Re: If coal is not a mineral because it is organic, why is diamond a mineral?

Great question!

In every domain of science, when scientists try to classify things, there are always a 
few things that sit on the fence.  In earth materials, it is the "mineraloids," things that 
look and behave like minerals, and are found with minerals, but are not themselves 
minerals.  Coal and amber are two good examples.

Organic, as used in the definition of minerals, is usually interpreted to mean 
hydrogen and carbon together.  Coal is made up of plant remains, so it has some of 
the hydrocarbon structure of the materials remains.  Now, I learned something new 
when I looked up some information for this question.  These hydrocarbon remains 
are called macerals:

"Macerals

Equivalent to minerals in rocks, macerals form from the coalification of the various 
parts of plants. There are three major groups of macerals:

Vitrinite - the most abundant maceral is the result of the decomposition of the cell 
walls of plant material (essentially decomposed cellulose and lignin)

Exinite - composed of spores, cuticles (the outer surfaces of leaves), resins, and 
waxes.

Inertinite - mostly charcoal, which usually forms in layers resulting from fires that 
periodically sweep through wetlands.

from http://smtc.uwyo.edu/coal/library/glossary.asp?show=Macerals#Macerals

These macerals also do not form regular, repeating arrangements of atoms 
(crystalline structure) and so they are not minerals on that count, either.


Diamond, on the other hand, is nearly pure carbon, rather than hydrocarbons, and 
does have a crystalline structure.  Therefore, it meets the definition.

One final note, diamonds do not, in general, come from coal, as widely believed.  
Most diamonds are found in rocks formed long before there were any plants on the 
earth.  The origin of the carbon is not entirely understood, but it could have been 
original meteorite material, incorporated into the mantle as the earth accreted from  
the original nebula.

Keep thinking up good questions,
David