MadSci Network: Earth Sciences |
The strength of any material, and so its hardness is related to how strongly the atoms that make up that material are attached to each other, in other words, how strong the chemical bonds within the material are. This strength is often different in different directions because of the orientation of bonds. Mineral hardness, as determined by scratch test, measures an average of sorts and is often controlled by a single set of weak bonds (the mineral version of a "weakest link"). For example, talc, the micas, and other sheet silicates all have low hardness because they have very weak bonds between the sheets of atoms that make up their structure even though the bonds within each sheet of atoms may be fairly strong. Graphite, which is pure carbon, also has a sheet structure with very weak bond between the sheets. Within a sheet, the carbon-carbon bonds are quite strong, but that does not contribute much to the hardness, because a scratch test just knocks those little sheets apart, no matter how strong they are inside and the mineral is very easy to scratch (it's worth remembering that you would have to stack up several million graphite sheets to make a stack as high as the period at the end of this sentence). Diamond is also pure carbon, but is at the opposite end of the scale from graphite. That's because it has a 3-D network of strong carbon-carbon bonds that gives it strength in all directions. With this network, there are no weak links, no sheets that are easily slid apart. In addition, the carbon-carbon bonds in diamond involve sharing of electrons between adjacent carbon atoms, rather than the donating of electrons as happens in salt. This covalent bonding (as chemists call the sharing arangement) is the strongest kind of bond. Scratching diamond, therefore, requires breaking very strong carbon-carbon bonds and that makes diamond the hardest mineral. Enjoy your studies, Dave Smith, Geology and Environmental Sciences, La Salle University, Philadelphia, PA ------ Admin Note: Be careful not to misinterpret this answer. Strength and hardness are not quite the same thing. Hardness is very closely related to shear strength. But tensile strength may be quite different. A nylon filament is one of the strongest materials known in tension. But it is really very soft.
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