Minakawaite

Chemical formula: RhSb

Minakawaite is an extremely rare rhodium antimonide, occurring as tiny, metallic grains with a pinkish-gray color.

## Characteristics Minakawaite is an extremely rare mineral from the antimonide group, chemically classified as rhodium antimonide (RhSb). It occurs as very small, anhedral (not exhibiting its own crystal form) grains, usually not exceeding several tens of micrometers in size. Its typical appearance is metallic, opaque inclusions within other minerals, most often in serpentinite. ## Physical Properties This mineral is characterized by a strong metallic luster. It is opaque. Its density is very high, at 10.04 g/cm³, which is characteristic of minerals containing platinum-group metals. Hardness has not been precisely measured due to the small size of the grains. ## Colors and Varieties In reflected light, minakawaite has a characteristic pinkish-gray color. No varieties are known. ## History and Name Minakawaite was first described in 2007 by A. K. Guda, M. Ohnishi, T. K. Kim, K. Kajiwara, and M. Shimizaki. The mineral is named in honor of Tetsuo Minakawa (born 1951), a professor of mineralogy at Ehime University in Japan, for his contributions to the study of ore deposit mineralogy in Japan. It was approved by the International Mineralogical Association (IMA) under number IMA2006-042. The type locality where this mineral was first identified is Haraigawa in Kumamoto Prefecture, Japan.

Properties

Luster
Metallic
Density
10.04
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of minakawaite is impossible without advanced laboratory techniques. It requires analysis in reflected light (ore microscopy), where it reveals its characteristic pinkish-gray color, and chemical analysis (e.g., using an electron microprobe) to confirm its composition – the presence of rhodium and antimony. High density is also a diagnostic feature. ## Distinguishing from Similar Minerals It can be confused with other microscopic platinum-group minerals or antimonides, such as cherepanovite (RhAs) or sudburyite (PdSb). Final differentiation is only possible based on precise chemical composition analysis. ## Crystal Forms Minakawaite crystallizes in the orthorhombic system. In nature, it is found exclusively as anhedral, rounded grains of microscopic size, without visible crystal faces.

Geological environment

## Genesis Minakawaite forms under specific geological conditions associated with ultramafic rocks. At its type locality (Haraigawa, Japan), it was found within serpentinite, which is a product of the alteration of olivine and pyroxene-rich rocks. Its presence is linked to hydrothermal or magmatic processes concentrating platinum-group metals. ## Mineral Associations This mineral co-occurs with other rare minerals, such as irarsite, laurite, cherepanovite, as well as chromite, pentlandite, and heazlewoodite. It occurs as inclusions in serpentinite rock. ## Localities The only confirmed and described locality for minakawaite in the world is its type locality: Haraigawa, Misato, in Kumamoto Prefecture on Kyushu Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, minakawaite is not evaluated based on typical collector criteria such as size or crystal form. Its value is purely scientific and systematic. The most valuable specimens are those with analytically verified presence of minakawaite, originating from the type locality, preferably with rich associations of other rare minerals. The value of the specimen lies in the host rock, not in the minakawaite grain itself, which is invisible to the naked eye.

Care and storage

## Cleaning Due to its extreme rarity and microscopic size, minakawaite specimens (usually as inclusions in host rock) practically do not require cleaning. If necessary, compressed air can be used to remove dust. All mechanical and chemical methods should be avoided. ## What to Avoid Avoid contact with chemicals, acids, and cleaning agents. Do not subject the specimen to ultrasound or sudden temperature changes. As a metallic mineral, it may be susceptible to oxidation in a humid environment, although its stability has not been fully investigated. ## Storage Specimens containing minakawaite should be stored under stable conditions, in a dry place, away from direct sunlight and contaminants. The safest way to store them is in a sealed display box or cabinet.

External references

Sources

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