Thorbastnäsite
Chemical formula: Th<sup>4+</sup>Ca(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>·3H<sub>2</sub>O
Thorbastnäsite is a rare thorium and calcium fluorocarbonate mineral, forming small, hexagonal crystals and aggregates with a vitreous or pearly luster.
Properties
- Mohs hardness
- 4-4.5
- Luster
- Vitreous, Pearly
- Streak
- White
- Density
- 3.98-4.02
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Hexagonal
Diagnostic features
## Identification Helpful identification features include: hexagonal crystal habit, often in tabular forms, rosette-like aggregates, pearly luster on cleavage planes, and relatively high density. However, its association with a specific geological environment – alkaline igneous rocks and carbonatites – is key. Final confirmation requires advanced chemical (EDS/WDS) or X-ray (XRD) analyses due to its similarity to other minerals in the bastnäsite group. ## Distinguishing from Similar Minerals Thorbastnäsite can be confused with other minerals from the bastnäsite group, such as bastnäsite-(Ce) or hydroxylbastnäsite-(Ce), which have similar habits and luster. "Hand specimen" differentiation is practically impossible and requires chemical analysis to confirm the dominance of thorium. It may also resemble some micas or zeolites, but differs significantly in its much higher density. ## Crystal Forms It crystallizes in thin, tabular crystals with a hexagonal outline. It often forms radial, spherulitic, or rosette-like clusters and aggregates.
Geological environment
## Genesis Thorbastnäsite is a hydrothermal mineral associated with alkaline rocks, such as nepheline syenites and carbonatites. It forms in the late stages of crystallization of these magmas, in veins and mineral pockets, often in association with other rare minerals. ## Mineral Associations It co-occurs with minerals such as fluorite, zircon, pyrochlore, bastnäsite, galena, sphalerite, and amphibole group minerals (arfvedsonite, riebeckite). ## Localities Key localities for this mineral include the type locality in an alkaline massif in Siberia (Russia), carbonatite complexes on the Kola Peninsula (Russia), and Mont Saint-Hilaire in Quebec (Canada), known for the occurrence of many rare alkaline minerals.
Rarity
Very rare
For collectors
## Quality Criteria For collectors, the most valuable specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. Crystal size is also important – the larger, the more valuable. A contrasting matrix background and co-occurrence with other rare minerals significantly enhance a specimen's attractiveness. ## Popular Localities The most prized specimens by collectors come from Mont Saint-Hilaire in Canada, which is famous for well-formed crystals of relatively large size. Specimens from Russian localities are also sought after, although they often appear as finer aggregates.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its brittleness and potential reactivity, contact with water and any chemical agents should be avoided. ## What to Avoid Contact with acids, which can damage it, must be strictly avoided. The mineral is brittle and sensitive to impacts and scratches. As a thorium-bearing mineral, it is radioactive – direct exposure should be limited, and it should be stored away from other radiation-sensitive minerals. ## Storage It is recommended to store specimens in separate, padded collector's boxes to prevent mechanical damage and contact with other minerals. Due to its radioactivity, it should be kept in a location away from permanent human presence, preferably in a ventilated display case.