Kobeite-(Y)

Chemical formula: (Y,U⁶⁺,U⁴⁺)(Ti⁴⁺,Nb⁵⁺)₂(O,OH)₆

Kobeite-(Y) is a rare, black oxide mineral containing niobium, titanium, and yttrium, found in granitic pegmatites.

## Characteristics Kobeite-(Y) is a complex oxide belonging to the euxenite group. It forms anhedral grains or poorly developed, prismatic crystals, usually small in size, embedded in the rock. Its appearance is inconspicuous, and its color is uniformly black. Due to its uranium content, this mineral is radioactive and undergoes metamictization over time, meaning the destruction of its internal crystalline structure under the influence of its own radiation. The surface of metamict specimens can be dull or covered with weathering products. ## Physical Properties Kobeite-(Y) has a hardness ranging from 5.5-6.5 on the Mohs scale, but it may be lower in metamict specimens. The luster is most often pitchy or vitreous, and the fracture is conchoidal to uneven. It is an opaque and relatively dense mineral, with a density of approximately 4.95 g/cm³. ## Colors and Varieties This mineral occurs in a uniform, black color. No colored or commercial varieties are distinguished. ## History and Name The name "kobeite" was given in 1950 by Jitsutaro Takubo and his colleagues in honor of the discovery locality – the Kobe district in Kyoto Prefecture, Japan. The suffix "-(Y)" was added later in accordance with rare-earth mineral nomenclature rules, indicating the dominance of yttrium (Y) in its chemical composition. The originally described material turned out to be a mixture of phases, and the mineral was formally redefined and approved in its current form in 1987. ## Uses Kobeite-(Y) has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals, rare-earth minerals, and radioactive minerals.

Properties

Mohs hardness
5.5
Color
Black to dark brown
Luster
Resinous, vitreous
Streak
Reddish brown
Density
4.60
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent,Translucent
Crystal system
Isometric

Diagnostic features

## Identification Identifying kobeite-(Y) under amateur conditions is practically impossible and requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical composition analysis (e.g., EDS/WDS). Preliminary identification is based on its black color, pitchy luster, conchoidal fracture, high density, and occurrence in granitic pegmatites. A key diagnostic feature is its radioactivity, which can be detected with a Geiger counter. ## Distinguishing from Similar Minerals Kobeite-(Y) is visually almost identical to other black, metamict minerals from the euxenite group, such as euxenite-(Y), samarskite-(Y), or polycrase-(Y). It can also be confused with columbite, tantalite, or uraninite. Certain differentiation from these minerals is only possible based on detailed laboratory analyses of chemical composition, especially the content of yttrium, niobium, and titanium. ## Crystal Forms Kobeite-(Y) most often forms irregular, massive aggregates or anhedral (shapeless) grains embedded in the host rock. Poorly developed, prismatic crystals with a nearly square cross-section are rarely observed.

Geological environment

## Genesis Kobeite-(Y) is an accessory mineral in granitic pegmatites, especially those enriched in rare-earth elements (REE), niobium, titanium, and uranium. It forms in the late stages of pegmatitic magma crystallization under conditions of high concentration of incompatible elements. ## Mineral Associations This mineral co-occurs with other minerals typical of granitic pegmatites, such as quartz, feldspars (especially microcline and albite), biotite, as well as other rare-earth and niobium minerals, e.g., fergusonite-(Y), allanite, zircon, monazite, and xenotime. ## Localities The most important and well-documented kobeite-(Y) localities worldwide include: - Japan - Okuma mine, Kobe district, Kyoto Prefecture (type locality). - Norway - pegmatites in the Setesdalen region (e.g., Iveland, Evje og Hornnes). - Sweden - pegmatite in Ytterby, near Stockholm. - USA - several localities in pegmatites in Colorado.

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of kobeite-(Y) is very specific and targeted at specialized collectors. Specimens with a well-defined locality, especially from the type locality in Japan, are most valued. Rarely found, partially developed crystals are much more desirable than shapeless masses. Specimen size also matters, although even small grains with confirmed identity are valuable. Purity is not a key criterion; association with other rare pegmatite minerals is more important. ## Popular Localities Specimens from Japan (type locality) and from classic pegmatite localities in Norway (Iveland, Setesdalen) are most sought after by collectors due to their historical significance and the quality of the research material.

Care and storage

## Cleaning Kobeite-(Y) specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Due to its brittleness and possible metamictization, avoid washing in water and using any chemical agents, including ultrasonics. ## What to Avoid The mineral is sensitive to chemicals and sudden temperature changes. As a radioactive mineral, it should be stored with appropriate precautions. Avoid direct, prolonged skin contact and inhalation of dust that may arise during its handling or cleaning. ## Storage It is recommended to store specimens in separate, padded boxes or containers, away from other minerals that could be damaged by radiation. Display should be in a closed display case, in a location not exposed to shocks, moisture, or direct sunlight. Due to its radioactivity, it should be stored away from areas of permanent human habitation.

External references

Sources

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