Iwashiroite-(Y)

Chemical formula: YTa⁵⁺O₄

Iwashiroite-(Y) is a rare, black or blackish-brown oxide mineral of the fergusonite group, containing yttrium and tantalum.

## Characteristics Iwashiroite-(Y) is a rare oxide mineral belonging to the fergusonite group. It forms very small, subhedral (partially developed faces) grains, typically not exceeding 100 micrometers in size. It occurs as inclusions within other minerals, mainly biotite. Its appearance is inconspicuous, and identification requires advanced analytical methods. ## Physical Properties This mineral is characterized by a adamantine to resinous luster. It is opaque. Due to the microscopic size of its crystals, many of its physical properties, such as hardness or density, have not been precisely measured, but only calculated based on its chemical composition and crystal structure. The calculated density is 7.36 g/cm³. ## Colors and Varieties Iwashiroite-(Y) is black to blackish-brown. No color varieties or commercial varieties are known. ## History and Name The mineral was discovered in the Suishoyama pegmatite, located in the Iizaka district (formerly part of Iwashiro Province), Fukushima Prefecture, Japan. It was described and approved by the International Mineralogical Association (IMA) in 2003. Its name comes from the former name of Iwashiro Province, where the discovery site is located, with the suffix "-(Y)" indicating the dominant rare-earth element – yttrium.

Properties

Mohs hardness
6
Color
Dark red-brown, amber brown to brown
Luster
Adamantine to resinous
Streak
Pale brown
Density
0
Cleavage
on {010}, poor on {001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of Iwashiroite-(Y) is impossible without specialized equipment. It requires chemical analysis (e.g., using an electron microprobe - EDS/WDS) to confirm the presence of yttrium and tantalum as dominant components. X-ray diffraction (XRD) is necessary to determine its tetragonal crystal structure. ## Distinguishing from Similar Minerals It can be confused with other dark, heavy minerals from the fergusonite group, such as formanite-(Y) (dominant niobium instead of tantalum) or fergusonite-(Y). Differentiation requires precise chemical composition analysis. Visually, it is indistinguishable from many other microscopic, dark mineral inclusions. ## Crystal Forms Iwashiroite-(Y) forms very small, poorly developed (subhedral) grains and aggregates. Well-formed, idiomorphic crystals are not observed.

Geological environment

## Genesis Iwashiroite-(Y) is an igneous mineral, crystallizing in granitic pegmatites. It forms in the late stages of crystallization of pegmatites rich in rare-earth elements, tantalum, and niobium. ## Mineral Associations This mineral most often occurs as inclusions in biotite. It co-occurs with minerals such as quartz, microcline, albite, biotite, zircon, xenotime-(Y), monazite-(Ce), and other tantalum and niobium minerals. ## Localities The only confirmed and described occurrence (type locality) is the Suishoyama pegmatite in Fukushima Prefecture on Honshu Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria Iwashiroite-(Y) is a mineral of purely scientific and systematic importance. It does not form visually attractive specimens for the typical collector. Its value lies in its rarity and being part of the mineral paragenesis in the host rock. Specimens with accurately studied and documented presence of this mineral are most valuable for research institutions.

Care and storage

## Cleaning Due to the extremely small size of its grains and its occurrence as inclusions in other minerals, Iwashiroite-(Y) is not an object that is cleaned in the traditional sense. Rock specimens containing this mineral can be cleaned with a soft brush and distilled water, taking care not to damage the host mineral. ## What to Avoid Contact with strong acids and chemicals that could damage both Iwashiroite-(Y) itself and its associated minerals should be avoided. As a mineral containing radioactive elements (albeit in small quantities), it should be stored away from radiation-sensitive materials. ## Storage Specimens containing Iwashiroite-(Y) should be stored in stable conditions, away from dust and moisture, preferably in sealed collector boxes. The label should precisely describe the location and associated minerals.

External references

Sources

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