mohs scale of mineral hardness

2 min read 10-03-2025

The Mohs Hardness Scale is a fundamental tool in mineralogy, used to determine the relative hardness of a mineral. Understanding this scale is crucial for mineral identification and various geological applications. This comprehensive guide will delve into the intricacies of the Mohs scale, explaining its use, limitations, and significance.

What is the Mohs Hardness Scale?

Developed in 1812 by German mineralogist Friedrich Mohs, the Mohs Hardness Scale is a qualitative ordinal scale, meaning it ranks minerals based on their relative resistance to scratching. It doesn't provide absolute hardness values, but rather compares the scratch resistance of one mineral against another. The scale consists of ten minerals, each ranked from 1 (softest) to 10 (hardest).

The Ten Minerals of the Mohs Scale:

1. Talc: Extremely soft, easily scratched by a fingernail.
2. Gypsum: Soft, easily scratched by a fingernail.
3. Calcite: Relatively soft, scratched by a copper penny.
4. Fluorite: Moderately hard, scratched by a steel knife.
5. Apatite: Harder than a steel knife, can be scratched by a file.
6. Orthoclase: Can scratch glass.
7. Quartz: Very hard, scratches glass easily.
8. Topaz: Extremely hard, only scratched by a few other minerals.
9. Corundum: Extremely hard, only scratched by diamond.
10. Diamond: The hardest naturally occurring substance.

Mohs Hardness Scale Chart showing the ten minerals and their relative hardness

How to Use the Mohs Hardness Scale

To determine a mineral's hardness, you attempt to scratch it with a known mineral from the scale, or a common object with a known hardness (like a fingernail, penny, or knife). If mineral A scratches mineral B, then A is harder than B. If neither scratches the other, they have approximately the same hardness.

Testing Mineral Hardness: Practical Applications

Fingernail (approx. 2.5): Can scratch gypsum and talc.
Copper Penny (approx. 3.5): Can scratch calcite.
Steel Knife (approx. 5.5): Can scratch apatite, fluorite, and softer minerals.
Glass (approx. 5.5): Can be scratched by apatite, orthoclase, quartz, and harder minerals.
Steel File (approx. 6.5): Can scratch orthoclase and softer minerals.

Limitations of the Mohs Hardness Scale

While incredibly useful, the Mohs Hardness Scale has limitations:

Qualitative, not quantitative: It only provides a relative comparison, not an absolute measure of hardness. The difference in hardness between successive minerals isn't consistent. For example, the difference between diamond (10) and corundum (9) is significantly greater than the difference between gypsum (2) and talc (1).
Anisotropy: Some minerals exhibit different hardness depending on the crystallographic direction.

Beyond the Mohs Scale: Other Hardness Measures

For more precise hardness measurements, scientists use other scales, such as the Knoop hardness scale and the Vickers hardness scale. These provide quantitative data, offering a more detailed understanding of a mineral's resistance to deformation.

Conclusion

The Mohs Hardness Scale remains a valuable tool for geologists, mineralogists, and anyone interested in identifying minerals. While it has limitations, its simplicity and ease of use make it an essential part of mineral identification procedures. Understanding the scale provides a basic understanding of mineral properties and their relative strengths. Remember to always handle minerals with care, and if unsure about identifying a specific mineral, consult a professional.