Changchengite

Chemical formula: Ir³⁺(BiS)³⁻

Changchengit is a very rare, metallic mineral of the cobaltite group, being an iridium bismuth sulfide, discovered in China.

## Characteristics Changchengit is an extremely rare mineral of the cobaltite group, composed of iridium, bismuth, and sulfur. It occurs as very small, rounded grains, usually not exceeding 0.2 mm in size. Its color is steel-gray, and on a fresh surface, it exhibits a strong, metallic luster. Due to its extreme rarity and small crystal sizes, it is a mineral known primarily in scientific circles and among specialized collectors. ## Physical Properties This mineral has a hardness of 3.5 on the Mohs scale. It is opaque and has a black streak. Its luster is distinctly metallic. Its density has not been precisely measured due to the scarce amount of research material. ## History and Name Changchengit was first described in 1997 by the Chinese mineralogist Yu Zuxiang. The mineral's name comes from the Great Wall of China (Cháng Chéng), as its discovery site is located near one of its sections in Hebei Province, China. ## Uses Due to its extreme rarity and microscopic size, changchengit has no industrial applications. Its significance is purely scientific and collectible.

Properties

Mohs hardness
3.5
Luster
Metallic
Streak
Black
Density
0
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Identification of changchengit is impossible without advanced laboratory techniques, such as electron microprobe analysis (EDS/WDS) to confirm its chemical composition (presence of iridium, bismuth, and sulfur). Visually, in the host rock, it appears as fine, steel-gray, metallic inclusions. ## Distinguishing from Similar Minerals It can be confused with other fine, metallic sulfides or platinum-group minerals, such as pyrite, galena, or other minerals from the cobaltite group. Definitive differentiation requires specialized chemical analysis. ## Crystal Forms Changchengit occurs as very small, rounded or irregular grains, often forming inclusions in other minerals, such as chromite. Well-formed crystals have not been observed.

Geological environment

## Genesis Changchengit forms in chromite and platinum-group element (PGE) deposits associated with ultramafic rocks. It forms under hydrothermal or magmatic conditions, crystallizing as one of the later ore minerals. ## Mineral Associations This mineral co-occurs with chromite, as well as with other platinum-group minerals such as irarsite, laurite, osarsite, ruarsite, and also with pyrite, chalcopyrite, bornite, and gold-silver alloys. ## Locations The only confirmed occurrence (type locality) of changchengit is the Gaositai chromite and PGE deposit in Chengde County, Hebei Province, China. It has been found there both in the host rock and in alluvium (river sediments) of the Wulie River.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a changchengit specimen depends solely on the confirmation of its presence through analysis and on the abundance and size of the microscopic grains in the host rock. Since the mineral does not form crystals visible to the naked eye, the aesthetics of the specimen are secondary and mainly related to the associated rock. ## Popular Localities The only source of specimens is the type locality in Hebei Province, China. Material from this location is extremely difficult to acquire and rarely appears on the collector's market.

Care and storage

## Cleaning Due to the microscopic size of the grains embedded in the host rock, mechanical cleaning is neither recommended nor possible. Any attempts at cleaning may damage or remove the tiny mineral grains. The specimen should be protected from dust. ## What to Avoid Avoid contact with chemicals, acids, and strong detergents. Do not subject the specimen to ultrasound or sudden temperature changes. As a sulfide, it may be susceptible to moisture and oxidation over the long term. ## Storage Specimens containing changchengit should be stored in stable conditions, in a closed display box or cabinet, away from dust, moisture, and direct sunlight. It is recommended to store them with a label indicating the location of the microscopic grains.

External references

Sources

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