Tritomite-(Y)

Chemical formula: Y₅(SiO₄,BO₄)₃(O,OH,F)

Tritomite-(Y) is a rare yttrium borosilicate, forming dark, resinous masses and poorly formed crystals with high radioactivity.

## Characteristics Tritomite-(Y) is a complex borosilicate from the apatite group, containing yttrium and other rare earth elements. It typically occurs as massive aggregates or poorly formed, rounded crystals with a pyramidal or tabular habit. Its surface is often dull and covered with alteration products. This mineral is strongly radioactive due to the presence of thorium and uranium in its structure, which leads to its metamictization – the destruction of the crystal lattice by its own radiation. ## Physical Properties Due to its metamict state, its physical properties are variable. Hardness ranges from 5 to 6.5 on the Mohs scale, and density is approximately 4.2 g/cm³. Luster is vitreous to resinous on fresh fracture surfaces, but generally dull on external surfaces. It is brittle and exhibits conchoidal or uneven fracture. ## Colors and Varieties It is most commonly found in shades of dark brown to almost black. Sometimes it can be reddish-brown or yellowish. Thin splinters may be translucent brown. No named varieties of this mineral are distinguished. ## History and Name The name "tritomite" comes from the Greek word *tritos* (τρίτος), meaning "third", because at the time of its discovery it was thought to be the third mineral exhibiting a trigonal pyramidal habit. The suffix "-(Y)" was added in 1987 to indicate yttrium (Y) as the dominant rare earth element in its composition. The mineral was originally described from specimens from Langesundsfjorden in Norway.

Properties

Mohs hardness: 5 - 6.5
Luster: Resinous to vitreous
Streak: Grayish-brown
Density: 4.2
Cleavage: Imperfect on {0001}
Fracture: Conchoidal to uneven
Transparency: Translucent to opaque
Crystal system: Trigonal

Diagnostic features

## Identification A key diagnostic feature is its strong radioactivity, which can be easily detected with a Geiger counter. Other features include its dark color (brown to black), resinous luster on fresh fracture, high density, and occurrence in nepheline syenite pegmatites. It is often metamict, meaning amorphous as a result of radiation. ## Distinguishing from Similar Minerals It can be confused with other dark, metamict minerals from pegmatites, such as euxenite, fergusonite, or gadolinite. Differentiation often requires chemical analysis (EDS) to confirm the presence of boron, silicon, and the dominance of yttrium. Tritomite-(Y) is a borosilicate, whereas many similar minerals are oxides or silicates without boron. ## Crystal Forms Crystals are rare and usually poorly formed, with hexagonal pyramidal or thick tabular shapes. Most often, it forms irregular masses and grains intergrown in the host rock.

Geological environment

## Genesis Tritomite-(Y) is an igneous mineral, crystallizing in the late stages from highly differentiated alkaline melts. It occurs mainly in nepheline syenite pegmatites, where rare earth elements, boron, and other incompatible elements are concentrated. ## Mineral Associations It often co-occurs with other minerals typical of alkaline pegmatites, such as microcline, nepheline, aegirine, eudialyte, lorenzenite, astrophyllite, zircon, and catapleiite. ## Localities The most important and classic locality, from which the best specimens originate, is the Langesundsfjorden complex in Norway. It is also found in Russia on the Kola Peninsula (Khibiny and Lovozero massifs), in Canada (Mont Saint-Hilaire, Quebec), and in Sweden (Norra Kärr area).

Rarity

Very rare

For collectors

## Quality Criteria The quality of a tritomite-(Y) specimen is primarily assessed based on crystal development – even imperfect but recognizable pyramidal forms are highly valued. The size of the crystal or mass and the degree of preservation (lack of cracks) are also important. Specimens well-embedded in a contrasting rock matrix (e.g., white nepheline) are more desirable than isolated fragments. ## Popular Localities By far the most prized specimens by collectors come from localities in Norway, in the Langesundsfjorden area, which are the type material for this mineral. Specimens from Mont Saint-Hilaire in Canada and from the Kola Peninsula in Russia are also sought after, although they often occur as smaller grains or masses.

Care and storage

## Cleaning Cleaning tritomite-(Y) specimens is generally not recommended due to their brittleness and radioactivity. If absolutely necessary, a soft brush can be used to remove loose dust. Contact with water and chemical agents should be avoided. ## What to Avoid The mineral is strongly radioactive – exposure should be limited, and inhalation of dust should be avoided. It should not be heated, as this can release dangerous gases, including radon. It should be protected from impacts and shocks due to its brittleness. ## Storage Tritomite-(Y) specimens must be stored with precautions for radioactive materials. It is recommended to keep them in separate, lead-lined or thick-walled plastic containers, away from other minerals that could be damaged by radiation. The container should be clearly marked with a radioactivity symbol. Store in a well-ventilated area, away from places of permanent human presence.

Sources

IMA Mineral List (CNMNC)