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Vein or Lump graphite is considered the rarest, most valuable, and highest quality form of natural graphite. It is formed from the direct deposition of solid, graphitic carbon from subterranean, high temperature fluids. Unlike diamond (another carbon allotrope), graphite is an electrical conductor, making it a semimetal. Graphite is the most stable form of carbon under standard conditions. As a result it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds.
Understanding Cast Irons - Malleable Iron. This type of iron is characterized by having the majority of its C content occur in the microstructure as irregularly shaped nodules of graphite. This form of graphite is called temper carbon because it is formed in the solid state during heat treatment. The iron is cast as a white iron of a suitable chemical composition. After the castings are removed from the mold, they are given an extended heat treatment starting at a temperature above 1650Â°F (900Â°C). This causes the iron carbide to dissociate and the free carbon precipitates in the solid iron as graphite.