Graphite: A dark appearance hides a soft heart

The English name of graphite, "Graphite", was named by Abraham Gottlob Werner in 1789 from the Greek word "graph", which originally meant "write". It can be seen that graphite has been used by people at a very early time.

In my country, when it comes to "ink", most people may think of calligraphy, ink painting or the free and unrestrained landscape in poetry and songs, but these are already artificial inks, not the protagonist we are going to introduce today. What we are going to share with you in this issue is the natural mineral element - graphite.

Flake graphite aggregate (stored in the Earth Science Museum of Kunming University of Science and Technology)

According to the data recorded by archaeologists, people used graphite to carry out simple religious activities as early as the primitive tribal period, such as divination and making simple marks. Graphite is a convenient writing material with clear handwriting. It can be used for calligraphy and painting without processing. In ancient times, it can even be used as a cosmetic. In the poem "On the Boat in Shixing" written by Tang Xianzu in the Ming Dynasty, it is written: "Graphite eyebrows are painted in spring, and someone sends sorrow back to the mountains and rivers." Such poems make us imagine the ancient women carefully drawing eyebrows with graphite, and the beautiful silhouette of their spring-like faces spread out in the dressing mirror. These historical traces are enough to prove that graphite plays an important role in the development of human civilization.

Graphite, like diamond, is a naturally occurring single carbon element. However, due to the different arrangements of their internal atoms, graphite and diamond are vastly different in both appearance and internal physical and chemical properties. The two are homogeneous minerals with different appearances.

Graphite produced in nature is rarely pure and is often mixed with other components (up to 10%-20%), such as clay, asphalt, Si, Al and Fe oxides, etc.

Graphite has a typical layered structure. Each layer is composed of carbon atoms arranged in a hexagonal ring network. The carbon atoms in the upper layer network face the center of the hexagonal ring of the lower layer network. There are covalent bonds-metal bonds between atoms in the layer, and molecular bonds between layers. This structure determines the obvious anisotropy of its physical properties.

The color of graphite is steel gray to iron black, with black streaks; semi-metallic luster; the luster of cryptocrystalline earthy aggregates is dull. Thin sheets are flexible; soft, slippery, and easy to stain hands. A group of very complete cleavage, Mohs hardness is very low, 1-2, and can be easily scratched by a fingernail (2.5). Relative density is 2.09-2.23, and it has good conductivity.

Blocky graphite aggregate (stored in the Earth Science Museum of Kunming University of Science and Technology)

Graphite in nature is mainly formed under high temperature environment, and is often produced in rock systems that have been strongly regionally metamorphic or contact metamorphic. It is formed by metamorphism of deposited organic carbon, coal series, etc. Graphite can also be produced in igneous rocks with different compositions, and the source of carbon often comes from carbon-containing surrounding rocks. It is also found in pegmatite veins.

Graphite is widely used in industry. It is used to make graphite crucibles in the metallurgical industry, lubricants in the mechanical industry, moderators in the atomic industry, pencil leads, coatings, dyes, etc. High-carbon graphite with pure ingredients can be used as neutron moderators in atomic reactors, and radiation-proof shells are important materials for national defense and nuclear energy industries. In particular, since the advent of the new material graphene in 2004, graphite has once again become popular among the public, and the market structure of the graphite resource industry has undergone earth-shaking changes.

Graphite is distributed all over the world, even in Antarctica. my country is the main distribution area of ​​graphite deposits in the world. Graphite resources are mainly found in the eastern circum-Pacific tectonic belt and the Qilian-Qinling-Huaiyang metallogenic belt, with reserves accounting for 77.5% of the world's total, ranking first in the world. my country's coal-based graphite resources have huge potential. The coal-based graphite that has been identified supports more than half of my country's annual graphite production. It is mainly distributed in the east and central parts of my country, with the basic rules of more in the east and less in the west, more in the south and less in the north.

Detail of natural graphite aggregate

Note: There are different understandings about the "graphite" (also known as thrush stone and black ink) used by women in ancient times as described in this article. The main ones are: one is that these ancient names correspond to the elemental mineral graphite described in this article; the second is that it is coal; the third is that people in ancient times confused graphite and coal, so both have the meanings of "thrush stone", "black ink" and "graphite".

The graphite specimens displayed in this issue are one of the world's rock and mineral specimens. They are one of the two sets of geological teaching specimens customized by Tsinghua University using the "Boxer Indemnity" in the early years of the Republic of China. They cover more than 1,700 rock and mineral specimens from five continents in the world. Many specimens were collected from the original naming places of rocks and minerals, classic mining areas and stratigraphic sections. Some mineral deposits no longer exist after long-term mining, so many collected specimens cannot be recovered. This set of specimens moved south with Tsinghua during the Anti-Japanese War, and after several twists and turns, it was preserved in Kunming University of Science and Technology through the National Southwest Associated University. It has been preserved almost intact to this day, witnessing the formation and development of China's modern geological disciplines and geoscience education, and has important scientific research, historical and cultural relic value.

Written by: Zhu Jun, Gao Danxia, ​​Shen Cen, Wang Lei

Scientific Advisor: Zhang Shitao

Photographed by: Yang Yuchen

Supporting institution: College of Land and Resources Engineering, Kunming University of Science and Technology