Geologists use hardness as a fundamental property to help identify minerals by comparing their relative hardness.
Understanding Mineral Hardness
One of the key physical properties geologists examine when identifying minerals is their hardness. This property relates to a mineral's resistance to scratching.
As stated in the reference, minerals can be identified by comparing their relative hardness. The principle is simple: If a substance is able to scratch another substance, it is harder.
Think of it like testing materials in everyday life – a diamond can scratch glass, indicating diamond is harder than glass. Geologists apply this same concept systematically to mineral samples.
The Mohs Scale of Hardness
To standardize this comparison, geologists widely use a system called the Mohs Scale of Hardness. This scale, named after the scientist Friedrich Mohs (1773-1839), ranks minerals from 1 (softest) to 10 (hardest).
The scale is relative, meaning each mineral on the scale can scratch the minerals below it but can be scratched by the minerals above it. It does not represent a linear increase in hardness (e.g., a mineral with a hardness of 4 is not twice as hard as a mineral with a hardness of 2).
Here is the standard Mohs Scale:
| Hardness | Mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
Practical Application of Hardness Testing
In the field or laboratory, geologists test a mineral's hardness by trying to scratch it with objects of known hardness, often using a set of standard minerals or common items with established positions on the Mohs scale.
Here are common test objects and their approximate hardness:
- Fingernail: ~2.5
- Copper Penny: ~3.5
- Iron Nail/Glass: ~5.5
- Steel File: ~6.5
Steps for Testing Hardness:
- Hold the mineral specimen firmly.
- Take a test object (like a fingernail or penny).
- Press the test object firmly against a smooth, inconspicuous surface of the mineral and try to make a scratch.
- Observe the result carefully. Is there a scratch, or is it just a powder streak from the test object rubbing off?
- Wipe the surface to see if a permanent groove was made.
Interpreting the Results:
- If your fingernail scratches the mineral, its hardness is less than 2.5 (likely 1 or 2).
- If your fingernail doesn't scratch it, but a penny does, its hardness is between 2.5 and 3.5.
- If a penny doesn't scratch it, but glass does, its hardness is between 3.5 and 5.5.
- If glass doesn't scratch it, but a steel file does, its hardness is between 5.5 and 6.5.
- If even a steel file doesn't scratch it, its hardness is greater than 6.5. You would then use minerals from the Mohs scale (Quartz, Topaz, Corundum, Diamond) to narrow down the hardness range further.
By determining where a mineral falls on the Mohs scale, geologists gain crucial information that helps them narrow down the possibilities and identify the mineral alongside other properties like color, streak, cleavage, fracture, and luster.
