Makotoite

Chemical formula: Ag₁₂(Cu₃Au)S₈

Makotoite is a very rare silver, copper, and gold sulfide, discovered in 2023 in the Funan gold deposit in China.

## Characteristics Makotoite is a newly described, very rare sulfide mineral. It was approved by the International Mineralogical Association (IMA) in 2023. It occurs as irregular grains up to 200 micrometers in size, most often intergrown with other minerals such as chalcopyrite, galena, pyrite, and native gold. It is opaque and has a metallic luster. ## Physical Properties Due to the small size of the grains and its rarity, many physical properties of makotoite, such as hardness, density, cleavage, or fracture, have not yet been precisely determined. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 7.73 g/cm³. ## Colors and Varieties This mineral has a gray color with a slight yellowish tint. In reflected light under a polarizing microscope, its color is described as creamy yellow. No varieties have been distinguished. ## History and Name The mineral's name honors Makoto Watanabe, a Japanese geologist and ore deposit researcher, in recognition of his contributions to economic geology. It was discovered in material from the Funan (Bunan) gold deposit, near Zhaoyuan city in Shandong Province, China, which is its type locality. It was described by a team of mineralogists led by Shoichi Kobayashi. ## Uses Due to its extreme rarity and microscopic size, makotoite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized micromineral collectors.

Properties

Luster
Metallic
Streak
Black
Density
7.73
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identification of makotoite is impossible without advanced laboratory techniques. It requires chemical composition analysis using electron microprobe (EDS/WDS) and crystal structure studies using X-ray diffraction (XRD). In reflected light under a microscope, it is characterized by a creamy yellow color and weak anisotropy. ## Distinction from Similar Minerals Makotoite occurs in paragenesis with other metallic minerals, such as native gold, pyrite, or chalcopyrite. Distinguishing it from these minerals with the naked eye is impossible. Native gold is much more yellow and malleable. Pyrite has a brighter, brassy yellow color. Chalcopyrite has a more intense yellow color, often with an iridescent tarnish. ## Crystal Forms Makotoite forms exclusively anhedral (irregular) grains ranging from 10 to 200 micrometers in size. No well-formed crystals have been observed.

Geological environment

## Genesis Makotoite forms under hydrothermal conditions. In its type locality (Funan gold deposit), it is associated with quartz-sulfide veins cutting Precambrian gneisses. It is believed to have crystallized in the late stage of deposit formation, at temperatures below 300°C, from solutions rich in silver, copper, gold, and sulfur. ## Mineral Associations This mineral co-occurs with many other sulfides and ore minerals. It is most commonly accompanied by: native gold, chalcopyrite, galena, pyrite, sphalerite, acanthite, tetrahedrite, and quartz. ## Localities The only confirmed occurrence of makotoite in the world is its type locality: the Funan (Bunan) gold deposit, located in Zhaoyuan city, Yantai prefecture, Shandong province in eastern China.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromineral, the collector's value of a makotoite specimen primarily depends on the confirmation of its identity by a reliable laboratory. The most valuable specimens are those where makotoite grains are relatively large (above 100 micrometers) and clearly visible against the rock matrix. An additional advantage is a rich association with other rare or well-formed minerals, especially native gold. ## Popular Localities The only source of makotoite specimens is its type locality in China (Funan gold deposit). Material from this location is extremely difficult to acquire on the collector's market.

Care and storage

## Cleaning Specimens containing makotoite, which are typically microminerals in a rock matrix, should not be cleaned mechanically or chemically. Any attempts at cleaning may damage or remove the microscopic mineral grains. If necessary, compressed air can be used to remove dust. ## What to Avoid Avoid contact with any chemicals, acids, and detergents. Do not subject the specimen to ultrasonic cleaning. Due to the presence of sulfur and silver, the mineral may be sensitive to moisture and atmospheric pollutants, which can cause its slow oxidation and color change. ## Storage Specimens containing makotoite are best stored in stable conditions, in closed, dry "micromount" containers, to protect them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight and sudden temperature changes.

External references

Sources

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