Vein graphite is known under various names including crystalline vein, Plumbago, Sri Lankan graphite, and Ceylon graphite. The name “Sri Lankan” and “Ceylon” are commonly used for vein graphite since the island nation of Sri Lanka (formally Ceylon) is the only area to produce this material in commercial quantities. Serious mining and exportation of Ceylon graphite began about 1824, however the unusual deposits of Ceylon have been known, and apparently used locally, since the middle of the1600s.
Of all the natural graphite materials vein graphite is probably the most difficult to describe geologically and various theories of its origin have been presented. As the name suggests, vein graphite is a true vein mineral as opposed to a seam mineral (amorphous graphite) or a mineral that is disseminated throughout the ore rock (as in flake graphite). Seam minerals have some unique properties including their being non-contemporaneous with the country rock, steeply inclined (vein orientation), and subjected to filling by a host of minerals, especially those of hydrothermal origin.
Vein graphite is not restricted to Sri Lanka. Many localities are known including the famous Borrowdale, Cumberland, Great Britain location where the first “pencils” were carved from solid graphite veins. Dillon Montana USA is another location where relatively thick vein fillings of graphite have been found. The author has even observed pegmatitic vein-like fracture fillings in northwestern New Jersey, USA. However, all currently available commercial vein graphite is mined in Sri Lanka.
The first photograph below is a specimen of vein graphite from the Borrowdale, Great Britain deposit. The second photograph below is a vein graphite specimen from Sri Lanka.
Crystalline vein graphite is unique, as it is believed to be naturally occurring pyrolytic (deposited from a fluid phase) graphite. Vein graphite gets its name from the fact that it is found in veins and fissures in the enclosing “ore” rock. This variety of graphite is formed from the direct deposition of solid, graphitic carbon from subterranean, high temperature fluids known as pegmatitic fluids. Pegmatites form regionally or locally when a mass of magma cools, or when some other source of “geologic energy” results in melting of the country rock. The fluids emanating from such sources are hot, under high pressure, and are erosive and may actually be in a state known as “supercritical”, which is a meta-stable fluid state that is not a liquid and not a gas. Pegmatites represent substances that distill out of heated rock due their low solubility in the local system. If such fluids invade a previously existing flake graphite deposit or other carbonaceous bearing rock the solid carbon present may become incorporated into the fluid as carbon dioxide, methane, carbon monoxide, or other carbonaceous fluid phase. Carbon containing gases can also form from the reaction of carbonate mineral species with magma or other energy source. If limestone, marble, or other carbonate rich mineral species is involved large volumes of carbonaceous gases can potentially form from the release of “carbon dioxide of crystallization”.
Regardless of the mode of gas/fluid generation, carbon is mobilized, and transported through fractured country rock to a location more or less remote from the location where the carbonaceous fluid was formed. Where equilibrium conditions are right, solid graphitic carbon “precipitates” directly from the fluid phase resulting in the beautiful masses of graphite vein fillings known as vein graphite.
This type of graphite typically shows needle-like macro morphology and flake like micro morphology. Close examination of fracture fillings show the presence of tightly packed, needle or acicular crystals aligned perpendicular to vein walls. Needle-like texture is clearly visible to the naked eye, but vein fillings are anhedral, yielding no well formed single crystals. The accompanying photograph shows a large, 6 X 6 X10 inch piece of vein graphite from Sri Lanka. Note the somewhat “top-to-bottom”,
elongated, preferred orientation of the piece. When emplaced in country rock this specimen was rotated 90o from its current position, the top and bottom of the specimen were perpendicular to the fissure walls. A close up of this same specimen shows the acicular structure of adjacent vein graphite crystals. This type of perpendicular “crystal-to-wall” orientation is typical of pyrolytic-type carbon deposition.
As with all graphite materials the micro-morphology of vein graphite converges to flake structure as the particle size is decreased. This convergence is illustrated by the accompanying series of scanning electron micrographs of Asbury grade 98DC milled vein graphite:
Particles imaged in the top SEM show the elongated, needle-shaped morphology characteristic of vein graphite at this scale. Moving to the bottom SEM it can clearly be seen that the individual needle shaped particles are made up of graphite having intrinsic flake morphology. This observation is especially apparent in the micrograph to the far right.
When viewed using polarized light microscopy, the oriented, needle shaped crystals that make up the mass of the vein are apparent as shown in the accompanying photomicrograph (field width approximately 300 micrometers).
Due to the natural fluid-to-solid deposition process, vein graphite deposits are typically above 90 percent pure with some vein graphite reaching 99.5% graphitic carbon in the “as found” state. This level of purity is possible because the deposition of carbon occurs as a precipitation of solid carbon from a geologic fluid that is traversing emplaced rock. There is no intimate mixing or association of the graphite with country rock as in conventional flake graphite deposits were the non-carbon and carbon phases may be deposited contemporaneously.
Typical veins measure from centimeters to nearly two meters in thickness with the highest purity material being located toward the center of the vein away from contact with the wall rock. Vein graphite is mined using conventional shaft or surface methods typically used to mine vein-type deposits.
Vein graphite is available in sizes ranging from 8-cm. lumps to powder as fine as 5-micrometers. Products covering the range of purity from 94% graphitic carbon to 99% graphitic carbon are commonly available. In many applications vein graphite may offer superior performance since it has slightly higher thermal and electrical conductivity, which result from its high degree of crystalline perfection. Vein graphite also has the highest degree of cohesive integrity of all natural graphite materials. High cohesive “energy” means that vein graphite is easy to mold and can be formed into solid shapes without the aid of a binder addition.