MadSci Network: Earth Sciences
Query:

Re: Why are diamonds so hard?

Date: Tue Nov 13 16:53:51 2001
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 1004381666.Es
Message:

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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