Hydroxylbastnäsite-(La)

Chemical formula: La(CO<sub>3</sub>)(OH)

Hydroxylbastnäsite-(La) is a rare lanthanide mineral, forming tiny, hexagonal crystals and aggregates, closely related to bastnäsite.

## Characteristics Hydroxylbastnäsite-(La) is a carbonate group mineral belonging to the bastnäsite group. Its composition is dominated by lanthanum, as indicated in its name. It typically forms very small, hexagonal, tabular or prismatic crystals, often grouped into rosetted or spherical aggregates. Specimens are most commonly found as coatings and accumulations within other rocks. ## Physical Properties Crystals exhibit a vitreous to greasy luster. The Mohs hardness is approximately 4. The mineral is relatively dense, with a specific gravity of about 4.79 g/cm³. It is brittle and exhibits an uneven fracture. ## Colors and Varieties Hydroxylbastnäsite-(La) is usually colorless, white, grayish-white, or pale yellow. No distinct color varieties or commercial forms are recognized. ## History and Name The mineral's name refers to its chemical composition – the presence of a hydroxyl group (OH) and the dominance of lanthanum (La) – and its structural similarity to bastnäsite. It was first described by A.S. Kiriukov and colleagues in 1964 based on material from the Burpala massif in Buryatia (Russia). ## Applications Due to its rarity and small crystal size, hydroxylbastnäsite-(La) has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness
4
Luster
Vitreous, Greasy
Streak
White
Density
4.79
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Identifying hydroxylbastnäsite-(La) is very difficult without advanced analytical methods (XRD, EDS/WDS). Preliminary identification is based on its occurrence environment (alkaline pegmatites, carbonatites), crystal habit (small hexagonal tablets), and associated minerals (other rare earth minerals). Reaction with hydrochloric acid (violent effervescence) is a diagnostic feature for carbonates, but it does not distinguish it from other minerals in this group. ## Distinguishing from Similar Minerals It can be confused with other minerals from the bastnäsite group, such as bastnäsite-(Ce), bastnäsite-(La), or hydroxylbastnäsite-(Ce). Distinguishing them from each other is only possible based on chemical analysis determining the dominant rare earth element and the presence of the hydroxyl group. Visually, it is indistinguishable from hydroxylbastnäsite-(Nd). ## Crystal Forms It forms small, hexagonal, tabular crystals. It often occurs in radial or spherulitic (spherical) aggregates, as well as granular masses.

Geological environment

## Genesis Hydroxylbastnäsite-(La) is a mineral of hydrothermal origin. It forms in the late stages of crystallization of nepheline-syenite pegmatites and in carbonatites. It develops in environments rich in rare earth elements, especially lanthanum. ## Mineral Associations It often co-occurs with other rare earth minerals. Typical associated minerals include eudialyte, lorenzenite, lamprophyllite, aegirine, microcline, nepheline, as well as other REE carbonates like bastnäsite-(Ce) and ancylite-(Ce). ## Localities Important occurrences of this mineral include the Burpala massif in Buryatia (Russia), where it was discovered. It is also known from the Ilimaussaq alkaline complex in Greenland, Mont Saint-Hilaire in Quebec (Canada), and from the Bayan Obo carbonatites in China.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals, even if small, are most valued by collectors. Their grouping into aesthetic aggregates, such as rosettes, is important. A contrasting matrix of another mineral, e.g., aegirine, significantly enhances the specimen's appeal. Purity and lack of damage are key. ## Popular Localities The best quality specimens, though still microscopic, come from Mont Saint-Hilaire in Canada. The type locality in Russia (Burpala massif) has also yielded well-defined specimens, which are scientifically important.

Care and storage

## Cleaning Utmost caution is recommended. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding vigorous rubbing. The specimen should be thoroughly dried immediately. ## What to Avoid The mineral is sensitive to acids, which cause it to effervesce violently and decompose. Avoid contact with all household chemicals and acids. It should not be heated or cleaned with ultrasonics. ## Storage Specimens should be stored in stable conditions, away from moisture and chemical contaminants. It is best to keep them in sealed display boxes to protect fragile crystals from mechanical damage and dust.

Sources

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