Rumoiite

Chemical formula: AuSn<sub>2</sub>

Rumoiite is a rare mineral from the native elements group, a natural gold-tin alloy, found as microscopic grains.

## Characteristics Rumoiite is a natural alloy (intermetallic compound) of gold and tin. It occurs as extremely small, rounded or irregular grains, usually not exceeding several tens of micrometers in size. Due to its size, visual features are observable only under high magnification. In reflected light, its surface is metallic. ## Physical Properties As an opaque mineral with a metallic luster, its hardness and other physical properties have not been precisely determined due to the microscopic size of the grains. The density calculated based on the chemical formula and unit cell parameters is approximately 11.25 g/cm³. ## Colors and Varieties In reflected light under a microscope, rumoiite is grayish-white with a slight yellowish tint. No varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Rumoi River in Hokkaido, Japan. It was described and approved as a new mineral by the International Mineralogical Association (IMA) in 1998. The discovery was made by a group of Japanese scientists, including H. Matsueda, Y. Shimizu, and S. Yui. ## Uses Rumoiite has no industrial applications. Its significance is purely scientific and collectible, as a rare and unusual natural alloy of precious metals.

Properties

Luster
Metallic
Density
11.25
Transparency
Opaque
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of rumoiite is possible only through advanced laboratory techniques. It requires analysis in reflected light (ore microscopy) and confirmation of chemical composition by electron microprobe (EDS/WDS). Its high density allows for its isolation in heavy mineral concentrates. ## Distinguishing from Similar Minerals Under a microscope, it can be confused with other natural gold-tin alloys or platinum-group metals. Definitive distinction is possible only based on precise chemical composition analysis. It differs from native gold by its significantly lighter, grayish-white color. ## Crystal Forms Rumoiite forms exclusively anhedral (not showing its own faces), rounded or irregular grains of microscopic size.

Geological environment

## Genesis Rumoiite is a secondary mineral, found in alluvial deposits (river sediments). It forms as a result of weathering and erosion of primary ultramafic rocks (serpentinites), which contain mineralization of platinum-group metals, gold, and tin. Rumoiite grains, due to their high density, concentrate in sediments along with other heavy minerals. ## Mineral Associations In alluvial deposits, it coexists with native gold, native platinum, iridium, osmium, rutheniridosmine, as well as chromite, magnetite, and zircon. ## Localities The only confirmed locality of rumoiite in the world is the alluvial sediments of the Rumoi River in Rumoi Province, Hokkaido Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of specimens containing rumoiite is not assessed in the traditional way, as this mineral does not form macroscopically visible crystals. Its scientific and collectible value lies in the mere confirmation of the presence of this rare mineral in a sample. The most valuable are microscopic preparations (polished sections) with well-isolated and studied grains, accompanied by documented chemical analysis. ## Popular Localities The only source of specimens is the type locality – the alluvium of the Rumoi River in Japan. Material from this location is extremely difficult to obtain and is almost exclusively available in the collections of scientific institutions.

Care and storage

## Cleaning Specimens containing rumoiite (typically heavy mineral concentrates) do not require special cleaning. Any attempts at mechanical or chemical cleaning may destroy or remove the microscopic mineral grains. ## What to Avoid Avoid contact with chemicals, especially strong acids, which can react with gold and tin. Do not heat specimens or subject them to ultrasound. ## Storage Rumoiite samples, most often in the form of polished petrographic sections or embedded in resin, should be stored in a dry place at a stable temperature. It is best to keep them in specialized microscope slide boxes to protect them from dust and mechanical damage.

Sources

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