A mineral is a naturally occuring, homogeneous, solid with a crystalline atomic structure. Crystallinity implies that a mineral has a definite and limited range of composition, and that the composition is expressible as a chemical formula. Some definitions of minerals give them as inorganic materials, however both diamonds and graphite are considered minerals, and both are primarily comprised of carbon, which would make them organic. So this leads me, as an engineer, to believe that mineralogists do not have a good, precise definition of a mineral, but rather a loose definition. The definition above, is the most inclusive and would include all substances currently described as minerals. The key items that make something a mineral are occurring naturally, and the definite crystal structure, that is expressible as a chemical formula. Rocks that do not meet this criteria are referred to as amorphis - not having a definite structure or expressible as a chemical formula. Some elements that occur naturally and are minerals are arsenic, bismuth, platinum, gold, silver, copper, and sulphur.

THE DEFINITION OF ORGANIC: Organic chemistry is the study of those substances containing carbon in combination with hydrogen (H), and a few other non metals, namely oxygen (O), nitrogen (N), sulfur (S) and the halogens (F2, Cl2, Br2, and I2).

Coal is not a mineral, because it can not be expressed as a chemical formula, and therefore, does not have a definate crystaline structure. Coal is predominately carbon. The specimen in the photo is Bituminous Coal, with a carbon content of about 90%, and a BTU rating of over 13,000 BTU/pound. Even though it is predominately carbon the other constituents of coal can be almost anything that was in the swamp when it was formed. I put it in the mineral section because numerous questions have been forwarded to this site asking why coal is not a mineral.

The carbon content of coal ranges from 40% for low ranked coal (lignite) to about 98% for Anthracite Coal.