The “hardness” of a material describes its ability to withstand surface indentation and permanent, localised deformation caused by direct contact with another substance. In simpler terms, it tells us how likely a material is to be scratched or abraded by other materials. The hardness of a mineral is controlled by the strength of its atomic bonds, and the structure of its crystal lattice. Considering bismuth’s metallic properties and density, you may be surprised at where bismuth stands in comparison to the hardness of other materials.
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Mohs Scale of Hardness is on a scale of 1 to 10, all minerals have their own hadness and rankings
One of the most common ways to measure hardness is the Mohs hardness scale. This is a relative scale, used in geology to compare different minerals’ resistances to being scratched. The scale ranges from 1 to 10, with the softest minerals, such as talc, at number 1, and the hardest minerals, like diamonds, at 10. The hardness of any mineral can be easily hypothesized by trying to scratch the mineral with different materials. If a mineral can be scratched with a fingernail, which has a hardness of 2.5, we know it is at the lower end of the scale. If a mineral can resist being scratched by a steel nail, which has a hardness of 6.5, we know it is on the higher end of the scale. This easy test is often used to help identify unknown minerals.
Bismuth a colorful metal, low on the Mohs scale of hardness.
The hardness of a mineral is also what determines if it leaves a residue behind. Some different minerals that all appear to be black, will leave different coloured streaks on a ceramic plate based on hardness and composition. A common example of this is the graphite used in pencils. Graphite has such weak bonds between its layers of carbon that it easily separates and leaves a streak of graphite on the paper. Alternatively, Diamond, which is also composed purely of carbon, has such a strong crystal lattice that it refuses to be pulled apart and cannot be easily scratched.