Tsumoite

Chemical formula: BiTe

Tsumoite is a rare bismuth telluride, characterized by a metallic luster, platy habit, and high density.

## Characteristics Tsumoite is a bismuth telluride with a metallic appearance. It typically forms small, thin plates, flakes, or granular, lamellar aggregates. It rarely occurs as hexagonal crystals. Its color is tin-white to steel-gray, often with a slight yellowish or brownish tarnish resulting from oxidation. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3. It exhibits a strong, metallic luster and is opaque. It is a very dense mineral, with a specific gravity exceeding 8 g/cm³. It has perfect, unidirectional cleavage, which causes it to easily split into thin, flexible, but inelastic plates. ## Colors and Varieties The main color of tsumoite is tin-white to steel-gray. A dull, yellowish tarnish may appear on surfaces exposed to air. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery location and first description – the Tsumo Mine in Shimane Prefecture, Japan. It was described as a new mineral species in 1962 by T. Shimazaki and T. Ozawa. ## Uses Due to its rarity, tsumoite has no industrial applications. It is solely an object of interest for collectors and scientists.

Properties

Mohs hardness
2.5-3
Luster
Metallic
Streak
Greyish black
Density
8.16
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Key features include metallic luster, tin-white color, very high density (the specimen is surprisingly heavy for its size), and perfect cleavage, causing it to break into thin plates. It is soft and can be scratched by a copper coin. ## Distinguishing from Similar Minerals Tsumoite is visually almost indistinguishable from other bismuth tellurides, such as tellurobismuthite (Bi₂Te₃) or tetradymite (Bi₂Te₂S). They often coexist. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS) or X-ray diffraction (XRD). It is distinguished from molybdenite by its higher density and slightly different color. ## Crystal Forms It most commonly occurs as platy, flaky, or lamellar aggregates. Well-formed, hexagonal tabular crystals are extremely rare and reach small sizes, usually less than a millimeter.

Geological environment

## Genesis Tsumoite forms under hydrothermal conditions, in ore veins of low to medium formation temperature. It is associated with igneous rock intrusions. ## Mineral Associations It often coexists with other tellurides and sulfides. Its typical associated minerals include: tellurobismuthite, tetradymite, native bismuth, bismuthinite, native gold, pyrite, chalcopyrite, and quartz. ## Locations The most important localities worldwide include the Tsumo Mine in Japan (type locality), the Dashkesan deposit in Azerbaijan, Baita Bihorului in Romania, the Zod deposit in Armenia, as well as some localities in Canada (Glacier Gulch, British Columbia) and the USA (Little Mildred Mine, New Mexico).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens are those with visible, even small, hexagonal crystals. Rich platy aggregates on a contrasting background (e.g., on quartz) and associations with other rare minerals, such as gold or other tellurides, are also highly prized. Surface cleanliness and absence of tarnish increase the specimen's value. ## Popular Localities Classic and most sought-after specimens come from the type locality in Japan. Material from Romania and Azerbaijan has also yielded good quality collector specimens.

Care and storage

## Cleaning It should only be cleaned dry, using a soft brush to remove dust. As a last resort, distilled water can be used, but the specimen must be thoroughly dried immediately. Ultrasonic cleaners should not be used. ## What to Avoid The mineral is sensitive to chemicals, especially acids. Contact with moisture, which can accelerate oxidation, should be avoided. As a soft mineral with perfect cleavage, it is very susceptible to mechanical damage and scratches. ## Storage Tsumoite specimens are best stored in separate, padded boxes to protect them from impacts and abrasions. A dry environment is recommended to minimize the risk of tarnish formation.

Sources

Read more