are more quantitative.
The method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. AD 77. The Mohs scale is useful for identification of minerals in the field, but is not an accurate predictor of how well materials endure in an industrial setting.
Contents
Examples
Below is a table of more materials by Mohs scale. Some of them have a hardness between two of the Mohs scale reference minerals. Some solid substances that are not minerals have been assigned a hardness on the Mohs scale. Hardness may be difficult to determine, or may be misleading or meaningless, if a material is a mixture of two or more substances; for example, some sources have assigned a Mohs hardness of 6 or 7 to granite but it is a rock made of several minerals, each with its own Mohs hardness (e.g. topaz-rich granite contains: topaz — Mohs 8, quartz — Mohs 7, orthoclase — Mohs 6, plagioclase — Mohs 6–6.5, mica — Mohs 2–4).
| Mohs hardness | Mineral | Chemical formula | Absolute hardness | Image | |
|---|---|---|---|---|---|
| 1 | Talc |
|
1 |  |
|
| 2 | Gypsum |
|
3 |  |
|
| 3 | Calcite | CaCO3 | 9 |  |
|
| 4 | Fluorite | CaF2 | 21 |  |
|
| 5 | Apatite | Ca5(PO4)3(OH–,Cl–,F–) | 48 |  |
|
| 6 | Feldspar | KAlSi3O8 | 72 |  |
|
| 7 | Quartz | SiO2 | 100 |  |
|
| 8 | Topaz | Al2SiO4(OH–,F–)2 | 200 |  |
|
| 9 | Corundum | Al2O3 | 400 |  |
|
| 10 | Diamond | C | 1600 |  |
|
Use
Despite its lack of precision, the Mohs scale is relevant for field geologists, who use it to roughly identify minerals using scratch kits. The Mohs scale hardness of minerals can be commonly found in reference sheets.
Mohs hardness is useful in milling. It allows the assessment of which type of mill and grinding medium will best reduce a given product whose hardness is known.
Electronic manufacturers use the scale for testing the resilience of flat panel display components (such as cover glass for LCDs or encapsulation for OLEDs), as well as to evaluate the hardness of touch screens in consumer electronics.
Comparison with Vickers scale
Comparison between Mohs hardness and Vickers hardness:
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Mineral
nameHardness (Mohs) Hardness (Vickers)
(kg/mm2)Tin 1.5 VHN10 = 7–9 Bismuth 2–2.5 VHN100 = 16–18 Gold 2.5 VHN10 = 30–34 Silver 2.5 VHN100 = 61–65 Chalcocite 2.5–3 VHN100 = 84–87 Copper 2.5–3 VHN100 = 77–99 Galena 2.5 VHN100 = 79–104 Sphalerite 3.5–4 VHN100 = 208–224 Heazlewoodite 4 VHN100 = 230–254 Goethite 5–5.5 VHN100 = 667 Chromite 5.5 VHN100 = 1,278–1,456 Anatase 5.5–6 VHN100 = 616–698 Rutile 6–6.5 VHN100 = 894–974 Pyrite 6–6.5 VHN100 = 1,505–1,520 Bowieite 7 VHN100 = 858–1,288 Euclase 7.5 VHN100 = 1,310 Chromium 8.5 VHN100 = 1,875–2,000
See also
In Spanish: Escalas de dureza para niños
- Brinell scale
- Geological Strength Index
- Hardnesses of the elements (data page)
- Knoop hardness test
- Meyer hardness test
- Pencil hardness
- Rockwell scale
- Rosiwal scale
- Scratch hardness
- Superhard material
