Polycrase-(Y)

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

Polycrase-(Y) is a rare, radioactive titanium and niobium mineral from the euxenite group, forming black, resinous crystals in granitic pegmatites.

## Characteristics Polycrase-(Y) is a complex oxide of titanium, niobium, and yttrium, belonging to the euxenite group. It occurs as well-formed, prismatic crystals with a square or rectangular cross-section, often with longitudinal striations on the faces. Specimens are typically black, opaque, and covered with a thin, yellowish-brown or reddish-brown layer of weathering products. Due to its uranium and thorium content, this mineral is radioactive and undergoes metamictization over time, meaning the destruction of its internal crystal structure while retaining the external crystal form. ## Physical Properties Fresh, unaltered surfaces of polycrase-(Y) exhibit a strong, resinous or nearly metallic luster. The mineral's hardness ranges from 5 to 6 on the Mohs scale, but it can be lower due to metamictization. Density is variable, usually in the range of 4.3 to 5.9 g/cm³, depending on chemical composition and the degree of metamict alteration. The mineral is brittle and characterized by a conchoidal or uneven fracture. ## Colors and Varieties Polycrase-(Y) is typically black, less commonly brownish-black or reddish-brown. It does not form color varieties or commercial variants. Its name reflects its complex chemical composition, and the suffix "-(Y)" indicates the dominance of yttrium as a rare-earth element. ## History and Name The name "polycrase" comes from the Greek words *polys* (many) and *krasis* (mixture), referring to its complex chemical composition. The mineral was first described in 1844. The suffix "-(Y)" was added later in accordance with the rules for naming minerals with a dominant rare-earth element, in this case, yttrium. ## Applications Due to its rarity and often small crystal sizes, polycrase-(Y) has no industrial applications. However, it is of interest to collectors specializing in rare-earth minerals, pegmatite minerals, and radioactive minerals.

Properties

Mohs hardness: 5-6
Luster: Resinous to submetallic
Streak: Brown, gray, yellowish-brown
Density: 4.3-5.9
Cleavage: Indistinct on {010} and {110}
Fracture: Conchoidal to uneven
Transparency: Opaque, thin splinters translucent brown
Crystal system: Orthorhombic

Diagnostic features

## Identification Polycrase-(Y) can be identified by its black color, strong resinous or submetallic luster on fresh fracture surfaces, and characteristic prismatic crystals. An important diagnostic feature is its high density, noticeable even in small specimens. Its radioactivity, easily detected with a Geiger counter, is also a key indicator. ## Differentiation from Similar Minerals Polycrase-(Y) is difficult to distinguish from other black, radioactive minerals of the euxenite group, such as euxenite-(Y) or samarskite-(Y), without advanced chemical analyses (EDS/WDS). It differs from euxenite-(Y) mainly in the titanium to niobium ratio. Visually, it can be confused with columbite, ilmenite, or schorl tourmaline. It is distinguished from columbite and ilmenite by its weaker metallic luster and often poorer crystal face development, and from schorl by its significantly higher density and lack of striations typical of tourmaline. ## Crystal Forms It most commonly forms single, elongated prismatic crystals, often with complex cross-sections. Crystals may be terminated by simple or complex pyramidal faces. It also occurs as irregular granular aggregates within the mass of other minerals, mainly in quartz or feldspars.

Geological environment

## Genesis Polycrase-(Y) is an accessory mineral, typical of granitic pegmatites, especially those enriched in rare-earth elements (REE), niobium, tantalum, and titanium. It forms in the late stages of pegmatitic magma crystallization under conditions of high temperature and pressure. ## Mineral Associations It often co-occurs with other pegmatite minerals such as albite, microcline, quartz, muscovite, biotite, zircon, euxenite-(Y), samarskite-(Y), fergusonite-(Y), monazite-(Ce), xenotime-(Y), and garnets (especially spessartine). ## Localities Important and historical localities from which well-formed polycrase-(Y) crystals originate include: - Norway: Arendal, Kragerø, Iveland, and Evje areas – classic sites that have yielded some of the best specimens. - United States: pegmatites in North Carolina (e.g., Mitchell County), Colorado (e.g., South Platte district), and Texas (e.g., Baringer Hill). - Brazil: numerous pegmatites in Minas Gerais state. - Madagascar: Antananarivo region, where it occurs in association with other REE minerals. - Poland: reported in pegmatites in the Strzegom area of Lower Silesia.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those of polycrase-(Y) with well-formed, sharp, and undamaged crystals. Large, single crystals with a distinct luster are highly prized. Specimens embedded in a contrasting rock matrix (e.g., in white albite or quartz) are much more desirable than loose crystals. The presence of distinct crystallographic faces and the absence of mechanical damage significantly increase collectible value. ## Popular Localities Classic and most sought-after specimens come from historical localities in Norway (Iveland, Arendal), which have provided many exemplary crystals now housed in museum collections. High-quality specimens have also been found in the United States (North Carolina) and Madagascar.

Care and storage

## Cleaning Specimens should only be dry-cleaned, using a soft brush to remove dust. Due to the potential presence of weathering products, avoid contact with water and other liquids that could remove or damage the specimen. ## What to Avoid As a radioactive mineral, polycrase-(Y) requires careful handling. Avoid prolonged, direct contact and storage in areas of permanent human habitation, such as bedrooms or desks. It should not be cut, polished, or subjected to other mechanical processes without appropriate personal protective equipment (dust mask) to avoid inhaling radioactive dust. The mineral is sensitive to strong acids. ## Storage It is recommended to store specimens in closed, labeled containers, away from other radiation-sensitive minerals (e.g., smoky quartz, some fluorites). Ideally, they should be kept in a ventilated room, in a special display cabinet or collector's box, away from flammable materials and out of reach of children and pets.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FPolycrase