GRAPHITE

Name: Graphite (from the Greek graphein, “to write”)

Symbol: C

Group: 14

Period: 2

Block: p

Category: Nonmetal (crystalline allotrope of carbon)

Atomic number: 6

Atomic mass: 12.011 u

Electrons per shell: 2.4

Electronegativity: 2.55

Density: 2.26 g/cm³

Melting point: Not defined (sublimes at ~3,650 °C in an inert atmosphere)

Boiling point: Not applicable (sublimes at high temperatures)

Thermal conductivity: 100–400 W/(m K) (anisotropic, varies with direction)

Electrical conductivity: ~10⁵–10⁶ S/m (high in the layer direction)

Magnetic order: Diamagnetic

State Ordinary: Solid

Oxidation states: 0 (mainly), +2, +4 (in compounds)

Mohs hardness: 1–2

Vickers hardness: ~10–20 MPa (low due to its softness)

Brinell hardness: Nonstandard (soft, anisotropic material)

Most stable isotopes: C-12 (98.93%), C-13 (1.07%)

Discoverer: Not attributed to a specific individual; known since antiquity, scientifically characterized by Carl Wilhelm Scheele (1779)

At standard temperatures and pressures, graphite is the most thermodynamically stable (allotropic) form of carbon, so it predominates over the rest by an overwhelming majority. Proof of this is that for every diamond mined, not to mention the rare cases in which we find fullerenes, we can find hundreds of tons of graphite around the world, easily recognizable and exploitable. However, given that throughout my life I have noticed that many people have difficulty distinguishing coal from elemental carbon, I will try to explain in the simplest way possible the differences between the two. Graphite (pure carbon) should not be confused with other minerals, also carbonoids, with elemental contents ranging from 40-95%, depending on their "carbon richness," a property that enumerates them according to their carbon content.

Coal is an impure source of carbon, easily combustible, used since the dawn of time as an energy source. It is divided into several minerals mined for their carbon content and follows a hierarchy from lowest to highest carbon content (%), which increases its price. These minerals are, in order: Peat, Lignite, Bituminous Coal, and Anthracite. All these minerals make up the traditionally known "coal," which has the same name in both Spain and Latin America and comes from the Latin "carbo," the source of the element's name.

The quality of coal (not to be confused with pure carbon) depends precisely on the percentage of carbon found by mass. Thus, minerals such as lignite never exceed 75% carbon by mass, while anthracite comes closest, with a typical limit of 95%. These minerals are combustible, and their vigorous oxidation, which releases heat with a flame, is what we know as coal or wood fires. It is by far the most primitive form of heat.

Fire is a chemical reaction in which carbon combines with oxygen to typically form CO2, the often infamous (due to its greenhouse effect) "carbon dioxide." Since the affinity between carbon and oxygen is very high, the reaction is easy and releases a lot of energy (although not as much as fossil hydrocarbons like oil). Coal is sometimes separated into what is called "wood charcoal" and "mineral coal." What is the difference? Is "mineral coal" a proper name for graphite?

The difference between charcoal and mineral charcoal is exactly what the adjective implies: different origins. While charcoal comes from animals and, above all, large remnants of dead plant biomass, often fossilized, mineral charcoal comes intrinsically from non-organic sources, or from sources that were once organic but have lost significant amounts of water and other elements. The reason we find large, increasingly "pure" carbon remains as the years go by, for example, when examining the trunk of a fallen tree, is that all other elements, mainly water and other more volatile hydrogen compounds, are progressively lost because their bonds are not chemically stable enough, so they "migrate" out of the sample, leaving more and more carbon as the wood "dries," systematically losing chemical compounds and leaving behind carbon (which is more stable and solidifies). This means that, except for the formation of hydrocarbons (such as natural gas and oil, for example), over the years we find the remains of the tree (or animal) separated in such a way that the carbon content is increasingly higher, to the detriment of other compounds. This is also the reason why the various sources of charcoal and/or animal charcoal appear black. Some examples of typical charcoal are virtually any combustible wood, animal remains, etc. Some forms of coal can be considered, depending on the perspective, as organic or inorganic. Jet, for example, is an organic fossil belonging to the lignite family, and due to its hardness, it has been used for centuries in jewelry.

Mineral coal, on the other hand, is always richer in pure carbon (hence its %C is higher). This is because it has not been part of any life form, or it has been, but over a period of time long enough to lose hydrogen and other elements that barely form, as in the case of anthracite, 5-6% in the form of impurities. Needless to say, mineral coal itself is more expensive than vegetable coal, as it generates more heat energy when burned due to its higher carbon content.

Having explained the characteristics of the minerals that fall into the category of "coal" itself, let's move on to differentiating them from graphite.

Graphite is elemental carbon, meaning a geologist or anyone who understands minerals will give you the same composition: "C."

This means we are dealing with pure, native carbon. Native means we can obtain it naturally in its elemental state, just as it is. In turn, "elemental state" means that what you have in your hand is exactly what the Periodic Table describes, with all its characteristics. Let me explain: most of the elements that appear on it always appear combined in nature. In fact, of the 94 elements (assuming we count up to plutonium), only about six or seven appear in their pure form, naturally. The reason for this is mainly due to their chemical stability, which allows them, so to speak, to "resist" combination with more electronegative elements. Carbon is one of the few elements that can be found in pure form, both in graphite and diamond. But since we're talking about the former for now, let's focus on defining some of its basic and more complex properties.

In its pure state, it has a "metallic" black-gray color, similar to that of hematite (a common iron ore), but opaque and with low reflectivity (in this context, a "mirror effect"). Sometimes, the color varies from "steel black" or "metallic black" to softer, bluer hues. It is extremely soft (Mohs hardness 1-2) and can be cut with an ordinary knife. It is brittle and easily pulverized. Contrary to what one might think due to carbon's ability to form all kinds of compounds (there are several million of them), chemically, graphite is more "noble" than metals such as iron or copper, and only reacts with corrosive agents at high temperatures.

In its highly pure state (+99.99%) and at temperatures up to 100°C, it resists attack by sulfuric and hydrochloric acids, and is not properly combustible until above 500°C (some sources raise it as high as 700°C). However, at this temperature, and in the presence of oxygen, it begins to react, releasing heat and forming the corresponding dioxide (CO2), sublimating. Graphite does not have a melting point; that is, it never converts to liquid form under normal pressures. However, its boiling point, which in this case is actually sublimation, is the highest of all elements, with 3642°C being necessary, at normal pressures, to convert it from a solid to a gaseous state. This process is only conceivable in the absence of other corrosive elements, especially oxygen. Considering carbon's atomic number (Z=6), it is quite dense (approximately 2.24 g/cc), denser than common quartz and elemental silicon. One of the characteristics that separates graphite from anthracite itself, considering the similarities between the two, is that the ignition process in graphite is more difficult, and it is generally considered less "combustible" than the carbon-rich minerals that make up coal itself.

Other characteristics that make graphite stand out compared to impure forms of carbon are its high electrical and thermal conductivity.

Structurally, graphite is composed of several layers of graphene (hence the name given to this allotrope), stacked layer by layer in such a way that the bonds are easily separable and weak (e.g., graphite's low Mohs hardness).

Probably the most famous application of graphite is the manufacture of pencils. Few people know that graphite rods aren't actually graphite, but rather are composed of a mixture rich in pure graphite and binders (substances that "shrink" the particles), often ceramic, which give them greater structural rigidity. The word "graphite" itself was coined by the German Abraham Gottlob Werner in 1789. Until then, the word that defined the mineral was "plumbago." The reason for this is the physical similarity between galena (the mineral of lead, "plumbum") and graphite itself. To this day, the graphite in pencils is called "lead," a word that also refers to lead. Aside from this, they have no major similarities except for the fact that they both belong to the same group on the Periodic Table. Another "mundane" use of graphite that any jeweler will be very familiar with is its use as a crucible. A crucible is the mold used to melt typical metals in a jewelry workshop, although it can also be used for other types. The reason for this is precisely the high resistance elemental carbon has to high temperatures, its low cost, and ease of molding. "Graphite" crucibles are not made of pure graphite per se, but rather of a mixture that, as in the case of pencils, increases its rigidity. Furthermore, carbon in this form is quite inert, but even if it were to react, for example, in an "accident" such as if it began to react (oxidize), it would not alter the vast majority of metals melted in it, since carbon does not form carbides with typical jewelry and costume jewelry metals, such as copper, silver, gold, tin, zinc, etc., which further increases its value. Another common application of graphite is "dry lubrication." As its name suggests, graphite is added in powder form to a mechanism to prevent wear from the typical contact between metal parts, such as, for example, an old safe mechanism. In these cases, graphite is used in its pure form, taking advantage of its very low Mohs hardness (1-2), which prevents it from wearing down other materials. Since graphite is added as a powder and without any additives, unlike many other lubricants such as resins, its use is favored where the watery/gelatinous characteristics of typical lubricants are desired to be avoided.

In applications related to the world of electricity and electronics, and due to its extremely high electrical conductivity, it was used (and still is) for many years as a conductor. Few people know that the first light bulbs were manufactured with graphite filaments, before switching to rhenium and ultimately, tungsten. Due to these characteristics, it is also used in high-responsibility electrodes.

Finally, the application of graphite that is least known to most people takes us to nuclear power plants: it is used, or was used, in the form of long rods as a particle moderator; that is, more simply put, as a shield against radiation resulting from the reactions at the plant itself. The reason is that the carbon atom, being so small and so concentrated in graphite (although not as much as in diamond, incidentally), can "trap" loose high-speed neutrons that become part of its nucleus, increasing the C-12 isotope by one percentage point, which is the most abundant naturally occurring (more than 98% of all carbon is C-12). This form of graphite is called "nuclear graphite," and while it is not intrinsically dangerous, it must be handled with care. Like all "shielding" systems in nuclear power plants, it has a fixed useful life of several cycles before it becomes unusable. Nowadays, it is used less and less, as the preference is for the use of so-called "heavy water," which also has the advantage of being mobile, as opposed to static graphite bars.

I don't include graphite as a component of steel, since steel doesn't consume it directly. Instead, it obtains its carbon content directly from coke during smelting and its purification in the refining process.