Furutobeite

Chemical formula: (Cu₁+,Ag₁+)₆Pb₂+S₂-₄

Furutobeite is a rare copper, silver, and lead sulfide, occurring as massive, metallic aggregates in ore veins.

## Characteristics Furutobeite is a rare mineral from the sulfosalt group, chemically a complex sulfide of copper, silver, and lead. It typically forms compact, massive, or granular aggregates, strongly intergrown with other ore minerals. It is rarely observed in the form of well-developed crystals, and these, if present, are microscopic in size. It has a characteristic metallic luster and opaque structure. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3. It has a high density, approximately 6.95 g/cm³, resulting from the presence of heavy metals like lead and silver. It is brittle and has an uneven fracture. It does not exhibit cleavage. ## Colors and Varieties Furutobeite ranges in color from steel-gray to grayish-black, often with a metallic luster. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Furutobe mine in Akita Prefecture, Japan. It was first described in 1981 by Japanese mineralogists A. Sugaki, A. Kitakaze, and K. Hayashi. ## Uses Due to its extreme rarity, furutobeite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness: 2.5-3
Luster: Metallic
Streak: Black
Density: 6.95
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Amateur identification of furutobeite is very difficult. Helpful characteristics include: steel-gray color, high density, black streak, and metallic luster. However, its co-occurrence with other sulfides and sulfosalts of silver, copper, and lead is key. Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals Furutobeite is visually almost impossible to distinguish from many other gray, metallic sulfosalts, such as stromeyerite, jalpaite, or mckinstryite, with which it often co-occurs. It differs from galena by the absence of distinct cleavage. It differs from chalcocite and acanthite in chemical composition, which requires laboratory confirmation. ## Crystal Forms It most commonly occurs as anhedral (undeveloped) grains and massive aggregates. Well-formed crystals are extremely rare and microscopic in size.

Geological environment

## Genesis Furutobeite forms in epithermal conditions, in low-temperature, polymetallic ore veins. It originates from hydrothermal processes, crystallizing from solutions rich in copper, silver, lead, and sulfur. ## Mineral Associations This mineral almost always occurs in association with other ore minerals. Its most common associations include: stromeyerite, mckinstryite, jalpaite, galena, sphalerite, pyrite, chalcopyrite, bornite, tetrahedrite, and with gangue minerals such as quartz, calcite, and barite. ## Localities The most important and classic localities for this mineral are in Japan, in Akita Prefecture (Furutobe, Ani, Osarizawa mines). It is also known from the Iriki mine in Kagoshima Prefecture. Outside Japan, its occurrence has been confirmed in volcanic exhalations in the La Fossa crater on Vulcano Island, Italy.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a furutobeite specimen is primarily assessed based on its abundance in the sample and confirmed identity. Since it does not form aesthetic crystals, its value lies in its rarity and scientific significance. Specimens from a well-documented locality, especially from the type locality (Furutobe mine), in association with other rare minerals, are most desired by systematic collectors. ## Popular Localities By far the most prized specimens come from Japanese mines in Akita Prefecture, which are classic localities for this mineral.

Care and storage

## Cleaning Furutobeite specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially tap water, can lead to slow oxidation. If liquid is necessary, isopropyl alcohol should be used, and the specimen immediately dried. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. As a sulfide, it is sensitive to humid air, which can cause its slow decomposition and surface tarnishing. It should not be heated or exposed to prolonged strong light. ## Storage It is recommended to store specimens in a dry place, preferably in closed, airtight containers or mineralogical display cases away from moisture. Using desiccants (e.g., silica gel) near the specimen is good practice.

Sources

IMA Mineral List (CNMNC)