The silica of the compound has been driven off, leaving a shell of graphitic substance formed from the coke.
A naturally occurring, steel-gray to black, crystalline form of carbon. The carbon atoms in graphite are strongly bonded together in sheets. Because the bonds between the sheets are weak, other atoms can easily fit between them, causing graphite to be soft and slippery to the touch. Graphite is used in pencils and paints and as a lubricant and electrode. It is also used to control chain reactions in nuclear reactors because of its ability to absorb neutrons.