Graphite, archaically referred to as plumbago, is a crystalline allotrope of carbon, a semimetal, a native element mineral, and a form of coal. Graphite is the most stable form of carbon under standard conditions. Therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds.Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc. As such, talc can easily be scratched by a fingernail. Talc has a specific gravity of 2.5ââ¬â2.8, a clear or dusty luster, and is translucent to opaque. Talc is not soluble in water, but is slightly soluble in dilute mineral acids.[citation needed] Its color ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white. Soapstone is a metamorphic rock composed predominantly of talc. It crystallizes in the tetragonal system, often occurring as stubby, pyramidal or tabular crystals. It also occurs as earthy, granular masses. It is found in many localities, associated with lead ores as a secondary mineral associated with the oxidized zone of lead deposits. It is also a secondary ore of molybdenum, and is sought by collectors.
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