Hardness
Hardness gauges how strongly a mineral resists surface scratching. [1] It is one of the most useful physical properties for field identification because it can be tested quickly, with no equipment other than materials of known hardness, and it gives a consistent and reproducible result for a given mineral. Two minerals of different hardness will always scratch each other in the same direction: the harder one scratches the softer one, not the other way around. [1]
The Mohs Scale
Hardness values are denoted via the Mohs scale, an ordinal ranking from 1 (the softest) to 10 (the hardest). [1] Specific minerals serve as reference points for each integer on this scale: [1]
| Hardness | Mineral | [1] |
|---|---|---|
| 1 | Talc | [1] |
| 2 | Gypsum | [1] |
| 3 | Calcite | [1] |
| 4 | Fluorite | [1] |
| 5 | Apatite | [1] |
| 6 | Orthoclase | [1] |
| 7 | Quartz | [1] |
| 8 | Topaz | [1] |
| 9 | Corundum | [1] |
| 10 | Diamond | [1] |
For instance, a specimen registering a 5 on this scale can easily abrade a softer material rated 4, yet it will itself suffer scratching from any substance rated 6 or higher. [1] The Mohs scale is ordinal: it ranks minerals relative to each other, but the steps are not equal in absolute hardness terms.
Testing in the Field
For field and laboratory work, mineral hardness is routinely tested with common materials of known hardness rather than with scale minerals. [1] A fingernail (2+) and a steel nail or knife blade (~5) provide two reference points that bracket the vast majority of rock-forming minerals encountered in the field. [1]
What Controls Hardness
At a structural level, hardness is dictated by the strength of the chemical bonds holding the crystalline framework together. [1] This is why minerals with different crystal structures but similar compositions can have very different hardnesses, and why hardness, like all physical properties, ultimately traces back to crystal chemistry.
References
- Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.
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References & Citations
- 1.Introduction to Mineralogy Nesse
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