Graphite
A variety of Minerals
What is Graphite?
Although graphite is a sister stone to diamond, formed by very similar earth forces and from the same material, they have very different end results. Graphite has a long list of uses, with one of the most common being the writing force behind pencils. It is appropriately named after the Greek word meaning "to write!"
Etymology & Origins
Historically, graphite was called black lead or plumbago. Plumbago was commonly used in its massive mineral form. Both of these names arise from confusion with the similar-appearing lead ores, particularly galena. The Latin word for lead, plumbum, gave its name to the English term for this grey metallic-sheened mineral and even to the leadworts or plumbagos, plants with flowers that resemble this colour. The term black lead usually refers to a powdered or processed graphite, matte black in color. Abraham Gottlob Werner coined the name graphite ("writing stone") in 1789. He attempted to clear up the confusion between molybdena, plumbago and black lead after Carl Wilhelm Scheele in 1778 proved that there are at least three different minerals. Scheele's analysis showed that the chemical compounds molybdenum sulfide (molybdenite), lead(II) sulfide (galena) and graphite were three different soft black minerals.
Uses & Applications
Though most popular for its presence in pencils, graphite has many applications including use in batteries, brake linings, electric motor brushes, carbon products, crucibles, nozzles, lubricants, refractories, rubber, and steelmaking. This mineral is also used to improve various manufacturing processes and their end products. Applications of graphite are dependent on whether the mineral is natural or synthetic, as well as if it is in solid or powder form.
Healing Properties
Graphite Is a high energy stone that is an excellent component in wands. Often referred to as the "Stone of personal freedom," it is believed to be beneficial in helping one make the life changes needed for personal growth. It is thought to improve writing skills and provide better understanding of mathematical equations. It helps to stimulate cognitive thinking skills, encouraging its user toward greater learning and improves communication.
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Key Characteristics
Rock Types of Graphite
The principal types of natural graphite, each occurring in different types of ore deposits, are Crystalline small flakes of graphite (or flake graphite) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken. When broken the edges can be irregular or angular; Amorphous graphite: very fine flake graphite is sometimes called amorphous; Lump graphite (or vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin. Highly ordered pyrolytic graphite refers to graphite with an angular spread between the graphite sheets of less than 1°. The name "graphite fiber" is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer.
Characteristics of Graphite
The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate quickly along the tightly bound planes, but are slower to travel from one plane to another. Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite readily oxidizes to form carbon dioxide at temperatures of 700 °C and above. Graphite is an electrical conductor, hence useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers (a phenomenon called aromaticity). These valence electrons are free to move, so are able to conduct electricity. However, the electricity is primarily conducted within the plane of the layers. The conductive properties of powdered graphite allow its use as pressure sensor in carbon microphones. Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are solely due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment (such as in technologies for use in space), graphite degrades as a lubricant, due to the hypoxic conditions. This observation led to the hypothesis that the lubrication is due to the presence of fluids between the layers, such as air and water, which are naturally adsorbed from the environment. This hypothesis has been refuted by studies showing that air and water are not absorbed. Recent studies suggest that an effect called superlubricity can also account for graphite's lubricating properties. The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steel, and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity). It is also corrosive to aluminium in the presence of moisture. For this reason, the US Air Force banned its use as a lubricant in aluminium aircraft, and discouraged its use in aluminium-containing automatic weapons. Even graphite pencil marks on aluminium parts may facilitate corrosion. Another high-temperature lubricant, hexagonal boron nitride, has the same molecular structure as graphite. It is sometimes called white graphite, due to its similar properties. When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic graphite. It is also highly anisotropic, and diamagnetic, thus it will float in mid-air above a strong magnet. If it is made in a fluidized bed at 1000–1300 °C then it is isotropic turbostratic, and is used in blood contacting devices like mechanical heart valves and is called pyrolytic carbon, and is not diamagnetic. Pyrolytic graphite and pyrolytic carbon are often confused but are very different materials. Natural and crystalline graphites are not often used in pure form as structural materials, due to their shear-planes, brittleness, and inconsistent mechanical properties.
Formation of Graphite
Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in igneous rocks and in meteorites. Minerals associated with graphite include quartz, calcite, micas and tourmaline. The principal export sources of mined graphite are in order of tonnage: China, Mexico, Canada, Brazil, and Madagascar. In meteorites, graphite occurs with troilite and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite. Some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar system. They are one of about 12 known types of mineral that predate the Solar System and have also been detected in molecular clouds. These minerals were formed in the ejecta when supernovae exploded or low- to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the second or third oldest mineral in the Universe.
Composition of Graphite
Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common are diamond and graphite (less common ones include buckminsterfullerene). In diamond the bonds are sp and the atoms form tetrahedra with each bound to four nearest neighbors. In graphite they are sp orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. The individual layers are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with a bond length of 0.142 nm, and the distance between planes is 0.335 nm. Atoms in the plane are bonded covalently, with only three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive. Bonding between layers is via weak van der Waals bonds, which allow layers of graphite to be easily separated, or to slide past each other. Electrical conductivity perpendicular to the layers is consequently about 1000 times lower.
Health & Safety Information
- ⚠️People can be exposed to graphite in the workplace by breathing it in, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for graphite exposure in the workplace as a time weighted average (TWA) of 15 million particles per cubic foot (1.5 mg/m³) over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of TWA 2.5 mg/m³ respirable dust over an 8-hour workday. At levels of 1250 mg/m³, graphite is immediately dangerous to life and health.
Quick Facts
Physical Properties
- Color
- Iron black to steel-grey
- Hardness (Mohs)
- 1 - 2
- Density
- 2.26 g/cm³
- Streak
- Black to steel gray
- Luster
- Metallic
- Crystal System
- Hexagonal
Chemical Properties
- Chemical Formula
- C
- Elements
- C
Also Known As
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