280 



UNITED STATES MINERAL RESOURCES 



that of most domestic producers. Since about 1960, 

 however, technological changes and increased prices 

 for raw graphite have changed the attitudes of im- 

 porters and suppliers. Development of the silicon 

 carbide-graphite crucible, which can use graphite 

 from almost any source, is an example of improved 

 technology. The net result is that domestic deposits, 

 particularly those of flake graphite, look much more 

 attractive now than they did in the 1950's or 1960's. 



GEOLOGIC ENVIRONMENT 



The form, occurrence, mineralogy, origin, and dis- 

 tribution of natural graphite have been discussed 

 in considerable detail elsewhere (Cameron and Weis, 

 1960, p. 206-208; 241-249) and will be described 

 only briefly here. 



GEOCHEMISTRY 



Carbon is 11th in abundance among the elements 

 that form the earth's crust, but the crustal abun- 

 dance of carbon bears little relationship to the abun- 

 dance or distribution of graphite. Most of the carbon 

 in the crust of the earth — probably 80-90 percent — 

 is in carbonate rocks, and most of the remaining 

 carbon occurs in living or fossil organic matter or 

 in the atmosphere as CO2; probably less than one- 

 half of 1 percent of the carbon in the earth's crust 

 occurs in elemental form, as amorphous carbon, 

 graphite, or diamond. 



Carbon's behavior in the geochemical cycle de- 

 pends largely on its form. More than 95 percent of 

 the earth's carbon occurs as CO2 or its compounds, 

 and most of the remainder occurs as one of the 

 hundreds of organic hydrocarbons. These compounds 

 have characteristic geochemical cycles that are un- 

 like that of elemental carbon. 



Carbon that occurs as graphite came at least in 

 part from organic material. Once in the form of 

 graphite, it is likely to remain unchanged. In most 

 chemical environments, graphite is one of the most 

 inert substances known to man. Acids, bases, oxidiz- 

 ing and reducing environments alike leave it un- 

 changed. Only at temperatures greater than about 

 600° C in an oxidizing environment does it slowly 

 change to CO2. Under surface weathering conditions 

 it remains unchanged. Metamorphism tends to re- 

 crystallize graphite to coarser grains. The graphite 

 cycle tends to become a closed system once the 

 mineral is formed. 



The cycle of erosion, transportation, deposition, 

 and metamorphism tends to disseminate graphite, 

 rather than to concentrate it. There are no workable 

 sedimentary deposits of graphite. Economic deposits 

 of flake or amorphous graphite occur only where 



concentrations of nongraphitic carbon have been 

 metamorphosed. 



GEOLOGY 



VEIN GRAPHITE 



Vein deposits of graphite are fracture fillings that 

 contain 75-100 percent graphitic carbon. Crustifica- 

 tion is common, with coarse, platy, or needlelike 

 interlocking crystals apparently grown inward from 

 both walls. Mineral impurities include quartz, feld- 

 spar, pyrite, pyroxene, apatite, and calcite. Veins 

 range from thin films to massive bodies more than 

 10 feet in thickness. Some extend along strike for 

 thousands of feet, and some have been mined down- 

 dip for more than 1,500 feet. The veins cut both 

 igneous and metamorphic rocks, generally of Pre- 

 cambrian age. 



Vein graphite deposits are known near Crown 

 Point, N.Y., and in the Dillon area, Montana, but 

 the only economically important world supply is 

 from Ceylon. 



The agents of transport and precipitation of car- 

 bon that lead to the formation of graphite veins are 

 not understood. Some geologists have suggested that 

 the veins are hydrothermal ; others, that they are 

 pneumatolytic. Neither the hydrothermal nor the 

 pneumatolytic hypothesis has enough supporting 

 field evidence to enable one to make a final choice. 



FLAKE GRAPHITE 



Flake graphite of commercial size is derived from 

 carbonaceous material in sedimentary rocks that 

 have metamorphosed at least to garnet grade. The 

 resulting gneisses and schists are the hosts for all 

 deposits of flake graphite. The grade of the deposit 

 is dependent on the carbon content of the original 

 sedimentary rocks. Graphitic metasedimentary rocks 

 contain from a trace to as much as 90 percent flake 

 graphite, but most of these gneisses and schists con- 

 tain less than 3 percent graphitic carbon. Distribu- 

 tion of the graphite in flakes, that may range from 

 1 to more than 250 millimeters in diameter, gener- 

 ally is more uniform along strike and dip than from 

 one layer to another. 



Impurities in flake graphite are the common min- 

 erals of high-grade metasedimentary rocks, espe- 

 cially quartz, feldspars, mica, amphibole, and garnet. 

 The suite of gangue minerals is an important eco- 

 nomic consideration. Most of these minerals are 

 easily separated from the graphite. Mica is a nota- 

 ble exception, especially when intergrown with 

 graphite, a common relation in many flake deposits. 

 The separation of these minerals becomes extremely 



