Chalcopyrite: Unveil your disguise as gold, you are the hidden boss of the electrification era

This issue introduces a chalcopyrite specimen from Scotland. The originally golden chalcopyrite has become a slightly dull brass yellow due to slight oxidation. The specimen is collected in the Earth Science Museum of Kunming University of Science and Technology and is a world-class rock and mineral specimen.

Chalcopyrite is a common copper-iron sulfide mineral, which is often mixed with trace amounts of gold, silver and other elements. The color of chalcopyrite is brass yellow, and blue, purple and brown mottled tint can often be seen on the surface. The appearance of this mottled color is mainly due to the formation of an oxide film on the surface of chalcopyrite due to oxidation. The film is subjected to light interference and presents a variety of colors to human vision. It is the same as the coloring principle of colorful soap bubbles, which is caused by physical optical effects. Therefore, tint is a false color of the mineral, while the brass yellow of chalcopyrite is the natural color of the mineral.

Pictured is a specimen of chalcopyrite from Scotland

Chalcopyrite has polymorphic variants, but the most common one belongs to the tetragonal crystal system. The main single forms are tetragonal tetrahedron, tetragonal bipyramid, and tetragonal scalenohedron. However, in reality, it is difficult to see chalcopyrite with complete crystal shapes. It is mostly in the form of irregular granular or dense block aggregates, as well as kidney-shaped and grape-shaped aggregates.

Chalcopyrite has green-black streaks and metallic luster. It is an opaque mineral with a Mohs hardness of 3-4 and a relative density of 4.1-4.3. It is brittle and conductive.

Chalcopyrite has similar appearance characteristics to pyrite and native gold, and always gives people a golden feeling. Pyrite and chalcopyrite are also called "fool's gold". Compared with pyrite, chalcopyrite has a stronger yellow tone, often has a porphyritic tin color, has a lower hardness (it can be scratched with a knife), and rarely has intact crystals; while pyrite has a slightly lighter yellow tone, usually has no tin color, has a higher hardness (it cannot be scratched with a knife), and often has intact crystals of cubes and pentagonal dodecahedrons. In addition, it is common for chalcopyrite and pyrite to coexist in ore deposits. Chalcopyrite is easily distinguished from native gold by its green-black streaks, brittleness, and solubility in nitric acid.

Chalcopyrite is the most important and widely distributed copper mineral. It can be formed in a variety of different environments, but it is mainly produced in copper-nickel sulfide deposits, skarn deposits and medium-temperature hydrothermal deposits. Chalcopyrite is easily oxidized on the surface and transformed into malachite and azurite; it can be altered into bornite, chalcocite and covellite in secondary enrichment zones.

Detail of a chalcopyrite specimen from Scotland

Compared with other industrial copper minerals (such as chalcocite, malachite, azurite, etc.), the copper content in chalcopyrite is not high, but it is the main mineral raw material for industrial copper smelting and the absolute protagonist of industrial copper minerals. Copper is a non-ferrous metal that has a very close relationship with humans. Humans went through a long Bronze Age from the Stone Age to the Iron Age. It is copper that has supported the development of human civilization for thousands of years. Today, the consumption of copper and aluminum accounts for a large proportion of non-ferrous metals in my country. Refined copper is widely used in electrical, light industry, machinery manufacturing, construction industry, transportation, national defense industry, emerging industries and other fields. Among them, the electrical and electronic industries use the most copper. For example, wires, transformers, switches, plug-in components, connectors, etc. for power transmission, motor manufacturing, communication cables, etc. are all made of copper. With the continuous development of electrification, non-ferrous metal copper will become more and more important.

Chalcopyrite is produced all over the world. The main production areas in China are concentrated in the middle and lower reaches of the Yangtze River, Sichuan and Yunnan, Zhongtiao Mountain in southern Shanxi, Hexi Corridor in Gansu, and the Tibetan Plateau. Among them, the copper deposits of Dexing in Jiangxi and Yulong in Tibet are the most famous. Other famous production areas in the world include Rio Tinto in Spain, Kalamazoo in Arizona, Bingham in Utah, Butte in Montana, Cananea in Mexico, Chuquicamata in Chile, etc.

Written by: Zhu Jun, Shen Cen, Wang Lei

Scientific Advisor: Zhang Shitao

Photographed by: Yang Yuchen

Poster: Jung Pil-yoon

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