Håleniusite-(La)

Chemical formula: LaOF

Håleniusite-(La) is an extremely rare lanthanum oxyfluoride, forming microscopic, hexagonal crystals known only from one locality in the world.

## Characteristics Håleniusite-(La) is a lanthanum oxyfluoride with the chemical formula LaOF. It belongs to the matlockite group. It forms very small, most often microscopic, tabular or short-prismatic crystals with a hexagonal outline. They are usually colorless to pale yellow and transparent. Due to their size, specimens are mainly visible under magnification and are primarily of scientific interest and for specialized collectors of rare minerals. ## Physical Properties Crystals exhibit adamantine luster. The Mohs hardness is approximately 3.5, and the density calculated based on chemical composition and unit cell parameters is 5.89 g/cm³. The mineral is transparent and shows perfect cleavage in one plane, perpendicular to the main crystal axis. ## Colors and Varieties The mineral is usually colorless, rarely taking on pale yellowish hues. No colored or commercial varieties have been distinguished. ## History and Name The mineral name was approved by the International Mineralogical Association (IMA) in 2010. The name "Håleniusite" honors Ulf Hålenius (born 1951), professor of mineralogy at the Swedish Museum of Natural History in Stockholm, for his contributions to mineral research. The "-(La)" suffix in the name (a so-called Levinson modifier) indicates lanthanum as the dominant rare-earth element in the mineral's chemical composition. The type locality (locus typicus) is the Östanmossa mine in Sweden.

Properties

Mohs hardness
3.5
Luster
Adamantine
Streak
White
Density
5.89
Cleavage
Perfect on {0001}
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of håleniusite-(La) based on visual characteristics is impossible. Recognition requires advanced analytical techniques, such as energy-dispersive spectroscopy (EDS) or wavelength-dispersive spectroscopy (WDS) to confirm the presence of lanthanum, oxygen, and fluorine. Final identification is possible through X-ray diffraction (XRD), which confirms its crystal structure. ## Distinguishing from Similar Minerals Visually, it can be confused with many other colorless or pale yellow, microscopic minerals occurring in similar environments, such as other REE minerals (e.g., bastnäsite), apatite, or calcite. Definitive distinction is only possible based on chemical analysis. ## Crystal Forms It forms hexagonal, tabular or short, prismatic crystals, often not exceeding several tens of micrometers in size. It occurs as single, scattered crystals on the surface of other minerals.

Geological environment

## Genesis Håleniusite-(La) forms as a result of hydrothermal processes within iron-manganese skarns. In the type locality, it occurs in vugs and fissures in skarn composed of dolomite and calcite. ## Mineral Associations In the type locality, it is associated with minerals such as calcite, hematite, dollaseite-(Ce), bastnäsite-(La), bastnäsite-(Ce), and fluorbritholite-(Ce). ## Localities The only confirmed occurrence of håleniusite-(La) in the world is its type locality – the historical iron and manganese ore mine Östanmossa, in Norberg Municipality, Västmanland region, Sweden.

Rarity

Extremely rare

For collectors

## Quality Criteria For such a rare micromineral, the main criterion of value is its mere presence and confirmed identity. Specimens with well-formed, sharp crystals, even if microscopic, have additional value. Rich clusters and associations with other rare minerals from this locality are also highly prized. ## Popular Localities The only source of specimens is the Östanmossa mine in Sweden. Material from this locality is extremely difficult to acquire and is highly sought after by specialized collectors of rare mineral species.

Care and storage

## Cleaning Håleniusite-(La) specimens are almost exclusively micromounts, which should not be cleaned mechanically or chemically. Any attempt at cleaning, even with water, risks irreversible loss or damage to the microscopic crystals. Only very careful dust removal using a stream of compressed air from a safe distance is permissible. ## What to Avoid Contact with acids, chemical agents, and ultrasonic cleaners must be strictly avoided. The mineral is brittle and sensitive to shocks and mechanical damage. It should not be heated or exposed to extreme temperature changes. ## Storage Specimens should be stored exclusively in specialized "micromount" boxes, which protect them from dust, moisture, and mechanical damage. They should not be removed from the box unnecessarily.

External references

Sources

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