Chenguodaite

Chemical formula: Ag<sub>9</sub>FeTe<sub>2</sub>S<sub>4</sub>

A new, extremely rare silver-iron sulfide-telluride, identified in a Chinese copper-gold deposit.

## Characteristics Chenguodait is a newly described ore mineral with a metallic luster and a grayish-black color. It occurs as very fine, anhedral grains, typically ranging from 10 to 100 micrometers in size. These grains often form aggregates with other similar minerals. ## Physical Properties As a microscopic mineral, many of its physical properties, such as hardness or cleavage, have not been determined. It has a black streak and is opaque. Its density, calculated based on chemical composition and crystal structure, is 7.53 g/cm³. ## Colors and Varieties The mineral is homogeneous in color – it occurs exclusively in shades of grayish-black. No varieties have been distinguished. ## History and Name Chenguodait was officially approved by the International Mineralogical Association (IMA) in 2022. Its name honors Professor Chen Guoda (1912–2003), a distinguished Chinese geologist and member of the Chinese Academy of Sciences, known for his contributions to the theory of diagenetic transformation of ores. The discovery locality is the Chengmenshan copper-gold deposit in Jiujiang Prefecture, Jiangxi Province, China. ## Applications Due to its extreme rarity and microscopic size, chenguodait has no industrial applications. It is solely an object of scientific interest and advanced systematic collecting.

Properties

Luster
Metallic
Streak
Black
Density
7.53
Cleavage
None
Transparency
Opaque
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of chenguodait under amateur conditions is impossible. Due to its microscopic size and occurrence in intergrowths with other ore minerals, its recognition requires advanced laboratory techniques, such as reflected light ore microscopy and chemical composition analysis using an electron microprobe (EDS/WDS). ## Distinguishing from Similar Minerals It can be easily confused with many other gray, metallic minerals with which it coexists, such as hessite, petzite, altaite, or chalcopyrite. Certain differentiation is possible only on the basis of chemical analysis, which reveals a unique combination of silver, iron, tellurium, and sulfur. ## Crystal Forms The mineral does not form well-developed crystals. It occurs exclusively as anhedral grains, which can form irregular aggregates and intergrowths with other minerals.

Geological environment

## Genesis Chenguodait is a hydrothermal mineral. It forms at low temperatures, crystallizing in the final stage of the formation of ore veins rich in tellurium, gold, and silver. ## Mineral Associations It most often occurs in close association with other tellurides and sulfides. Coexisting minerals include: hessite, petzite, sylvanite, calaverite, krennerite, altaite, as well as more common pyrite and chalcopyrite. ## Localities The only known worldwide locality for chenguodait is its type locality – the Chengmenshan Cu-Au deposit, located in Jiujiang Prefecture, Jiangxi Province, China.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, chenguodait is not evaluated according to typical aesthetic criteria such as color or luster. The collectible value of a specimen depends on the richness of chenguodait grain aggregates on the rock matrix and on the presence of well-defined associated minerals, especially rare gold and silver tellurides. ## Market Prices The mineral is not traded on the commercial collector's market. Specimens are available almost exclusively through exchange between scientific institutions and specialized micromineral collectors. ## Popular Localities All known specimens originate from a single locality in the world – the Chengmenshan deposit in China.

Care and storage

## Cleaning Specimens containing chenguodait should not be cleaned with water or any chemical agents. Due to the microscopic nature of the mineral, any attempt at cleaning may lead to its damage or loss. Only very careful dust removal with a soft brush under a microscope is permissible. ## What to Avoid Avoid contact with acids, moisture, and sudden temperature changes. As a sulfide-telluride, it is potentially susceptible to oxidation in a humid environment, which can lead to changes in color and luster. ## Storage Specimens should be stored in a dry and stable environment. The best solution is to place them in sealed micromineral boxes (so-called perky boxes), which protect against dust, moisture, and mechanical damage.

Sources

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